diff --git a/Cargo.lock b/Cargo.lock
index be5ceeb5f1341..e199e700fedf2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3574,6 +3574,7 @@ dependencies = [
  "rustc_errors",
  "rustc_hir",
  "rustc_index",
+ "rustc_macros",
  "rustc_middle",
  "rustc_query_system",
  "rustc_session",
diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
index f687bfeced841..db4c5fe6b4426 100644
--- a/compiler/rustc_ast/src/visit.rs
+++ b/compiler/rustc_ast/src/visit.rs
@@ -244,14 +244,12 @@ pub trait Visitor<'ast>: Sized {
 
 #[macro_export]
 macro_rules! walk_list {
-    ($visitor: expr, $method: ident, $list: expr) => {
-        for elem in $list {
-            $visitor.$method(elem)
-        }
-    };
-    ($visitor: expr, $method: ident, $list: expr, $($extra_args: expr),*) => {
-        for elem in $list {
-            $visitor.$method(elem, $($extra_args,)*)
+    ($visitor: expr, $method: ident, $list: expr $(, $($extra_args: expr),* )?) => {
+        {
+            #[cfg_attr(not(bootstrap), allow(for_loop_over_fallibles))]
+            for elem in $list {
+                $visitor.$method(elem $(, $($extra_args,)* )?)
+            }
         }
     }
 }
diff --git a/compiler/rustc_ast_lowering/Cargo.toml b/compiler/rustc_ast_lowering/Cargo.toml
index 39ba62ef2ffc5..474aff2e2aac0 100644
--- a/compiler/rustc_ast_lowering/Cargo.toml
+++ b/compiler/rustc_ast_lowering/Cargo.toml
@@ -15,6 +15,7 @@ rustc_target = { path = "../rustc_target" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_index = { path = "../rustc_index" }
 rustc_middle = { path = "../rustc_middle" }
+rustc_macros = { path = "../rustc_macros" }
 rustc_query_system = { path = "../rustc_query_system" }
 rustc_span = { path = "../rustc_span" }
 rustc_errors = { path = "../rustc_errors" }
diff --git a/compiler/rustc_ast_lowering/src/asm.rs b/compiler/rustc_ast_lowering/src/asm.rs
index 4166b4fc2e5bc..0dba9da63da2a 100644
--- a/compiler/rustc_ast_lowering/src/asm.rs
+++ b/compiler/rustc_ast_lowering/src/asm.rs
@@ -1,11 +1,17 @@
 use crate::{ImplTraitContext, ImplTraitPosition, ParamMode, ResolverAstLoweringExt};
 
+use super::errors::{
+    AbiSpecifiedMultipleTimes, AttSyntaxOnlyX86, ClobberAbiNotSupported,
+    InlineAsmUnsupportedTarget, InvalidAbiClobberAbi, InvalidAsmTemplateModifierConst,
+    InvalidAsmTemplateModifierRegClass, InvalidAsmTemplateModifierRegClassSub,
+    InvalidAsmTemplateModifierSym, InvalidRegister, InvalidRegisterClass, RegisterClassOnlyClobber,
+    RegisterConflict,
+};
 use super::LoweringContext;
 
 use rustc_ast::ptr::P;
 use rustc_ast::*;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
-use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::definitions::DefPathData;
@@ -26,13 +32,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let asm_arch =
             if self.tcx.sess.opts.actually_rustdoc { None } else { self.tcx.sess.asm_arch };
         if asm_arch.is_none() && !self.tcx.sess.opts.actually_rustdoc {
-            struct_span_err!(
-                self.tcx.sess,
-                sp,
-                E0472,
-                "inline assembly is unsupported on this target"
-            )
-            .emit();
+            self.tcx.sess.emit_err(InlineAsmUnsupportedTarget { span: sp });
         }
         if let Some(asm_arch) = asm_arch {
             // Inline assembly is currently only stable for these architectures.
@@ -59,10 +59,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             && !matches!(asm_arch, Some(asm::InlineAsmArch::X86 | asm::InlineAsmArch::X86_64))
             && !self.tcx.sess.opts.actually_rustdoc
         {
-            self.tcx
-                .sess
-                .struct_span_err(sp, "the `att_syntax` option is only supported on x86")
-                .emit();
+            self.tcx.sess.emit_err(AttSyntaxOnlyX86 { span: sp });
         }
         if asm.options.contains(InlineAsmOptions::MAY_UNWIND) && !self.tcx.features().asm_unwind {
             feature_err(
@@ -82,51 +79,37 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         // If the abi was already in the list, emit an error
                         match clobber_abis.get(&abi) {
                             Some((prev_name, prev_sp)) => {
-                                let mut err = self.tcx.sess.struct_span_err(
-                                    *abi_span,
-                                    &format!("`{}` ABI specified multiple times", prev_name),
-                                );
-                                err.span_label(*prev_sp, "previously specified here");
-
                                 // Multiple different abi names may actually be the same ABI
                                 // If the specified ABIs are not the same name, alert the user that they resolve to the same ABI
                                 let source_map = self.tcx.sess.source_map();
-                                if source_map.span_to_snippet(*prev_sp)
-                                    != source_map.span_to_snippet(*abi_span)
-                                {
-                                    err.note("these ABIs are equivalent on the current target");
-                                }
+                                let equivalent = (source_map.span_to_snippet(*prev_sp)
+                                    != source_map.span_to_snippet(*abi_span))
+                                .then_some(());
 
-                                err.emit();
+                                self.tcx.sess.emit_err(AbiSpecifiedMultipleTimes {
+                                    abi_span: *abi_span,
+                                    prev_name: *prev_name,
+                                    prev_span: *prev_sp,
+                                    equivalent,
+                                });
                             }
                             None => {
-                                clobber_abis.insert(abi, (abi_name, *abi_span));
+                                clobber_abis.insert(abi, (*abi_name, *abi_span));
                             }
                         }
                     }
                     Err(&[]) => {
-                        self.tcx
-                            .sess
-                            .struct_span_err(
-                                *abi_span,
-                                "`clobber_abi` is not supported on this target",
-                            )
-                            .emit();
+                        self.tcx.sess.emit_err(ClobberAbiNotSupported { abi_span: *abi_span });
                     }
                     Err(supported_abis) => {
-                        let mut err = self
-                            .tcx
-                            .sess
-                            .struct_span_err(*abi_span, "invalid ABI for `clobber_abi`");
                         let mut abis = format!("`{}`", supported_abis[0]);
                         for m in &supported_abis[1..] {
                             let _ = write!(abis, ", `{}`", m);
                         }
-                        err.note(&format!(
-                            "the following ABIs are supported on this target: {}",
-                            abis
-                        ));
-                        err.emit();
+                        self.tcx.sess.emit_err(InvalidAbiClobberAbi {
+                            abi_span: *abi_span,
+                            supported_abis: abis,
+                        });
                     }
                 }
             }
@@ -141,24 +124,28 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             .iter()
             .map(|(op, op_sp)| {
                 let lower_reg = |reg| match reg {
-                    InlineAsmRegOrRegClass::Reg(s) => {
+                    InlineAsmRegOrRegClass::Reg(reg) => {
                         asm::InlineAsmRegOrRegClass::Reg(if let Some(asm_arch) = asm_arch {
-                            asm::InlineAsmReg::parse(asm_arch, s).unwrap_or_else(|e| {
-                                let msg = format!("invalid register `{}`: {}", s, e);
-                                sess.struct_span_err(*op_sp, &msg).emit();
+                            asm::InlineAsmReg::parse(asm_arch, reg).unwrap_or_else(|error| {
+                                sess.emit_err(InvalidRegister { op_span: *op_sp, reg, error });
                                 asm::InlineAsmReg::Err
                             })
                         } else {
                             asm::InlineAsmReg::Err
                         })
                     }
-                    InlineAsmRegOrRegClass::RegClass(s) => {
+                    InlineAsmRegOrRegClass::RegClass(reg_class) => {
                         asm::InlineAsmRegOrRegClass::RegClass(if let Some(asm_arch) = asm_arch {
-                            asm::InlineAsmRegClass::parse(asm_arch, s).unwrap_or_else(|e| {
-                                let msg = format!("invalid register class `{}`: {}", s, e);
-                                sess.struct_span_err(*op_sp, &msg).emit();
-                                asm::InlineAsmRegClass::Err
-                            })
+                            asm::InlineAsmRegClass::parse(asm_arch, reg_class).unwrap_or_else(
+                                |error| {
+                                    sess.emit_err(InvalidRegisterClass {
+                                        op_span: *op_sp,
+                                        reg_class,
+                                        error,
+                                    });
+                                    asm::InlineAsmRegClass::Err
+                                },
+                            )
                         } else {
                             asm::InlineAsmRegClass::Err
                         })
@@ -282,50 +269,39 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         }
                         let valid_modifiers = class.valid_modifiers(asm_arch.unwrap());
                         if !valid_modifiers.contains(&modifier) {
-                            let mut err = sess.struct_span_err(
-                                placeholder_span,
-                                "invalid asm template modifier for this register class",
-                            );
-                            err.span_label(placeholder_span, "template modifier");
-                            err.span_label(op_sp, "argument");
-                            if !valid_modifiers.is_empty() {
+                            let sub = if !valid_modifiers.is_empty() {
                                 let mut mods = format!("`{}`", valid_modifiers[0]);
                                 for m in &valid_modifiers[1..] {
                                     let _ = write!(mods, ", `{}`", m);
                                 }
-                                err.note(&format!(
-                                    "the `{}` register class supports \
-                                     the following template modifiers: {}",
-                                    class.name(),
-                                    mods
-                                ));
+                                InvalidAsmTemplateModifierRegClassSub::SupportModifier {
+                                    class_name: class.name(),
+                                    modifiers: mods,
+                                }
                             } else {
-                                err.note(&format!(
-                                    "the `{}` register class does not support template modifiers",
-                                    class.name()
-                                ));
-                            }
-                            err.emit();
+                                InvalidAsmTemplateModifierRegClassSub::DoesNotSupportModifier {
+                                    class_name: class.name(),
+                                }
+                            };
+                            sess.emit_err(InvalidAsmTemplateModifierRegClass {
+                                placeholder_span,
+                                op_span: op_sp,
+                                sub,
+                            });
                         }
                     }
                     hir::InlineAsmOperand::Const { .. } => {
-                        let mut err = sess.struct_span_err(
+                        sess.emit_err(InvalidAsmTemplateModifierConst {
                             placeholder_span,
-                            "asm template modifiers are not allowed for `const` arguments",
-                        );
-                        err.span_label(placeholder_span, "template modifier");
-                        err.span_label(op_sp, "argument");
-                        err.emit();
+                            op_span: op_sp,
+                        });
                     }
                     hir::InlineAsmOperand::SymFn { .. }
                     | hir::InlineAsmOperand::SymStatic { .. } => {
-                        let mut err = sess.struct_span_err(
+                        sess.emit_err(InvalidAsmTemplateModifierSym {
                             placeholder_span,
-                            "asm template modifiers are not allowed for `sym` arguments",
-                        );
-                        err.span_label(placeholder_span, "template modifier");
-                        err.span_label(op_sp, "argument");
-                        err.emit();
+                            op_span: op_sp,
+                        });
                     }
                 }
             }
@@ -346,12 +322,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 // require that the operand name an explicit register, not a
                 // register class.
                 if reg_class.is_clobber_only(asm_arch.unwrap()) && !op.is_clobber() {
-                    let msg = format!(
-                        "register class `{}` can only be used as a clobber, \
-                             not as an input or output",
-                        reg_class.name()
-                    );
-                    sess.struct_span_err(op_sp, &msg).emit();
+                    sess.emit_err(RegisterClassOnlyClobber {
+                        op_span: op_sp,
+                        reg_class_name: reg_class.name(),
+                    });
                     continue;
                 }
 
@@ -391,16 +365,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                         unreachable!();
                                     };
 
-                                    let msg = format!(
-                                        "register `{}` conflicts with register `{}`",
-                                        reg.name(),
-                                        reg2.name()
-                                    );
-                                    let mut err = sess.struct_span_err(op_sp, &msg);
-                                    err.span_label(op_sp, &format!("register `{}`", reg.name()));
-                                    err.span_label(op_sp2, &format!("register `{}`", reg2.name()));
-
-                                    match (op, op2) {
+                                    let in_out = match (op, op2) {
                                         (
                                             hir::InlineAsmOperand::In { .. },
                                             hir::InlineAsmOperand::Out { late, .. },
@@ -411,14 +376,18 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                         ) => {
                                             assert!(!*late);
                                             let out_op_sp = if input { op_sp2 } else { op_sp };
-                                            let msg = "use `lateout` instead of \
-                                                       `out` to avoid conflict";
-                                            err.span_help(out_op_sp, msg);
-                                        }
-                                        _ => {}
-                                    }
+                                            Some(out_op_sp)
+                                        },
+                                        _ => None,
+                                    };
 
-                                    err.emit();
+                                    sess.emit_err(RegisterConflict {
+                                        op_span1: op_sp,
+                                        op_span2: op_sp2,
+                                        reg1_name: reg.name(),
+                                        reg2_name: reg2.name(),
+                                        in_out
+                                    });
                                 }
                                 Entry::Vacant(v) => {
                                     if r == reg {
diff --git a/compiler/rustc_ast_lowering/src/errors.rs b/compiler/rustc_ast_lowering/src/errors.rs
new file mode 100644
index 0000000000000..59f1b7180e4f4
--- /dev/null
+++ b/compiler/rustc_ast_lowering/src/errors.rs
@@ -0,0 +1,329 @@
+use rustc_errors::{fluent, AddSubdiagnostic, Applicability, Diagnostic, DiagnosticArgFromDisplay};
+use rustc_macros::{SessionDiagnostic, SessionSubdiagnostic};
+use rustc_span::{symbol::Ident, Span, Symbol};
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::generic_type_with_parentheses, code = "E0214")]
+pub struct GenericTypeWithParentheses {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[subdiagnostic]
+    pub sub: Option<UseAngleBrackets>,
+}
+
+#[derive(Clone, Copy)]
+pub struct UseAngleBrackets {
+    pub open_param: Span,
+    pub close_param: Span,
+}
+
+impl AddSubdiagnostic for UseAngleBrackets {
+    fn add_to_diagnostic(self, diag: &mut Diagnostic) {
+        diag.multipart_suggestion(
+            fluent::ast_lowering::use_angle_brackets,
+            vec![(self.open_param, String::from("<")), (self.close_param, String::from(">"))],
+            Applicability::MaybeIncorrect,
+        );
+    }
+}
+
+#[derive(SessionDiagnostic)]
+#[help]
+#[diag(ast_lowering::invalid_abi, code = "E0703")]
+pub struct InvalidAbi {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub abi: Symbol,
+    pub valid_abis: String,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::assoc_ty_parentheses)]
+pub struct AssocTyParentheses {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub sub: AssocTyParenthesesSub,
+}
+
+#[derive(Clone, Copy)]
+pub enum AssocTyParenthesesSub {
+    Empty { parentheses_span: Span },
+    NotEmpty { open_param: Span, close_param: Span },
+}
+
+impl AddSubdiagnostic for AssocTyParenthesesSub {
+    fn add_to_diagnostic(self, diag: &mut Diagnostic) {
+        match self {
+            Self::Empty { parentheses_span } => diag.multipart_suggestion(
+                fluent::ast_lowering::remove_parentheses,
+                vec![(parentheses_span, String::new())],
+                Applicability::MaybeIncorrect,
+            ),
+            Self::NotEmpty { open_param, close_param } => diag.multipart_suggestion(
+                fluent::ast_lowering::use_angle_brackets,
+                vec![(open_param, String::from("<")), (close_param, String::from(">"))],
+                Applicability::MaybeIncorrect,
+            ),
+        };
+    }
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_lowering::misplaced_impl_trait, code = "E0562")]
+pub struct MisplacedImplTrait<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub position: DiagnosticArgFromDisplay<'a>,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::rustc_box_attribute_error)]
+pub struct RustcBoxAttributeError {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::underscore_expr_lhs_assign)]
+pub struct UnderscoreExprLhsAssign {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::base_expression_double_dot)]
+pub struct BaseExpressionDoubleDot {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::await_only_in_async_fn_and_blocks, code = "E0728")]
+pub struct AwaitOnlyInAsyncFnAndBlocks {
+    #[primary_span]
+    #[label]
+    pub dot_await_span: Span,
+    #[label(ast_lowering::this_not_async)]
+    pub item_span: Option<Span>,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::generator_too_many_parameters, code = "E0628")]
+pub struct GeneratorTooManyParameters {
+    #[primary_span]
+    pub fn_decl_span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::closure_cannot_be_static, code = "E0697")]
+pub struct ClosureCannotBeStatic {
+    #[primary_span]
+    pub fn_decl_span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[help]
+#[diag(ast_lowering::async_non_move_closure_not_supported, code = "E0708")]
+pub struct AsyncNonMoveClosureNotSupported {
+    #[primary_span]
+    pub fn_decl_span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::functional_record_update_destructuring_assignment)]
+pub struct FunctionalRecordUpdateDestructuringAssignemnt {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::async_generators_not_supported, code = "E0727")]
+pub struct AsyncGeneratorsNotSupported {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::inline_asm_unsupported_target, code = "E0472")]
+pub struct InlineAsmUnsupportedTarget {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::att_syntax_only_x86)]
+pub struct AttSyntaxOnlyX86 {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::abi_specified_multiple_times)]
+pub struct AbiSpecifiedMultipleTimes {
+    #[primary_span]
+    pub abi_span: Span,
+    pub prev_name: Symbol,
+    #[label]
+    pub prev_span: Span,
+    #[note]
+    pub equivalent: Option<()>,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::clobber_abi_not_supported)]
+pub struct ClobberAbiNotSupported {
+    #[primary_span]
+    pub abi_span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[note]
+#[diag(ast_lowering::invalid_abi_clobber_abi)]
+pub struct InvalidAbiClobberAbi {
+    #[primary_span]
+    pub abi_span: Span,
+    pub supported_abis: String,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::invalid_register)]
+pub struct InvalidRegister<'a> {
+    #[primary_span]
+    pub op_span: Span,
+    pub reg: Symbol,
+    pub error: &'a str,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::invalid_register_class)]
+pub struct InvalidRegisterClass<'a> {
+    #[primary_span]
+    pub op_span: Span,
+    pub reg_class: Symbol,
+    pub error: &'a str,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(ast_lowering::invalid_asm_template_modifier_reg_class)]
+pub struct InvalidAsmTemplateModifierRegClass {
+    #[primary_span]
+    #[label(ast_lowering::template_modifier)]
+    pub placeholder_span: Span,
+    #[label(ast_lowering::argument)]
+    pub op_span: Span,
+    #[subdiagnostic]
+    pub sub: InvalidAsmTemplateModifierRegClassSub,
+}
+
+#[derive(SessionSubdiagnostic)]
+pub enum InvalidAsmTemplateModifierRegClassSub {
+    #[note(ast_lowering::support_modifiers)]
+    SupportModifier { class_name: Symbol, modifiers: String },
+    #[note(ast_lowering::does_not_support_modifiers)]
+    DoesNotSupportModifier { class_name: Symbol },
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::invalid_asm_template_modifier_const)]
+pub struct InvalidAsmTemplateModifierConst {
+    #[primary_span]
+    #[label(ast_lowering::template_modifier)]
+    pub placeholder_span: Span,
+    #[label(ast_lowering::argument)]
+    pub op_span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::invalid_asm_template_modifier_sym)]
+pub struct InvalidAsmTemplateModifierSym {
+    #[primary_span]
+    #[label(ast_lowering::template_modifier)]
+    pub placeholder_span: Span,
+    #[label(ast_lowering::argument)]
+    pub op_span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::register_class_only_clobber)]
+pub struct RegisterClassOnlyClobber {
+    #[primary_span]
+    pub op_span: Span,
+    pub reg_class_name: Symbol,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::register_conflict)]
+pub struct RegisterConflict<'a> {
+    #[primary_span]
+    #[label(ast_lowering::register1)]
+    pub op_span1: Span,
+    #[label(ast_lowering::register2)]
+    pub op_span2: Span,
+    pub reg1_name: &'a str,
+    pub reg2_name: &'a str,
+    #[help]
+    pub in_out: Option<Span>,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[help]
+#[diag(ast_lowering::sub_tuple_binding)]
+pub struct SubTupleBinding<'a> {
+    #[primary_span]
+    #[label]
+    #[suggestion_verbose(
+        ast_lowering::sub_tuple_binding_suggestion,
+        code = "..",
+        applicability = "maybe-incorrect"
+    )]
+    pub span: Span,
+    pub ident: Ident,
+    pub ident_name: Symbol,
+    pub ctx: &'a str,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::extra_double_dot)]
+pub struct ExtraDoubleDot<'a> {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[label(ast_lowering::previously_used_here)]
+    pub prev_span: Span,
+    pub ctx: &'a str,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[note]
+#[diag(ast_lowering::misplaced_double_dot)]
+pub struct MisplacedDoubleDot {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::misplaced_relax_trait_bound)]
+pub struct MisplacedRelaxTraitBound {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::not_supported_for_lifetime_binder_async_closure)]
+pub struct NotSupportedForLifetimeBinderAsyncClosure {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic, Clone, Copy)]
+#[diag(ast_lowering::arbitrary_expression_in_pattern)]
+pub struct ArbitraryExpressionInPattern {
+    #[primary_span]
+    pub span: Span,
+}
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index bd61f4fa87ab8..61f8c0216f1cf 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -1,3 +1,9 @@
+use super::errors::{
+    AsyncGeneratorsNotSupported, AsyncNonMoveClosureNotSupported, AwaitOnlyInAsyncFnAndBlocks,
+    BaseExpressionDoubleDot, ClosureCannotBeStatic, FunctionalRecordUpdateDestructuringAssignemnt,
+    GeneratorTooManyParameters, NotSupportedForLifetimeBinderAsyncClosure, RustcBoxAttributeError,
+    UnderscoreExprLhsAssign,
+};
 use super::ResolverAstLoweringExt;
 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
 use crate::{FnDeclKind, ImplTraitPosition};
@@ -6,7 +12,6 @@ use rustc_ast::attr;
 use rustc_ast::ptr::P as AstP;
 use rustc_ast::*;
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_hir::definitions::DefPathData;
@@ -45,13 +50,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             let hir_id = self.lower_node_id(e.id);
                             return hir::Expr { hir_id, kind, span: self.lower_span(e.span) };
                         } else {
-                            self.tcx.sess
-                                .struct_span_err(
-                                    e.span,
-                                    "#[rustc_box] requires precisely one argument \
-                                    and no other attributes are allowed",
-                                )
-                                .emit();
+                            self.tcx.sess.emit_err(RustcBoxAttributeError { span: e.span });
                             hir::ExprKind::Err
                         }
                     } else if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
@@ -211,13 +210,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
                 }
                 ExprKind::Underscore => {
-                    self.tcx
-                        .sess.struct_span_err(
-                            e.span,
-                            "in expressions, `_` can only be used on the left-hand side of an assignment",
-                        )
-                        .span_label(e.span, "`_` not allowed here")
-                        .emit();
+                    self.tcx.sess.emit_err(UnderscoreExprLhsAssign { span: e.span });
                     hir::ExprKind::Err
                 }
                 ExprKind::Path(ref qself, ref path) => {
@@ -249,11 +242,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let rest = match &se.rest {
                         StructRest::Base(e) => Some(self.lower_expr(e)),
                         StructRest::Rest(sp) => {
-                            self.tcx
-                                .sess
-                                .struct_span_err(*sp, "base expression required after `..`")
-                                .span_label(*sp, "add a base expression here")
-                                .emit();
+                            self.tcx.sess.emit_err(BaseExpressionDoubleDot { span: *sp });
                             Some(&*self.arena.alloc(self.expr_err(*sp)))
                         }
                         StructRest::None => None,
@@ -662,17 +651,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         match self.generator_kind {
             Some(hir::GeneratorKind::Async(_)) => {}
             Some(hir::GeneratorKind::Gen) | None => {
-                let mut err = struct_span_err!(
-                    self.tcx.sess,
+                self.tcx.sess.emit_err(AwaitOnlyInAsyncFnAndBlocks {
                     dot_await_span,
-                    E0728,
-                    "`await` is only allowed inside `async` functions and blocks"
-                );
-                err.span_label(dot_await_span, "only allowed inside `async` functions and blocks");
-                if let Some(item_sp) = self.current_item {
-                    err.span_label(item_sp, "this is not `async`");
-                }
-                err.emit();
+                    item_span: self.current_item,
+                });
             }
         }
         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
@@ -892,13 +874,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         match generator_kind {
             Some(hir::GeneratorKind::Gen) => {
                 if decl.inputs.len() > 1 {
-                    struct_span_err!(
-                        self.tcx.sess,
-                        fn_decl_span,
-                        E0628,
-                        "too many parameters for a generator (expected 0 or 1 parameters)"
-                    )
-                    .emit();
+                    self.tcx.sess.emit_err(GeneratorTooManyParameters { fn_decl_span });
                 }
                 Some(movability)
             }
@@ -907,13 +883,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             }
             None => {
                 if movability == Movability::Static {
-                    struct_span_err!(
-                        self.tcx.sess,
-                        fn_decl_span,
-                        E0697,
-                        "closures cannot be static"
-                    )
-                    .emit();
+                    self.tcx.sess.emit_err(ClosureCannotBeStatic { fn_decl_span });
                 }
                 None
             }
@@ -946,10 +916,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         fn_decl_span: Span,
     ) -> hir::ExprKind<'hir> {
         if let &ClosureBinder::For { span, .. } = binder {
-            self.tcx.sess.span_err(
-                span,
-                "`for<...>` binders on `async` closures are not currently supported",
-            );
+            self.tcx.sess.emit_err(NotSupportedForLifetimeBinderAsyncClosure { span });
         }
 
         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
@@ -960,17 +927,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let body = self.with_new_scopes(|this| {
             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
-                struct_span_err!(
-                    this.tcx.sess,
-                    fn_decl_span,
-                    E0708,
-                    "`async` non-`move` closures with parameters are not currently supported",
-                )
-                .help(
-                    "consider using `let` statements to manually capture \
-                    variables by reference before entering an `async move` closure",
-                )
-                .emit();
+                this.tcx.sess.emit_err(AsyncNonMoveClosureNotSupported { fn_decl_span });
             }
 
             // Transform `async |x: u8| -> X { ... }` into
@@ -1210,20 +1167,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 );
                 let fields_omitted = match &se.rest {
                     StructRest::Base(e) => {
-                        self.tcx
-                            .sess
-                            .struct_span_err(
-                                e.span,
-                                "functional record updates are not allowed in destructuring \
-                                    assignments",
-                            )
-                            .span_suggestion(
-                                e.span,
-                                "consider removing the trailing pattern",
-                                "",
-                                rustc_errors::Applicability::MachineApplicable,
-                            )
-                            .emit();
+                        self.tcx.sess.emit_err(FunctionalRecordUpdateDestructuringAssignemnt {
+                            span: e.span,
+                        });
                         true
                     }
                     StructRest::Rest(_) => true,
@@ -1420,13 +1366,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         match self.generator_kind {
             Some(hir::GeneratorKind::Gen) => {}
             Some(hir::GeneratorKind::Async(_)) => {
-                struct_span_err!(
-                    self.tcx.sess,
-                    span,
-                    E0727,
-                    "`async` generators are not yet supported"
-                )
-                .emit();
+                self.tcx.sess.emit_err(AsyncGeneratorsNotSupported { span });
             }
             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
         }
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 0f1bab24f96ca..fd338ffc0c5e8 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -1,3 +1,4 @@
+use super::errors::{InvalidAbi, MisplacedRelaxTraitBound};
 use super::ResolverAstLoweringExt;
 use super::{AstOwner, ImplTraitContext, ImplTraitPosition};
 use super::{FnDeclKind, LoweringContext, ParamMode};
@@ -7,7 +8,6 @@ use rustc_ast::visit::AssocCtxt;
 use rustc_ast::*;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sorted_map::SortedMap;
-use rustc_errors::struct_span_err;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
@@ -1260,10 +1260,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     fn error_on_invalid_abi(&self, abi: StrLit) {
-        struct_span_err!(self.tcx.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
-            .span_label(abi.span, "invalid ABI")
-            .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
-            .emit();
+        self.tcx.sess.emit_err(InvalidAbi {
+            span: abi.span,
+            abi: abi.symbol,
+            valid_abis: abi::all_names().join(", "),
+        });
     }
 
     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
@@ -1338,11 +1339,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }
                 let is_param = *is_param.get_or_insert_with(compute_is_param);
                 if !is_param {
-                    self.diagnostic().span_err(
-                        bound.span(),
-                        "`?Trait` bounds are only permitted at the \
-                        point where a type parameter is declared",
-                    );
+                    self.tcx.sess.emit_err(MisplacedRelaxTraitBound { span: bound.span() });
                 }
             }
         }
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 8ab1daf23e86e..becb67d116546 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -39,6 +39,8 @@
 #[macro_use]
 extern crate tracing;
 
+use crate::errors::{AssocTyParentheses, AssocTyParenthesesSub, MisplacedImplTrait};
+
 use rustc_ast::ptr::P;
 use rustc_ast::visit;
 use rustc_ast::{self as ast, *};
@@ -49,7 +51,7 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{struct_span_err, Applicability, Handler, StashKey};
+use rustc_errors::{DiagnosticArgFromDisplay, Handler, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, LifetimeRes, Namespace, PartialRes, PerNS, Res};
 use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
@@ -75,6 +77,7 @@ macro_rules! arena_vec {
 
 mod asm;
 mod block;
+mod errors;
 mod expr;
 mod index;
 mod item;
@@ -1070,19 +1073,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     fn emit_bad_parenthesized_trait_in_assoc_ty(&self, data: &ParenthesizedArgs) {
-        let mut err = self.tcx.sess.struct_span_err(
-            data.span,
-            "parenthesized generic arguments cannot be used in associated type constraints",
-        );
         // Suggest removing empty parentheses: "Trait()" -> "Trait"
-        if data.inputs.is_empty() {
+        let sub = if data.inputs.is_empty() {
             let parentheses_span =
                 data.inputs_span.shrink_to_lo().to(data.inputs_span.shrink_to_hi());
-            err.multipart_suggestion(
-                "remove these parentheses",
-                vec![(parentheses_span, String::new())],
-                Applicability::MaybeIncorrect,
-            );
+            AssocTyParenthesesSub::Empty { parentheses_span }
         }
         // Suggest replacing parentheses with angle brackets `Trait(params...)` to `Trait<params...>`
         else {
@@ -1096,13 +1091,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             // End of last argument to end of parameters
             let close_param =
                 data.inputs.last().unwrap().span.shrink_to_hi().to(data.inputs_span.shrink_to_hi());
-            err.multipart_suggestion(
-                &format!("use angle brackets instead",),
-                vec![(open_param, String::from("<")), (close_param, String::from(">"))],
-                Applicability::MaybeIncorrect,
-            );
-        }
-        err.emit();
+            AssocTyParenthesesSub::NotEmpty { open_param, close_param }
+        };
+        self.tcx.sess.emit_err(AssocTyParentheses { span: data.span, sub });
     }
 
     #[instrument(level = "debug", skip(self))]
@@ -1341,14 +1332,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         path
                     }
                     ImplTraitContext::Disallowed(position) => {
-                        let mut err = struct_span_err!(
-                            self.tcx.sess,
-                            t.span,
-                            E0562,
-                            "`impl Trait` only allowed in function and inherent method return types, not in {}",
-                            position
-                        );
-                        err.emit();
+                        self.tcx.sess.emit_err(MisplacedImplTrait {
+                            span: t.span,
+                            position: DiagnosticArgFromDisplay(&position),
+                        });
                         hir::TyKind::Err
                     }
                 }
diff --git a/compiler/rustc_ast_lowering/src/pat.rs b/compiler/rustc_ast_lowering/src/pat.rs
index 51f67e505f4ee..1efa19a3a8286 100644
--- a/compiler/rustc_ast_lowering/src/pat.rs
+++ b/compiler/rustc_ast_lowering/src/pat.rs
@@ -1,3 +1,6 @@
+use super::errors::{
+    ArbitraryExpressionInPattern, ExtraDoubleDot, MisplacedDoubleDot, SubTupleBinding,
+};
 use super::ResolverAstLoweringExt;
 use super::{ImplTraitContext, LoweringContext, ParamMode};
 use crate::ImplTraitPosition;
@@ -5,7 +8,6 @@ use crate::ImplTraitPosition;
 use rustc_ast::ptr::P;
 use rustc_ast::*;
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_errors::Applicability;
 use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_span::symbol::Ident;
@@ -134,20 +136,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 // This is not allowed as a sub-tuple pattern
                 PatKind::Ident(ref _bm, ident, Some(ref sub)) if sub.is_rest() => {
                     let sp = pat.span;
-                    self.diagnostic()
-                        .struct_span_err(
-                            sp,
-                            &format!("`{} @` is not allowed in a {}", ident.name, ctx),
-                        )
-                        .span_label(sp, "this is only allowed in slice patterns")
-                        .help("remove this and bind each tuple field independently")
-                        .span_suggestion_verbose(
-                            sp,
-                            &format!("if you don't need to use the contents of {}, discard the tuple's remaining fields", ident),
-                            "..",
-                            Applicability::MaybeIncorrect,
-                        )
-                        .emit();
+                    self.tcx.sess.emit_err(SubTupleBinding {
+                        span: sp,
+                        ident_name: ident.name,
+                        ident,
+                        ctx,
+                    });
                 }
                 _ => {}
             }
@@ -296,19 +290,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
     /// Emit a friendly error for extra `..` patterns in a tuple/tuple struct/slice pattern.
     pub(crate) fn ban_extra_rest_pat(&self, sp: Span, prev_sp: Span, ctx: &str) {
-        self.diagnostic()
-            .struct_span_err(sp, &format!("`..` can only be used once per {} pattern", ctx))
-            .span_label(sp, &format!("can only be used once per {} pattern", ctx))
-            .span_label(prev_sp, "previously used here")
-            .emit();
+        self.tcx.sess.emit_err(ExtraDoubleDot { span: sp, prev_span: prev_sp, ctx });
     }
 
     /// Used to ban the `..` pattern in places it shouldn't be semantically.
     fn ban_illegal_rest_pat(&self, sp: Span) -> hir::PatKind<'hir> {
-        self.diagnostic()
-            .struct_span_err(sp, "`..` patterns are not allowed here")
-            .note("only allowed in tuple, tuple struct, and slice patterns")
-            .emit();
+        self.tcx.sess.emit_err(MisplacedDoubleDot { span: sp });
 
         // We're not in a list context so `..` can be reasonably treated
         // as `_` because it should always be valid and roughly matches the
@@ -345,8 +332,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             ExprKind::Path(..) if allow_paths => {}
             ExprKind::Unary(UnOp::Neg, ref inner) if matches!(inner.kind, ExprKind::Lit(_)) => {}
             _ => {
-                self.diagnostic()
-                    .span_err(expr.span, "arbitrary expressions aren't allowed in patterns");
+                self.tcx.sess.emit_err(ArbitraryExpressionInPattern { span: expr.span });
                 return self.arena.alloc(self.expr_err(expr.span));
             }
         }
diff --git a/compiler/rustc_ast_lowering/src/path.rs b/compiler/rustc_ast_lowering/src/path.rs
index 393be3b454c37..5874d08a94fe0 100644
--- a/compiler/rustc_ast_lowering/src/path.rs
+++ b/compiler/rustc_ast_lowering/src/path.rs
@@ -1,11 +1,11 @@
 use crate::ImplTraitPosition;
 
+use super::errors::{GenericTypeWithParentheses, UseAngleBrackets};
 use super::ResolverAstLoweringExt;
 use super::{GenericArgsCtor, LifetimeRes, ParenthesizedGenericArgs};
 use super::{ImplTraitContext, LoweringContext, ParamMode};
 
 use rustc_ast::{self as ast, *};
-use rustc_errors::{struct_span_err, Applicability};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, PartialRes, Res};
 use rustc_hir::GenericArg;
@@ -185,7 +185,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     ) -> hir::PathSegment<'hir> {
         debug!("path_span: {:?}, lower_path_segment(segment: {:?})", path_span, segment,);
         let (mut generic_args, infer_args) = if let Some(ref generic_args) = segment.args {
-            let msg = "parenthesized type parameters may only be used with a `Fn` trait";
             match **generic_args {
                 GenericArgs::AngleBracketed(ref data) => {
                     self.lower_angle_bracketed_parameter_data(data, param_mode, itctx)
@@ -193,10 +192,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 GenericArgs::Parenthesized(ref data) => match parenthesized_generic_args {
                     ParenthesizedGenericArgs::Ok => self.lower_parenthesized_parameter_data(data),
                     ParenthesizedGenericArgs::Err => {
-                        let mut err = struct_span_err!(self.tcx.sess, data.span, E0214, "{}", msg);
-                        err.span_label(data.span, "only `Fn` traits may use parentheses");
                         // Suggest replacing parentheses with angle brackets `Trait(params...)` to `Trait<params...>`
-                        if !data.inputs.is_empty() {
+                        let sub = if !data.inputs.is_empty() {
                             // Start of the span to the 1st character of 1st argument
                             let open_param = data.inputs_span.shrink_to_lo().to(data
                                 .inputs
@@ -212,16 +209,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                 .span
                                 .shrink_to_hi()
                                 .to(data.inputs_span.shrink_to_hi());
-                            err.multipart_suggestion(
-                                &format!("use angle brackets instead",),
-                                vec![
-                                    (open_param, String::from("<")),
-                                    (close_param, String::from(">")),
-                                ],
-                                Applicability::MaybeIncorrect,
-                            );
-                        }
-                        err.emit();
+
+                            Some(UseAngleBrackets { open_param, close_param })
+                        } else {
+                            None
+                        };
+                        self.tcx.sess.emit_err(GenericTypeWithParentheses { span: data.span, sub });
                         (
                             self.lower_angle_bracketed_parameter_data(
                                 &data.as_angle_bracketed_args(),
diff --git a/compiler/rustc_borrowck/src/diagnostics/region_errors.rs b/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
index 0dce5be953e1a..00fdf331ca60c 100644
--- a/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
+++ b/compiler/rustc_borrowck/src/diagnostics/region_errors.rs
@@ -1,3 +1,5 @@
+#![deny(rustc::untranslatable_diagnostic)]
+#![deny(rustc::diagnostic_outside_of_impl)]
 //! Error reporting machinery for lifetime errors.
 
 use rustc_data_structures::fx::FxHashSet;
@@ -23,7 +25,10 @@ use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::Span;
 
 use crate::borrowck_errors;
-use crate::session_diagnostics::GenericDoesNotLiveLongEnough;
+use crate::session_diagnostics::{
+    FnMutError, FnMutReturnTypeErr, GenericDoesNotLiveLongEnough, LifetimeOutliveErr,
+    LifetimeReturnCategoryErr, RequireStaticErr, VarHereDenote,
+};
 
 use super::{OutlivesSuggestionBuilder, RegionName};
 use crate::region_infer::BlameConstraint;
@@ -488,12 +493,6 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         let ErrorConstraintInfo { outlived_fr, span, .. } = errci;
 
-        let mut diag = self
-            .infcx
-            .tcx
-            .sess
-            .struct_span_err(*span, "captured variable cannot escape `FnMut` closure body");
-
         let mut output_ty = self.regioncx.universal_regions().unnormalized_output_ty;
         if let ty::Opaque(def_id, _) = *output_ty.kind() {
             output_ty = self.infcx.tcx.type_of(def_id)
@@ -501,19 +500,20 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
         debug!("report_fnmut_error: output_ty={:?}", output_ty);
 
-        let message = match output_ty.kind() {
-            ty::Closure(_, _) => {
-                "returns a closure that contains a reference to a captured variable, which then \
-                 escapes the closure body"
-            }
-            ty::Adt(def, _) if self.infcx.tcx.is_diagnostic_item(sym::gen_future, def.did()) => {
-                "returns an `async` block that contains a reference to a captured variable, which then \
-                 escapes the closure body"
-            }
-            _ => "returns a reference to a captured variable which escapes the closure body",
+        let err = FnMutError {
+            span: *span,
+            ty_err: match output_ty.kind() {
+                ty::Closure(_, _) => FnMutReturnTypeErr::ReturnClosure { span: *span },
+                ty::Adt(def, _)
+                    if self.infcx.tcx.is_diagnostic_item(sym::gen_future, def.did()) =>
+                {
+                    FnMutReturnTypeErr::ReturnAsyncBlock { span: *span }
+                }
+                _ => FnMutReturnTypeErr::ReturnRef { span: *span },
+            },
         };
 
-        diag.span_label(*span, message);
+        let mut diag = self.infcx.tcx.sess.create_err(err);
 
         if let ReturnConstraint::ClosureUpvar(upvar_field) = kind {
             let def_id = match self.regioncx.universal_regions().defining_ty {
@@ -532,20 +532,15 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 let upvars_map = self.infcx.tcx.upvars_mentioned(def_id).unwrap();
                 let upvar_def_span = self.infcx.tcx.hir().span(def_hir);
                 let upvar_span = upvars_map.get(&def_hir).unwrap().span;
-                diag.span_label(upvar_def_span, "variable defined here");
-                diag.span_label(upvar_span, "variable captured here");
+                diag.subdiagnostic(VarHereDenote::Defined { span: upvar_def_span });
+                diag.subdiagnostic(VarHereDenote::Captured { span: upvar_span });
             }
         }
 
         if let Some(fr_span) = self.give_region_a_name(*outlived_fr).unwrap().span() {
-            diag.span_label(fr_span, "inferred to be a `FnMut` closure");
+            diag.subdiagnostic(VarHereDenote::FnMutInferred { span: fr_span });
         }
 
-        diag.note(
-            "`FnMut` closures only have access to their captured variables while they are \
-             executing...",
-        );
-        diag.note("...therefore, they cannot allow references to captured variables to escape");
         self.suggest_move_on_borrowing_closure(&mut diag);
 
         diag
@@ -681,39 +676,33 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             ..
         } = errci;
 
-        let mut diag =
-            self.infcx.tcx.sess.struct_span_err(*span, "lifetime may not live long enough");
-
         let (_, mir_def_name) =
             self.infcx.tcx.article_and_description(self.mir_def_id().to_def_id());
 
+        let err = LifetimeOutliveErr { span: *span };
+        let mut diag = self.infcx.tcx.sess.create_err(err);
+
         let fr_name = self.give_region_a_name(*fr).unwrap();
         fr_name.highlight_region_name(&mut diag);
         let outlived_fr_name = self.give_region_a_name(*outlived_fr).unwrap();
         outlived_fr_name.highlight_region_name(&mut diag);
 
-        match (category, outlived_fr_is_local, fr_is_local) {
-            (ConstraintCategory::Return(_), true, _) => {
-                diag.span_label(
-                    *span,
-                    format!(
-                        "{mir_def_name} was supposed to return data with lifetime `{outlived_fr_name}` but it is returning \
-                         data with lifetime `{fr_name}`",
-                    ),
-                );
-            }
-            _ => {
-                diag.span_label(
-                    *span,
-                    format!(
-                        "{}requires that `{}` must outlive `{}`",
-                        category.description(),
-                        fr_name,
-                        outlived_fr_name,
-                    ),
-                );
-            }
-        }
+        let err_category = match (category, outlived_fr_is_local, fr_is_local) {
+            (ConstraintCategory::Return(_), true, _) => LifetimeReturnCategoryErr::WrongReturn {
+                span: *span,
+                mir_def_name,
+                outlived_fr_name,
+                fr_name: &fr_name,
+            },
+            _ => LifetimeReturnCategoryErr::ShortReturn {
+                span: *span,
+                category_desc: category.description(),
+                free_region_name: &fr_name,
+                outlived_fr_name,
+            },
+        };
+
+        diag.subdiagnostic(err_category);
 
         self.add_static_impl_trait_suggestion(&mut diag, *fr, fr_name, *outlived_fr);
         self.suggest_adding_lifetime_params(&mut diag, *fr, *outlived_fr);
@@ -862,7 +851,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                     ident.span,
                     "calling this method introduces the `impl`'s 'static` requirement",
                 );
-                err.span_note(multi_span, "the used `impl` has a `'static` requirement");
+                err.subdiagnostic(RequireStaticErr::UsedImpl { multi_span });
                 err.span_suggestion_verbose(
                     span.shrink_to_hi(),
                     "consider relaxing the implicit `'static` requirement",
diff --git a/compiler/rustc_borrowck/src/lib.rs b/compiler/rustc_borrowck/src/lib.rs
index 4ad9a970bc1be..0f8afb038f455 100644
--- a/compiler/rustc_borrowck/src/lib.rs
+++ b/compiler/rustc_borrowck/src/lib.rs
@@ -18,7 +18,7 @@ extern crate tracing;
 
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::graph::dominators::Dominators;
-use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_errors::{Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
 use rustc_hir as hir;
 use rustc_hir::def_id::LocalDefId;
 use rustc_index::bit_set::ChunkedBitSet;
@@ -50,6 +50,8 @@ use rustc_mir_dataflow::move_paths::{InitLocation, LookupResult, MoveData, MoveE
 use rustc_mir_dataflow::Analysis;
 use rustc_mir_dataflow::MoveDataParamEnv;
 
+use crate::session_diagnostics::VarNeedNotMut;
+
 use self::diagnostics::{AccessKind, RegionName};
 use self::location::LocationTable;
 use self::prefixes::PrefixSet;
@@ -424,17 +426,9 @@ fn do_mir_borrowck<'a, 'tcx>(
             continue;
         }
 
-        tcx.struct_span_lint_hir(UNUSED_MUT, lint_root, span, |lint| {
-            let mut_span = tcx.sess.source_map().span_until_non_whitespace(span);
-            lint.build("variable does not need to be mutable")
-                .span_suggestion_short(
-                    mut_span,
-                    "remove this `mut`",
-                    "",
-                    Applicability::MachineApplicable,
-                )
-                .emit();
-        })
+        let mut_span = tcx.sess.source_map().span_until_non_whitespace(span);
+
+        tcx.emit_spanned_lint(UNUSED_MUT, lint_root, span, VarNeedNotMut { span: mut_span })
     }
 
     let tainted_by_errors = mbcx.emit_errors();
diff --git a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
index d6712b6a4799c..127cb4e408372 100644
--- a/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
+++ b/compiler/rustc_borrowck/src/region_infer/opaque_types.rs
@@ -16,6 +16,8 @@ use rustc_span::Span;
 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
 use rustc_trait_selection::traits::TraitEngineExt as _;
 
+use crate::session_diagnostics::ConstNotUsedTraitAlias;
+
 use super::RegionInferenceContext;
 
 impl<'tcx> RegionInferenceContext<'tcx> {
@@ -639,17 +641,10 @@ impl<'tcx> TypeFolder<'tcx> for ReverseMapper<'tcx> {
                     Some(GenericArgKind::Const(c1)) => c1,
                     Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
                     None => {
-                        self.tcx
-                            .sess
-                            .struct_span_err(
-                                self.span,
-                                &format!(
-                                    "const parameter `{}` is part of concrete type but not \
-                                          used in parameter list for the `impl Trait` type alias",
-                                    ct
-                                ),
-                            )
-                            .emit();
+                        self.tcx.sess.emit_err(ConstNotUsedTraitAlias {
+                            ct: ct.to_string(),
+                            span: self.span,
+                        });
 
                         self.tcx().const_error(ct.ty())
                     }
diff --git a/compiler/rustc_borrowck/src/session_diagnostics.rs b/compiler/rustc_borrowck/src/session_diagnostics.rs
index 8c9676e4bfa85..5d750c6ca8c7b 100644
--- a/compiler/rustc_borrowck/src/session_diagnostics.rs
+++ b/compiler/rustc_borrowck/src/session_diagnostics.rs
@@ -1,7 +1,10 @@
-use rustc_macros::{SessionDiagnostic, SessionSubdiagnostic};
+use rustc_errors::{IntoDiagnosticArg, MultiSpan};
+use rustc_macros::{LintDiagnostic, SessionDiagnostic, SessionSubdiagnostic};
 use rustc_middle::ty::Ty;
 use rustc_span::Span;
 
+use crate::diagnostics::RegionName;
+
 #[derive(SessionDiagnostic)]
 #[diag(borrowck::move_unsized, code = "E0161")]
 pub(crate) struct MoveUnsized<'tcx> {
@@ -42,3 +45,115 @@ pub(crate) struct GenericDoesNotLiveLongEnough {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(borrowck::var_does_not_need_mut)]
+pub(crate) struct VarNeedNotMut {
+    #[suggestion_short(applicability = "machine-applicable", code = "")]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(borrowck::const_not_used_in_type_alias)]
+pub(crate) struct ConstNotUsedTraitAlias {
+    pub ct: String,
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(borrowck::var_cannot_escape_closure)]
+#[note]
+#[note(borrowck::cannot_escape)]
+pub(crate) struct FnMutError {
+    #[primary_span]
+    pub span: Span,
+    #[subdiagnostic]
+    pub ty_err: FnMutReturnTypeErr,
+}
+
+#[derive(SessionSubdiagnostic)]
+pub(crate) enum VarHereDenote {
+    #[label(borrowck::var_here_captured)]
+    Captured {
+        #[primary_span]
+        span: Span,
+    },
+    #[label(borrowck::var_here_defined)]
+    Defined {
+        #[primary_span]
+        span: Span,
+    },
+    #[label(borrowck::closure_inferred_mut)]
+    FnMutInferred {
+        #[primary_span]
+        span: Span,
+    },
+}
+
+#[derive(SessionSubdiagnostic)]
+pub(crate) enum FnMutReturnTypeErr {
+    #[label(borrowck::returned_closure_escaped)]
+    ReturnClosure {
+        #[primary_span]
+        span: Span,
+    },
+    #[label(borrowck::returned_async_block_escaped)]
+    ReturnAsyncBlock {
+        #[primary_span]
+        span: Span,
+    },
+    #[label(borrowck::returned_ref_escaped)]
+    ReturnRef {
+        #[primary_span]
+        span: Span,
+    },
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(borrowck::lifetime_constraints_error)]
+pub(crate) struct LifetimeOutliveErr {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(SessionSubdiagnostic)]
+pub(crate) enum LifetimeReturnCategoryErr<'a> {
+    #[label(borrowck::returned_lifetime_wrong)]
+    WrongReturn {
+        #[primary_span]
+        span: Span,
+        mir_def_name: &'a str,
+        outlived_fr_name: RegionName,
+        fr_name: &'a RegionName,
+    },
+    #[label(borrowck::returned_lifetime_short)]
+    ShortReturn {
+        #[primary_span]
+        span: Span,
+        category_desc: &'static str,
+        free_region_name: &'a RegionName,
+        outlived_fr_name: RegionName,
+    },
+}
+
+impl IntoDiagnosticArg for &RegionName {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        format!("{}", self).into_diagnostic_arg()
+    }
+}
+
+impl IntoDiagnosticArg for RegionName {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        format!("{}", self).into_diagnostic_arg()
+    }
+}
+
+#[derive(SessionSubdiagnostic)]
+pub(crate) enum RequireStaticErr {
+    #[note(borrowck::used_impl_require_static)]
+    UsedImpl {
+        #[primary_span]
+        multi_span: MultiSpan,
+    },
+}
diff --git a/compiler/rustc_borrowck/src/type_check/input_output.rs b/compiler/rustc_borrowck/src/type_check/input_output.rs
index 4431a2e8ec60d..84d2bfe04de05 100644
--- a/compiler/rustc_borrowck/src/type_check/input_output.rs
+++ b/compiler/rustc_borrowck/src/type_check/input_output.rs
@@ -225,7 +225,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
         debug!("{:?} normalized to {:?}", t, norm_ty);
 
-        for data in constraints {
+        if let Some(data) = constraints {
             ConstraintConversion::new(
                 self.infcx,
                 &self.borrowck_context.universal_regions,
diff --git a/compiler/rustc_const_eval/src/interpret/validity.rs b/compiler/rustc_const_eval/src/interpret/validity.rs
index f1b1855c3ec74..0e60f0c7ef131 100644
--- a/compiler/rustc_const_eval/src/interpret/validity.rs
+++ b/compiler/rustc_const_eval/src/interpret/validity.rs
@@ -8,6 +8,7 @@ use std::convert::TryFrom;
 use std::fmt::Write;
 use std::num::NonZeroUsize;
 
+use rustc_ast::Mutability;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_middle::mir::interpret::InterpError;
@@ -411,34 +412,51 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
             // Proceed recursively even for ZST, no reason to skip them!
             // `!` is a ZST and we want to validate it.
             if let Ok((alloc_id, _offset, _prov)) = self.ecx.ptr_try_get_alloc_id(place.ptr) {
-                // Special handling for pointers to statics (irrespective of their type).
+                // Let's see what kind of memory this points to.
                 let alloc_kind = self.ecx.tcx.try_get_global_alloc(alloc_id);
-                if let Some(GlobalAlloc::Static(did)) = alloc_kind {
-                    assert!(!self.ecx.tcx.is_thread_local_static(did));
-                    assert!(self.ecx.tcx.is_static(did));
-                    if matches!(
-                        self.ctfe_mode,
-                        Some(CtfeValidationMode::Const { allow_static_ptrs: false, .. })
-                    ) {
-                        // See const_eval::machine::MemoryExtra::can_access_statics for why
-                        // this check is so important.
-                        // This check is reachable when the const just referenced the static,
-                        // but never read it (so we never entered `before_access_global`).
-                        throw_validation_failure!(self.path,
-                            { "a {} pointing to a static variable", kind }
-                        );
+                match alloc_kind {
+                    Some(GlobalAlloc::Static(did)) => {
+                        // Special handling for pointers to statics (irrespective of their type).
+                        assert!(!self.ecx.tcx.is_thread_local_static(did));
+                        assert!(self.ecx.tcx.is_static(did));
+                        if matches!(
+                            self.ctfe_mode,
+                            Some(CtfeValidationMode::Const { allow_static_ptrs: false, .. })
+                        ) {
+                            // See const_eval::machine::MemoryExtra::can_access_statics for why
+                            // this check is so important.
+                            // This check is reachable when the const just referenced the static,
+                            // but never read it (so we never entered `before_access_global`).
+                            throw_validation_failure!(self.path,
+                                { "a {} pointing to a static variable in a constant", kind }
+                            );
+                        }
+                        // We skip recursively checking other statics. These statics must be sound by
+                        // themselves, and the only way to get broken statics here is by using
+                        // unsafe code.
+                        // The reasons we don't check other statics is twofold. For one, in all
+                        // sound cases, the static was already validated on its own, and second, we
+                        // trigger cycle errors if we try to compute the value of the other static
+                        // and that static refers back to us.
+                        // We might miss const-invalid data,
+                        // but things are still sound otherwise (in particular re: consts
+                        // referring to statics).
+                        return Ok(());
                     }
-                    // We skip checking other statics. These statics must be sound by
-                    // themselves, and the only way to get broken statics here is by using
-                    // unsafe code.
-                    // The reasons we don't check other statics is twofold. For one, in all
-                    // sound cases, the static was already validated on its own, and second, we
-                    // trigger cycle errors if we try to compute the value of the other static
-                    // and that static refers back to us.
-                    // We might miss const-invalid data,
-                    // but things are still sound otherwise (in particular re: consts
-                    // referring to statics).
-                    return Ok(());
+                    Some(GlobalAlloc::Memory(alloc)) => {
+                        if alloc.inner().mutability == Mutability::Mut
+                            && matches!(self.ctfe_mode, Some(CtfeValidationMode::Const { .. }))
+                        {
+                            // This should be unreachable, but if someone manages to copy a pointer
+                            // out of a `static`, then that pointer might point to mutable memory,
+                            // and we would catch that here.
+                            throw_validation_failure!(self.path,
+                                { "a {} pointing to mutable memory in a constant", kind }
+                            );
+                        }
+                    }
+                    // Nothing to check for these.
+                    None | Some(GlobalAlloc::Function(..) | GlobalAlloc::VTable(..)) => {}
                 }
             }
             let path = &self.path;
@@ -544,7 +562,7 @@ impl<'rt, 'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> ValidityVisitor<'rt, 'mir, '
             }
             ty::Ref(_, ty, mutbl) => {
                 if matches!(self.ctfe_mode, Some(CtfeValidationMode::Const { .. }))
-                    && *mutbl == hir::Mutability::Mut
+                    && *mutbl == Mutability::Mut
                 {
                     // A mutable reference inside a const? That does not seem right (except if it is
                     // a ZST).
diff --git a/compiler/rustc_error_messages/locales/en-US/ast_lowering.ftl b/compiler/rustc_error_messages/locales/en-US/ast_lowering.ftl
new file mode 100644
index 0000000000000..dcb1e2b08306f
--- /dev/null
+++ b/compiler/rustc_error_messages/locales/en-US/ast_lowering.ftl
@@ -0,0 +1,131 @@
+ast_lowering_generic_type_with_parentheses =
+    parenthesized type parameters may only be used with a `Fn` trait
+    .label = only `Fn` traits may use parentheses
+
+ast_lowering_use_angle_brackets = use angle brackets instead
+
+ast_lowering_invalid_abi =
+    invalid ABI: found `{$abi}`
+    .label = invalid ABI
+    .help = valid ABIs: {$valid_abis}
+
+ast_lowering_assoc_ty_parentheses =
+    parenthesized generic arguments cannot be used in associated type constraints
+
+ast_lowering_remove_parentheses = remove these parentheses
+
+ast_lowering_misplaced_impl_trait =
+    `impl Trait` only allowed in function and inherent method return types, not in {$position}
+
+ast_lowering_rustc_box_attribute_error =
+    #[rustc_box] requires precisely one argument and no other attributes are allowed
+
+ast_lowering_underscore_expr_lhs_assign =
+    in expressions, `_` can only be used on the left-hand side of an assignment
+    .label = `_` not allowed here
+
+ast_lowering_base_expression_double_dot =
+    base expression required after `..`
+    .label = add a base expression here
+
+ast_lowering_await_only_in_async_fn_and_blocks =
+    `await` is only allowed inside `async` functions and blocks
+    .label = only allowed inside `async` functions and blocks
+
+ast_lowering_this_not_async = this is not `async`
+
+ast_lowering_generator_too_many_parameters =
+    too many parameters for a generator (expected 0 or 1 parameters)
+
+ast_lowering_closure_cannot_be_static = closures cannot be static
+
+ast_lowering_async_non_move_closure_not_supported =
+    `async` non-`move` closures with parameters are not currently supported
+    .help = consider using `let` statements to manually capture variables by reference before entering an `async move` closure
+
+ast_lowering_functional_record_update_destructuring_assignment =
+    functional record updates are not allowed in destructuring assignments
+    .suggestion = consider removing the trailing pattern
+
+ast_lowering_async_generators_not_supported =
+    `async` generators are not yet supported
+
+ast_lowering_inline_asm_unsupported_target =
+    inline assembly is unsupported on this target
+
+ast_lowering_att_syntax_only_x86 =
+    the `att_syntax` option is only supported on x86
+
+ast_lowering_abi_specified_multiple_times =
+    `{$prev_name}` ABI specified multiple times
+    .label = previously specified here
+    .note = these ABIs are equivalent on the current target
+
+ast_lowering_clobber_abi_not_supported =
+    `clobber_abi` is not supported on this target
+
+ast_lowering_invalid_abi_clobber_abi =
+    invalid ABI for `clobber_abi`
+    .note = the following ABIs are supported on this target: {$supported_abis}
+
+ast_lowering_invalid_register =
+    invalid register `{$reg}`: {$error}
+
+ast_lowering_invalid_register_class =
+    invalid register class `{$reg_class}`: {$error}
+
+ast_lowering_invalid_asm_template_modifier_reg_class =
+    invalid asm template modifier for this register class
+
+ast_lowering_argument = argument
+
+ast_lowering_template_modifier = template modifier
+
+ast_lowering_support_modifiers =
+    the `{$class_name}` register class supports the following template modifiers: {$modifiers}
+
+ast_lowering_does_not_support_modifiers =
+    the `{$class_name}` register class does not support template modifiers
+
+ast_lowering_invalid_asm_template_modifier_const =
+    asm template modifiers are not allowed for `const` arguments
+
+ast_lowering_invalid_asm_template_modifier_sym =
+    asm template modifiers are not allowed for `sym` arguments
+
+ast_lowering_register_class_only_clobber =
+    register class `{$reg_class_name}` can only be used as a clobber, not as an input or output
+
+ast_lowering_register_conflict =
+    register `{$reg1_name}` conflicts with register `{$reg2_name}`
+    .help = use `lateout` instead of `out` to avoid conflict
+
+ast_lowering_register1 = register `{$reg1_name}`
+
+ast_lowering_register2 = register `{$reg2_name}`
+
+ast_lowering_sub_tuple_binding =
+    `{$ident_name} @` is not allowed in a {$ctx}
+    .label = this is only allowed in slice patterns
+    .help = remove this and bind each tuple field independently
+
+ast_lowering_sub_tuple_binding_suggestion = if you don't need to use the contents of {$ident}, discard the tuple's remaining fields
+
+ast_lowering_extra_double_dot =
+    `..` can only be used once per {$ctx} pattern
+    .label = can only be used once per {$ctx} pattern
+
+ast_lowering_previously_used_here = previously used here
+
+ast_lowering_misplaced_double_dot =
+    `..` patterns are not allowed here
+    .note = only allowed in tuple, tuple struct, and slice patterns
+
+ast_lowering_misplaced_relax_trait_bound =
+    `?Trait` bounds are only permitted at the point where a type parameter is declared
+
+ast_lowering_not_supported_for_lifetime_binder_async_closure =
+    `for<...>` binders on `async` closures are not currently supported
+
+ast_lowering_arbitrary_expression_in_pattern =
+    arbitrary expressions aren't allowed in patterns
diff --git a/compiler/rustc_error_messages/locales/en-US/borrowck.ftl b/compiler/rustc_error_messages/locales/en-US/borrowck.ftl
index a96fe7c8a05d5..67f2156f32e50 100644
--- a/compiler/rustc_error_messages/locales/en-US/borrowck.ftl
+++ b/compiler/rustc_error_messages/locales/en-US/borrowck.ftl
@@ -16,3 +16,45 @@ borrowck_higher_ranked_subtype_error =
 
 borrowck_generic_does_not_live_long_enough =
     `{$kind}` does not live long enough
+
+borrowck_move_borrowed =
+    cannot move out of `{$desc}` beacause it is borrowed
+
+borrowck_var_does_not_need_mut =
+    variable does not need to be mutable
+    .suggestion = remove this `mut`
+
+borrowck_const_not_used_in_type_alias =
+    const parameter `{$ct}` is part of concrete type but not used in parameter list for the `impl Trait` type alias
+
+borrowck_var_cannot_escape_closure =
+    captured variable cannot escape `FnMut` closure body
+    .note = `FnMut` closures only have access to their captured variables while they are executing...
+    .cannot_escape = ...therefore, they cannot allow references to captured variables to escape
+
+borrowck_var_here_defined = variable defined here
+
+borrowck_var_here_captured = variable captured here
+
+borrowck_closure_inferred_mut =  inferred to be a `FnMut` closure
+
+borrowck_returned_closure_escaped =
+    returns a closure that contains a reference to a captured variable, which then escapes the closure body
+
+borrowck_returned_async_block_escaped =
+    returns an `async` block that contains a reference to a captured variable, which then escapes the closure body
+
+borrowck_returned_ref_escaped =
+    returns a reference to a captured variable which escapes the closure body
+
+borrowck_lifetime_constraints_error =
+    lifetime may not live long enough
+
+borrowck_returned_lifetime_wrong =
+    {$mir_def_name} was supposed to return data with lifetime `{$outlived_fr_name}` but it is returning data with lifetime `{$fr_name}`
+
+borrowck_returned_lifetime_short =
+    {$category_desc}requires that `{$free_region_name}` must outlive `{$outlived_fr_name}`
+
+borrowck_used_impl_require_static =
+    the used `impl` has a `'static` requirement
diff --git a/compiler/rustc_error_messages/src/lib.rs b/compiler/rustc_error_messages/src/lib.rs
index 2d001d445be02..75dbbb2d557a7 100644
--- a/compiler/rustc_error_messages/src/lib.rs
+++ b/compiler/rustc_error_messages/src/lib.rs
@@ -32,6 +32,7 @@ pub use unic_langid::{langid, LanguageIdentifier};
 
 // Generates `DEFAULT_LOCALE_RESOURCES` static and `fluent_generated` module.
 fluent_messages! {
+    ast_lowering => "../locales/en-US/ast_lowering.ftl",
     ast_passes => "../locales/en-US/ast_passes.ftl",
     borrowck => "../locales/en-US/borrowck.ftl",
     builtin_macros => "../locales/en-US/builtin_macros.ftl",
diff --git a/compiler/rustc_infer/src/infer/error_reporting/mod.rs b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
index 59ea1f3f9de45..16cffb45f0f06 100644
--- a/compiler/rustc_infer/src/infer/error_reporting/mod.rs
+++ b/compiler/rustc_infer/src/infer/error_reporting/mod.rs
@@ -1588,9 +1588,14 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                             Mismatch::Variable(infer::ExpectedFound { expected, found }),
                         )
                     }
+                    ValuePairs::Terms(infer::ExpectedFound {
+                        expected: ty::Term::Const(_),
+                        found: ty::Term::Const(_),
+                    }) => (false, Mismatch::Fixed("constant")),
                     ValuePairs::TraitRefs(_) | ValuePairs::PolyTraitRefs(_) => {
                         (false, Mismatch::Fixed("trait"))
                     }
+                    ValuePairs::Regions(_) => (false, Mismatch::Fixed("lifetime")),
                     _ => (false, Mismatch::Fixed("type")),
                 };
                 let vals = match self.values_str(values) {
diff --git a/compiler/rustc_lint/src/for_loop_over_fallibles.rs b/compiler/rustc_lint/src/for_loop_over_fallibles.rs
new file mode 100644
index 0000000000000..2253546b5d357
--- /dev/null
+++ b/compiler/rustc_lint/src/for_loop_over_fallibles.rs
@@ -0,0 +1,188 @@
+use crate::{LateContext, LateLintPass, LintContext};
+
+use hir::{Expr, Pat};
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_infer::traits::TraitEngine;
+use rustc_infer::{infer::TyCtxtInferExt, traits::ObligationCause};
+use rustc_middle::ty::{self, List};
+use rustc_span::{sym, Span};
+use rustc_trait_selection::traits::TraitEngineExt;
+
+declare_lint! {
+    /// The `for_loop_over_fallibles` lint checks for `for` loops over `Option` or `Result` values.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// let opt = Some(1);
+    /// for x in opt { /* ... */}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Both `Option` and `Result` implement `IntoIterator` trait, which allows using them in a `for` loop.
+    /// `for` loop over `Option` or `Result` will iterate either 0 (if the value is `None`/`Err(_)`)
+    /// or 1 time (if the value is `Some(_)`/`Ok(_)`). This is not very useful and is more clearly expressed
+    /// via `if let`.
+    ///
+    /// `for` loop can also be accidentally written with the intention to call a function multiple times,
+    /// while the function returns `Some(_)`, in these cases `while let` loop should be used instead.
+    ///
+    /// The "intended" use of `IntoIterator` implementations for `Option` and `Result` is passing them to
+    /// generic code that expects something implementing `IntoIterator`. For example using `.chain(option)`
+    /// to optionally add a value to an iterator.
+    pub FOR_LOOP_OVER_FALLIBLES,
+    Warn,
+    "for-looping over an `Option` or a `Result`, which is more clearly expressed as an `if let`"
+}
+
+declare_lint_pass!(ForLoopOverFallibles => [FOR_LOOP_OVER_FALLIBLES]);
+
+impl<'tcx> LateLintPass<'tcx> for ForLoopOverFallibles {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+        let Some((pat, arg)) = extract_for_loop(expr) else { return };
+
+        let ty = cx.typeck_results().expr_ty(arg);
+
+        let &ty::Adt(adt, substs) = ty.kind() else { return };
+
+        let (article, ty, var) = match adt.did() {
+            did if cx.tcx.is_diagnostic_item(sym::Option, did) => ("an", "Option", "Some"),
+            did if cx.tcx.is_diagnostic_item(sym::Result, did) => ("a", "Result", "Ok"),
+            _ => return,
+        };
+
+        let msg = format!(
+            "for loop over {article} `{ty}`. This is more readably written as an `if let` statement",
+        );
+
+        cx.struct_span_lint(FOR_LOOP_OVER_FALLIBLES, arg.span, |diag| {
+            let mut warn = diag.build(msg);
+
+            if let Some(recv) = extract_iterator_next_call(cx, arg)
+            && let Ok(recv_snip) = cx.sess().source_map().span_to_snippet(recv.span)
+            {
+                warn.span_suggestion(
+                    recv.span.between(arg.span.shrink_to_hi()),
+                    format!("to iterate over `{recv_snip}` remove the call to `next`"),
+                    ".by_ref()",
+                    Applicability::MaybeIncorrect
+                );
+            } else {
+                warn.multipart_suggestion_verbose(
+                    format!("to check pattern in a loop use `while let`"),
+                    vec![
+                        // NB can't use `until` here because `expr.span` and `pat.span` have different syntax contexts
+                        (expr.span.with_hi(pat.span.lo()), format!("while let {var}(")),
+                        (pat.span.between(arg.span), format!(") = ")),
+                    ],
+                    Applicability::MaybeIncorrect
+                );
+            }
+
+            if suggest_question_mark(cx, adt, substs, expr.span) {
+                warn.span_suggestion(
+                    arg.span.shrink_to_hi(),
+                    "consider unwrapping the `Result` with `?` to iterate over its contents",
+                    "?",
+                    Applicability::MaybeIncorrect,
+                );
+            }
+
+            warn.multipart_suggestion_verbose(
+                "consider using `if let` to clear intent",
+                vec![
+                    // NB can't use `until` here because `expr.span` and `pat.span` have different syntax contexts
+                    (expr.span.with_hi(pat.span.lo()), format!("if let {var}(")),
+                    (pat.span.between(arg.span), format!(") = ")),
+                ],
+                Applicability::MaybeIncorrect,
+            );
+
+            warn.emit()
+        })
+    }
+}
+
+fn extract_for_loop<'tcx>(expr: &Expr<'tcx>) -> Option<(&'tcx Pat<'tcx>, &'tcx Expr<'tcx>)> {
+    if let hir::ExprKind::DropTemps(e) = expr.kind
+    && let hir::ExprKind::Match(iterexpr, [arm], hir::MatchSource::ForLoopDesugar) = e.kind
+    && let hir::ExprKind::Call(_, [arg]) = iterexpr.kind
+    && let hir::ExprKind::Loop(block, ..) = arm.body.kind
+    && let [stmt] = block.stmts
+    && let hir::StmtKind::Expr(e) = stmt.kind
+    && let hir::ExprKind::Match(_, [_, some_arm], _) = e.kind
+    && let hir::PatKind::Struct(_, [field], _) = some_arm.pat.kind
+    {
+        Some((field.pat, arg))
+    } else {
+        None
+    }
+}
+
+fn extract_iterator_next_call<'tcx>(
+    cx: &LateContext<'_>,
+    expr: &Expr<'tcx>,
+) -> Option<&'tcx Expr<'tcx>> {
+    // This won't work for `Iterator::next(iter)`, is this an issue?
+    if let hir::ExprKind::MethodCall(_, [recv], _) = expr.kind
+    && cx.typeck_results().type_dependent_def_id(expr.hir_id) == cx.tcx.lang_items().next_fn()
+    {
+        Some(recv)
+    } else {
+        return None
+    }
+}
+
+fn suggest_question_mark<'tcx>(
+    cx: &LateContext<'tcx>,
+    adt: ty::AdtDef<'tcx>,
+    substs: &List<ty::GenericArg<'tcx>>,
+    span: Span,
+) -> bool {
+    let Some(body_id) = cx.enclosing_body else { return false };
+    let Some(into_iterator_did) = cx.tcx.get_diagnostic_item(sym::IntoIterator) else { return false };
+
+    if !cx.tcx.is_diagnostic_item(sym::Result, adt.did()) {
+        return false;
+    }
+
+    // Check that the function/closure/constant we are in has a `Result` type.
+    // Otherwise suggesting using `?` may not be a good idea.
+    {
+        let ty = cx.typeck_results().expr_ty(&cx.tcx.hir().body(body_id).value);
+        let ty::Adt(ret_adt, ..) = ty.kind() else { return false };
+        if !cx.tcx.is_diagnostic_item(sym::Result, ret_adt.did()) {
+            return false;
+        }
+    }
+
+    let ty = substs.type_at(0);
+    let is_iterator = cx.tcx.infer_ctxt().enter(|infcx| {
+        let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
+
+        let cause = ObligationCause::new(
+            span,
+            body_id.hir_id,
+            rustc_infer::traits::ObligationCauseCode::MiscObligation,
+        );
+        fulfill_cx.register_bound(
+            &infcx,
+            ty::ParamEnv::empty(),
+            // Erase any region vids from the type, which may not be resolved
+            infcx.tcx.erase_regions(ty),
+            into_iterator_did,
+            cause,
+        );
+
+        // Select all, including ambiguous predicates
+        let errors = fulfill_cx.select_all_or_error(&infcx);
+
+        errors.is_empty()
+    });
+
+    is_iterator
+}
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index f087c624917e8..d9c5d47cadbf5 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/compiler/rustc_lint/src/lib.rs
@@ -48,6 +48,7 @@ mod context;
 mod early;
 mod enum_intrinsics_non_enums;
 mod expect;
+mod for_loop_over_fallibles;
 pub mod hidden_unicode_codepoints;
 mod internal;
 mod late;
@@ -80,6 +81,7 @@ use rustc_span::Span;
 use array_into_iter::ArrayIntoIter;
 use builtin::*;
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
+use for_loop_over_fallibles::*;
 use hidden_unicode_codepoints::*;
 use internal::*;
 use methods::*;
@@ -178,6 +180,7 @@ macro_rules! late_lint_mod_passes {
         $macro!(
             $args,
             [
+                ForLoopOverFallibles: ForLoopOverFallibles,
                 HardwiredLints: HardwiredLints,
                 ImproperCTypesDeclarations: ImproperCTypesDeclarations,
                 ImproperCTypesDefinitions: ImproperCTypesDefinitions,
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index 9b82320e556b3..ab7e5ba3a1067 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -469,6 +469,13 @@ impl<'tcx> ObligationCauseCode<'tcx> {
             _ => None,
         }
     }
+
+    pub fn peel_match_impls(&self) -> &Self {
+        match self {
+            MatchImpl(cause, _) => cause.code(),
+            _ => self,
+        }
+    }
 }
 
 // `ObligationCauseCode` is used a lot. Make sure it doesn't unintentionally get bigger.
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
index 70fac83325a9c..e4af7022239b8 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs
@@ -1506,13 +1506,28 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
                 .emit();
             }
             FulfillmentErrorCode::CodeConstEquateError(ref expected_found, ref err) => {
-                self.report_mismatched_consts(
+                let mut diag = self.report_mismatched_consts(
                     &error.obligation.cause,
                     expected_found.expected,
                     expected_found.found,
                     err.clone(),
-                )
-                .emit();
+                );
+                let code = error.obligation.cause.code().peel_derives().peel_match_impls();
+                if let ObligationCauseCode::BindingObligation(..)
+                | ObligationCauseCode::ItemObligation(..)
+                | ObligationCauseCode::ExprBindingObligation(..)
+                | ObligationCauseCode::ExprItemObligation(..) = code
+                {
+                    self.note_obligation_cause_code(
+                        &mut diag,
+                        &error.obligation.predicate,
+                        error.obligation.param_env,
+                        code,
+                        &mut vec![],
+                        &mut Default::default(),
+                    );
+                }
+                diag.emit();
             }
         }
     }
diff --git a/library/core/tests/option.rs b/library/core/tests/option.rs
index 9f5e537dcefc0..84eb4fc0aa329 100644
--- a/library/core/tests/option.rs
+++ b/library/core/tests/option.rs
@@ -57,6 +57,7 @@ fn test_get_resource() {
 }
 
 #[test]
+#[cfg_attr(not(bootstrap), allow(for_loop_over_fallibles))]
 fn test_option_dance() {
     let x = Some(());
     let mut y = Some(5);
diff --git a/library/std/src/fs.rs b/library/std/src/fs.rs
index c8e131b6ee14c..6e8cc15af64d0 100644
--- a/library/std/src/fs.rs
+++ b/library/std/src/fs.rs
@@ -377,6 +377,35 @@ impl File {
         OpenOptions::new().write(true).create(true).truncate(true).open(path.as_ref())
     }
 
+    /// Creates a new file in read-write mode; error if the file exists.
+    ///
+    /// This function will create a file if it does not exist, or return an error if it does. This
+    /// way, if the call succeeds, the file returned is guaranteed to be new.
+    ///
+    /// This option is useful because it is atomic. Otherwise between checking whether a file
+    /// exists and creating a new one, the file may have been created by another process (a TOCTOU
+    /// race condition / attack).
+    ///
+    /// This can also be written using
+    /// `File::options().read(true).write(true).create_new(true).open(...)`.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(file_create_new)]
+    ///
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let mut f = File::create_new("foo.txt")?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[unstable(feature = "file_create_new", issue = "none")]
+    pub fn create_new<P: AsRef<Path>>(path: P) -> io::Result<File> {
+        OpenOptions::new().read(true).write(true).create_new(true).open(path.as_ref())
+    }
+
     /// Returns a new OpenOptions object.
     ///
     /// This function returns a new OpenOptions object that you can use to
diff --git a/library/std/src/net/ip/display_buffer.rs b/library/std/src/net/display_buffer.rs
similarity index 86%
rename from library/std/src/net/ip/display_buffer.rs
rename to library/std/src/net/display_buffer.rs
index bd852d5da8ec5..7aadf06e92fc6 100644
--- a/library/std/src/net/ip/display_buffer.rs
+++ b/library/std/src/net/display_buffer.rs
@@ -3,12 +3,12 @@ use crate::mem::MaybeUninit;
 use crate::str;
 
 /// Used for slow path in `Display` implementations when alignment is required.
-pub struct IpDisplayBuffer<const SIZE: usize> {
+pub struct DisplayBuffer<const SIZE: usize> {
     buf: [MaybeUninit<u8>; SIZE],
     len: usize,
 }
 
-impl<const SIZE: usize> IpDisplayBuffer<SIZE> {
+impl<const SIZE: usize> DisplayBuffer<SIZE> {
     #[inline]
     pub const fn new() -> Self {
         Self { buf: MaybeUninit::uninit_array(), len: 0 }
@@ -25,7 +25,7 @@ impl<const SIZE: usize> IpDisplayBuffer<SIZE> {
     }
 }
 
-impl<const SIZE: usize> fmt::Write for IpDisplayBuffer<SIZE> {
+impl<const SIZE: usize> fmt::Write for DisplayBuffer<SIZE> {
     fn write_str(&mut self, s: &str) -> fmt::Result {
         let bytes = s.as_bytes();
 
diff --git a/library/std/src/net/ip.rs b/library/std/src/net/ip_addr.rs
similarity index 99%
rename from library/std/src/net/ip.rs
rename to library/std/src/net/ip_addr.rs
index 6004810655ebb..a670f7168334a 100644
--- a/library/std/src/net/ip.rs
+++ b/library/std/src/net/ip_addr.rs
@@ -8,8 +8,7 @@ use crate::mem::transmute;
 use crate::sys::net::netc as c;
 use crate::sys_common::{FromInner, IntoInner};
 
-mod display_buffer;
-use display_buffer::IpDisplayBuffer;
+use super::display_buffer::DisplayBuffer;
 
 /// An IP address, either IPv4 or IPv6.
 ///
@@ -997,7 +996,7 @@ impl fmt::Display for Ipv4Addr {
         } else {
             const LONGEST_IPV4_ADDR: &str = "255.255.255.255";
 
-            let mut buf = IpDisplayBuffer::<{ LONGEST_IPV4_ADDR.len() }>::new();
+            let mut buf = DisplayBuffer::<{ LONGEST_IPV4_ADDR.len() }>::new();
             // Buffer is long enough for the longest possible IPv4 address, so this should never fail.
             write!(buf, "{}.{}.{}.{}", octets[0], octets[1], octets[2], octets[3]).unwrap();
 
@@ -1844,7 +1843,7 @@ impl fmt::Display for Ipv6Addr {
         } else {
             const LONGEST_IPV6_ADDR: &str = "ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff";
 
-            let mut buf = IpDisplayBuffer::<{ LONGEST_IPV6_ADDR.len() }>::new();
+            let mut buf = DisplayBuffer::<{ LONGEST_IPV6_ADDR.len() }>::new();
             // Buffer is long enough for the longest possible IPv6 address, so this should never fail.
             write!(buf, "{}", self).unwrap();
 
diff --git a/library/std/src/net/ip/tests.rs b/library/std/src/net/ip_addr/tests.rs
similarity index 100%
rename from library/std/src/net/ip/tests.rs
rename to library/std/src/net/ip_addr/tests.rs
diff --git a/library/std/src/net/mod.rs b/library/std/src/net/mod.rs
index e7a40bdaf8e99..cc2e0d9742fea 100644
--- a/library/std/src/net/mod.rs
+++ b/library/std/src/net/mod.rs
@@ -24,11 +24,11 @@
 use crate::io::{self, ErrorKind};
 
 #[stable(feature = "rust1", since = "1.0.0")]
-pub use self::addr::{SocketAddr, SocketAddrV4, SocketAddrV6, ToSocketAddrs};
-#[stable(feature = "rust1", since = "1.0.0")]
-pub use self::ip::{IpAddr, Ipv4Addr, Ipv6Addr, Ipv6MulticastScope};
+pub use self::ip_addr::{IpAddr, Ipv4Addr, Ipv6Addr, Ipv6MulticastScope};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::parser::AddrParseError;
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use self::socket_addr::{SocketAddr, SocketAddrV4, SocketAddrV6, ToSocketAddrs};
 #[unstable(feature = "tcplistener_into_incoming", issue = "88339")]
 pub use self::tcp::IntoIncoming;
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -36,9 +36,10 @@ pub use self::tcp::{Incoming, TcpListener, TcpStream};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::udp::UdpSocket;
 
-mod addr;
-mod ip;
+mod display_buffer;
+mod ip_addr;
 mod parser;
+mod socket_addr;
 mod tcp;
 #[cfg(test)]
 mod test;
diff --git a/library/std/src/net/parser.rs b/library/std/src/net/parser.rs
index 069b660998559..a38031c48c862 100644
--- a/library/std/src/net/parser.rs
+++ b/library/std/src/net/parser.rs
@@ -39,8 +39,8 @@ struct Parser<'a> {
 }
 
 impl<'a> Parser<'a> {
-    fn new(input: &'a str) -> Parser<'a> {
-        Parser { state: input.as_bytes() }
+    fn new(input: &'a [u8]) -> Parser<'a> {
+        Parser { state: input }
     }
 
     /// Run a parser, and restore the pre-parse state if it fails.
@@ -273,32 +273,106 @@ impl<'a> Parser<'a> {
     }
 }
 
+impl IpAddr {
+    /// Parse an IP address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{IpAddr, Ipv4Addr, Ipv6Addr};
+    ///
+    /// let localhost_v4 = IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1));
+    /// let localhost_v6 = IpAddr::V6(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1));
+    ///
+    /// assert_eq!(IpAddr::parse_ascii(b"127.0.0.1"), Ok(localhost_v4));
+    /// assert_eq!(IpAddr::parse_ascii(b"::1"), Ok(localhost_v6));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_ip_addr(), AddrKind::Ip)
+    }
+}
+
 #[stable(feature = "ip_addr", since = "1.7.0")]
 impl FromStr for IpAddr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<IpAddr, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_ip_addr(), AddrKind::Ip)
+        Self::parse_ascii(s.as_bytes())
     }
 }
 
-#[stable(feature = "rust1", since = "1.0.0")]
-impl FromStr for Ipv4Addr {
-    type Err = AddrParseError;
-    fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
+impl Ipv4Addr {
+    /// Parse an IPv4 address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::Ipv4Addr;
+    ///
+    /// let localhost = Ipv4Addr::new(127, 0, 0, 1);
+    ///
+    /// assert_eq!(Ipv4Addr::parse_ascii(b"127.0.0.1"), Ok(localhost));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
         // don't try to parse if too long
-        if s.len() > 15 {
+        if b.len() > 15 {
             Err(AddrParseError(AddrKind::Ipv4))
         } else {
-            Parser::new(s).parse_with(|p| p.read_ipv4_addr(), AddrKind::Ipv4)
+            Parser::new(b).parse_with(|p| p.read_ipv4_addr(), AddrKind::Ipv4)
         }
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl FromStr for Ipv4Addr {
+    type Err = AddrParseError;
+    fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl Ipv6Addr {
+    /// Parse an IPv6 address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::Ipv6Addr;
+    ///
+    /// let localhost = Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1);
+    ///
+    /// assert_eq!(Ipv6Addr::parse_ascii(b"::1"), Ok(localhost));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_ipv6_addr(), AddrKind::Ipv6)
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl FromStr for Ipv6Addr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<Ipv6Addr, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_ipv6_addr(), AddrKind::Ipv6)
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl SocketAddrV4 {
+    /// Parse an IPv4 socket address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{Ipv4Addr, SocketAddrV4};
+    ///
+    /// let socket = SocketAddrV4::new(Ipv4Addr::new(127, 0, 0, 1), 8080);
+    ///
+    /// assert_eq!(SocketAddrV4::parse_ascii(b"127.0.0.1:8080"), Ok(socket));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_socket_addr_v4(), AddrKind::SocketV4)
     }
 }
 
@@ -306,7 +380,25 @@ impl FromStr for Ipv6Addr {
 impl FromStr for SocketAddrV4 {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddrV4, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_socket_addr_v4(), AddrKind::SocketV4)
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl SocketAddrV6 {
+    /// Parse an IPv6 socket address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{Ipv6Addr, SocketAddrV6};
+    ///
+    /// let socket = SocketAddrV6::new(Ipv6Addr::new(0x2001, 0xdb8, 0, 0, 0, 0, 0, 1), 8080, 0, 0);
+    ///
+    /// assert_eq!(SocketAddrV6::parse_ascii(b"[2001:db8::1]:8080"), Ok(socket));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_socket_addr_v6(), AddrKind::SocketV6)
     }
 }
 
@@ -314,7 +406,27 @@ impl FromStr for SocketAddrV4 {
 impl FromStr for SocketAddrV6 {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddrV6, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_socket_addr_v6(), AddrKind::SocketV6)
+        Self::parse_ascii(s.as_bytes())
+    }
+}
+
+impl SocketAddr {
+    /// Parse a socket address from a slice of bytes.
+    ///
+    /// ```
+    /// #![feature(addr_parse_ascii)]
+    ///
+    /// use std::net::{IpAddr, Ipv4Addr, Ipv6Addr, SocketAddr};
+    ///
+    /// let socket_v4 = SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)), 8080);
+    /// let socket_v6 = SocketAddr::new(IpAddr::V6(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0, 1)), 8080);
+    ///
+    /// assert_eq!(SocketAddr::parse_ascii(b"127.0.0.1:8080"), Ok(socket_v4));
+    /// assert_eq!(SocketAddr::parse_ascii(b"[::1]:8080"), Ok(socket_v6));
+    /// ```
+    #[unstable(feature = "addr_parse_ascii", issue = "101035")]
+    pub fn parse_ascii(b: &[u8]) -> Result<Self, AddrParseError> {
+        Parser::new(b).parse_with(|p| p.read_socket_addr(), AddrKind::Socket)
     }
 }
 
@@ -322,7 +434,7 @@ impl FromStr for SocketAddrV6 {
 impl FromStr for SocketAddr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<SocketAddr, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_socket_addr(), AddrKind::Socket)
+        Self::parse_ascii(s.as_bytes())
     }
 }
 
diff --git a/library/std/src/net/addr.rs b/library/std/src/net/socket_addr.rs
similarity index 94%
rename from library/std/src/net/addr.rs
rename to library/std/src/net/socket_addr.rs
index 53fee952a7a7a..33b0dfa03e0ed 100644
--- a/library/std/src/net/addr.rs
+++ b/library/std/src/net/socket_addr.rs
@@ -2,9 +2,9 @@
 mod tests;
 
 use crate::cmp::Ordering;
-use crate::fmt;
+use crate::fmt::{self, Write};
 use crate::hash;
-use crate::io::{self, Write};
+use crate::io;
 use crate::iter;
 use crate::mem;
 use crate::net::{IpAddr, Ipv4Addr, Ipv6Addr};
@@ -15,6 +15,8 @@ use crate::sys_common::net::LookupHost;
 use crate::sys_common::{FromInner, IntoInner};
 use crate::vec;
 
+use super::display_buffer::DisplayBuffer;
+
 /// An internet socket address, either IPv4 or IPv6.
 ///
 /// Internet socket addresses consist of an [IP address], a 16-bit port number, as well
@@ -616,25 +618,18 @@ impl fmt::Debug for SocketAddr {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Display for SocketAddrV4 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        // Fast path: if there's no alignment stuff, write to the output buffer
-        // directly
+        // If there are no alignment requirements, write the socket address directly to `f`.
+        // Otherwise, write it to a local buffer and then use `f.pad`.
         if f.precision().is_none() && f.width().is_none() {
             write!(f, "{}:{}", self.ip(), self.port())
         } else {
-            const IPV4_SOCKET_BUF_LEN: usize = (3 * 4)  // the segments
-                + 3  // the separators
-                + 1 + 5; // the port
-            let mut buf = [0; IPV4_SOCKET_BUF_LEN];
-            let mut buf_slice = &mut buf[..];
-
-            // Unwrap is fine because writing to a sufficiently-sized
-            // buffer is infallible
-            write!(buf_slice, "{}:{}", self.ip(), self.port()).unwrap();
-            let len = IPV4_SOCKET_BUF_LEN - buf_slice.len();
-
-            // This unsafe is OK because we know what is being written to the buffer
-            let buf = unsafe { crate::str::from_utf8_unchecked(&buf[..len]) };
-            f.pad(buf)
+            const LONGEST_IPV4_SOCKET_ADDR: &str = "255.255.255.255:65536";
+
+            let mut buf = DisplayBuffer::<{ LONGEST_IPV4_SOCKET_ADDR.len() }>::new();
+            // Buffer is long enough for the longest possible IPv4 socket address, so this should never fail.
+            write!(buf, "{}:{}", self.ip(), self.port()).unwrap();
+
+            f.pad(buf.as_str())
         }
     }
 }
@@ -649,35 +644,26 @@ impl fmt::Debug for SocketAddrV4 {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl fmt::Display for SocketAddrV6 {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        // Fast path: if there's no alignment stuff, write to the output
-        // buffer directly
+        // If there are no alignment requirements, write the socket address directly to `f`.
+        // Otherwise, write it to a local buffer and then use `f.pad`.
         if f.precision().is_none() && f.width().is_none() {
             match self.scope_id() {
                 0 => write!(f, "[{}]:{}", self.ip(), self.port()),
                 scope_id => write!(f, "[{}%{}]:{}", self.ip(), scope_id, self.port()),
             }
         } else {
-            const IPV6_SOCKET_BUF_LEN: usize = (4 * 8)  // The address
-            + 7  // The colon separators
-            + 2  // The brackets
-            + 1 + 10 // The scope id
-            + 1 + 5; // The port
-
-            let mut buf = [0; IPV6_SOCKET_BUF_LEN];
-            let mut buf_slice = &mut buf[..];
+            const LONGEST_IPV6_SOCKET_ADDR: &str =
+                "[ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff%4294967296]:65536";
 
+            let mut buf = DisplayBuffer::<{ LONGEST_IPV6_SOCKET_ADDR.len() }>::new();
             match self.scope_id() {
-                0 => write!(buf_slice, "[{}]:{}", self.ip(), self.port()),
-                scope_id => write!(buf_slice, "[{}%{}]:{}", self.ip(), scope_id, self.port()),
+                0 => write!(buf, "[{}]:{}", self.ip(), self.port()),
+                scope_id => write!(buf, "[{}%{}]:{}", self.ip(), scope_id, self.port()),
             }
-            // Unwrap is fine because writing to a sufficiently-sized
-            // buffer is infallible
+            // Buffer is long enough for the longest possible IPv6 socket address, so this should never fail.
             .unwrap();
-            let len = IPV6_SOCKET_BUF_LEN - buf_slice.len();
 
-            // This unsafe is OK because we know what is being written to the buffer
-            let buf = unsafe { crate::str::from_utf8_unchecked(&buf[..len]) };
-            f.pad(buf)
+            f.pad(buf.as_str())
         }
     }
 }
diff --git a/library/std/src/net/addr/tests.rs b/library/std/src/net/socket_addr/tests.rs
similarity index 79%
rename from library/std/src/net/addr/tests.rs
rename to library/std/src/net/socket_addr/tests.rs
index 585a17451a0b7..15211f81981ba 100644
--- a/library/std/src/net/addr/tests.rs
+++ b/library/std/src/net/socket_addr/tests.rs
@@ -51,6 +51,75 @@ fn to_socket_addr_string() {
     // s has been moved into the tsa call
 }
 
+#[test]
+fn ipv4_socket_addr_to_string() {
+    // Shortest possible IPv4 length.
+    assert_eq!(SocketAddrV4::new(Ipv4Addr::new(0, 0, 0, 0), 0).to_string(), "0.0.0.0:0");
+
+    // Longest possible IPv4 length.
+    assert_eq!(
+        SocketAddrV4::new(Ipv4Addr::new(255, 255, 255, 255), u16::MAX).to_string(),
+        "255.255.255.255:65535"
+    );
+
+    // Test padding.
+    assert_eq!(
+        &format!("{:16}", SocketAddrV4::new(Ipv4Addr::new(1, 1, 1, 1), 53)),
+        "1.1.1.1:53      "
+    );
+    assert_eq!(
+        &format!("{:>16}", SocketAddrV4::new(Ipv4Addr::new(1, 1, 1, 1), 53)),
+        "      1.1.1.1:53"
+    );
+}
+
+#[test]
+fn ipv6_socket_addr_to_string() {
+    // IPv4-mapped address.
+    assert_eq!(
+        SocketAddrV6::new(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0xc000, 0x280), 8080, 0, 0)
+            .to_string(),
+        "[::ffff:192.0.2.128]:8080"
+    );
+
+    // IPv4-compatible address.
+    assert_eq!(
+        SocketAddrV6::new(Ipv6Addr::new(0, 0, 0, 0, 0, 0, 0xc000, 0x280), 8080, 0, 0).to_string(),
+        "[::192.0.2.128]:8080"
+    );
+
+    // IPv6 address with no zero segments.
+    assert_eq!(
+        SocketAddrV6::new(Ipv6Addr::new(8, 9, 10, 11, 12, 13, 14, 15), 80, 0, 0).to_string(),
+        "[8:9:a:b:c:d:e:f]:80"
+    );
+
+    // Shortest possible IPv6 length.
+    assert_eq!(SocketAddrV6::new(Ipv6Addr::UNSPECIFIED, 0, 0, 0).to_string(), "[::]:0");
+
+    // Longest possible IPv6 length.
+    assert_eq!(
+        SocketAddrV6::new(
+            Ipv6Addr::new(0x1111, 0x2222, 0x3333, 0x4444, 0x5555, 0x6666, 0x7777, 0x8888),
+            u16::MAX,
+            u32::MAX,
+            u32::MAX,
+        )
+        .to_string(),
+        "[1111:2222:3333:4444:5555:6666:7777:8888%4294967295]:65535"
+    );
+
+    // Test padding.
+    assert_eq!(
+        &format!("{:22}", SocketAddrV6::new(Ipv6Addr::new(1, 2, 3, 4, 5, 6, 7, 8), 9, 0, 0)),
+        "[1:2:3:4:5:6:7:8]:9   "
+    );
+    assert_eq!(
+        &format!("{:>22}", SocketAddrV6::new(Ipv6Addr::new(1, 2, 3, 4, 5, 6, 7, 8), 9, 0, 0)),
+        "   [1:2:3:4:5:6:7:8]:9"
+    );
+}
+
 #[test]
 fn bind_udp_socket_bad() {
     // rust-lang/rust#53957: This is a regression test for a parsing problem
diff --git a/src/librustdoc/html/static/css/rustdoc.css b/src/librustdoc/html/static/css/rustdoc.css
index 38cdcd61e88cb..c25c16ef9e8d3 100644
--- a/src/librustdoc/html/static/css/rustdoc.css
+++ b/src/librustdoc/html/static/css/rustdoc.css
@@ -777,7 +777,7 @@ pre, .rustdoc.source .example-wrap {
 .content .docblock >.impl-items table td {
 	padding: 0;
 }
-.content .docblock > .impl-items .table-display, .impl-items table td {
+.content .docblock > .impl-items .table-display {
 	border: none;
 }
 
diff --git a/src/test/rustdoc-gui/docblock-table.goml b/src/test/rustdoc-gui/docblock-table.goml
new file mode 100644
index 0000000000000..7263156ab2bb3
--- /dev/null
+++ b/src/test/rustdoc-gui/docblock-table.goml
@@ -0,0 +1,4 @@
+goto: file://|DOC_PATH|/test_docs/doc_block_table/struct.DocBlockTable.html#method.func
+
+compare-elements-css: (".impl-items .docblock table th", ".top-doc .docblock table th", ["border"])
+compare-elements-css: (".impl-items .docblock table td", ".top-doc .docblock table td", ["border"])
diff --git a/src/test/rustdoc-gui/src/test_docs/lib.rs b/src/test/rustdoc-gui/src/test_docs/lib.rs
index 1b26aaecb5ef3..a02d5934cc245 100644
--- a/src/test/rustdoc-gui/src/test_docs/lib.rs
+++ b/src/test/rustdoc-gui/src/test_docs/lib.rs
@@ -293,3 +293,29 @@ pub mod details {
     /// </details>
     pub struct Details;
 }
+
+pub mod doc_block_table {
+
+    pub trait DocBlockTableTrait {
+        fn func();
+    }
+
+    /// Struct doc.
+    ///
+    /// | header1                  | header2                  |
+    /// |--------------------------|--------------------------|
+    /// | Lorem Ipsum, Lorem Ipsum | Lorem Ipsum, Lorem Ipsum |
+    pub struct DocBlockTable {}
+
+    impl DocBlockTableTrait for DocBlockTable {
+        /// Trait impl func doc for struct.
+        ///
+        /// | header1                  | header2                  |
+        /// |--------------------------|--------------------------|
+        /// | Lorem Ipsum, Lorem Ipsum | Lorem Ipsum, Lorem Ipsum |
+        fn func() {
+            println!();
+        }
+    }
+
+}
diff --git a/src/test/ui/const-generics/defaults/generic-expr-default-concrete.stderr b/src/test/ui/const-generics/defaults/generic-expr-default-concrete.stderr
index e8826ce4335e7..61b3551182c90 100644
--- a/src/test/ui/const-generics/defaults/generic-expr-default-concrete.stderr
+++ b/src/test/ui/const-generics/defaults/generic-expr-default-concrete.stderr
@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     Foo::<10, 12>
    |     ^^^^^^^^^^^^^ expected `11`, found `12`
    |
-   = note: expected type `11`
-              found type `12`
+   = note: expected constant `11`
+              found constant `12`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/const-generics/defaults/generic-expr-default-mismatched-types.stderr b/src/test/ui/const-generics/defaults/generic-expr-default-mismatched-types.stderr
index d5a3071b77d15..e83f89a60333f 100644
--- a/src/test/ui/const-generics/defaults/generic-expr-default-mismatched-types.stderr
+++ b/src/test/ui/const-generics/defaults/generic-expr-default-mismatched-types.stderr
@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     Foo::<N, { N + 2 }>
    |     ^^^^^^^^^^^^^^^^^^^ expected `{ N + 1 }`, found `{ N + 2 }`
    |
-   = note: expected type `{ N + 1 }`
-              found type `{ N + 2 }`
+   = note: expected constant `{ N + 1 }`
+              found constant `{ N + 2 }`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/const-generics/generic_const_exprs/abstract-const-as-cast-3.stderr b/src/test/ui/const-generics/generic_const_exprs/abstract-const-as-cast-3.stderr
index 615dc875f67a3..ada1050d35f35 100644
--- a/src/test/ui/const-generics/generic_const_exprs/abstract-const-as-cast-3.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/abstract-const-as-cast-3.stderr
@@ -22,8 +22,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as u128 }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as u128 }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as u128 }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:20:19
@@ -49,8 +54,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as _ }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as _ }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as _ }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:23:5
@@ -58,8 +68,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `12`, found `13`
    |
-   = note: expected type `12`
-              found type `13`
+   = note: expected constant `12`
+              found constant `13`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:25:5
@@ -67,8 +82,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<14, 13>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `13`, found `14`
    |
-   = note: expected type `13`
-              found type `14`
+   = note: expected constant `13`
+              found constant `14`
+note: required by a bound in `use_trait_impl::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:14:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:35:19
@@ -94,8 +114,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as u128 }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as u128 }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as u128 }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error: unconstrained generic constant
   --> $DIR/abstract-const-as-cast-3.rs:38:19
@@ -121,8 +146,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<{ N + 1 }, { N as _ }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N as _ }`, found `{ O as u128 }`
    |
-   = note: expected type `{ N as _ }`
-              found type `{ O as u128 }`
+   = note: expected constant `{ N as _ }`
+              found constant `{ O as u128 }`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:41:5
@@ -130,8 +160,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<13, { 12 as u128 }>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `12`, found `13`
    |
-   = note: expected type `12`
-              found type `13`
+   = note: expected constant `12`
+              found constant `13`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error[E0308]: mismatched types
   --> $DIR/abstract-const-as-cast-3.rs:43:5
@@ -139,8 +174,13 @@ error[E0308]: mismatched types
 LL |     assert_impl::<HasCastInTraitImpl<14, 13>>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `13`, found `14`
    |
-   = note: expected type `13`
-              found type `14`
+   = note: expected constant `13`
+              found constant `14`
+note: required by a bound in `use_trait_impl_2::assert_impl`
+  --> $DIR/abstract-const-as-cast-3.rs:32:23
+   |
+LL |     fn assert_impl<T: Trait>() {}
+   |                       ^^^^^ required by this bound in `use_trait_impl_2::assert_impl`
 
 error: aborting due to 12 previous errors
 
diff --git a/src/test/ui/const-generics/generic_const_exprs/different-fn.stderr b/src/test/ui/const-generics/generic_const_exprs/different-fn.stderr
index 2aeb9b961ffde..83a2f3740b146 100644
--- a/src/test/ui/const-generics/generic_const_exprs/different-fn.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/different-fn.stderr
@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     [0; size_of::<Foo<T>>()]
    |     ^^^^^^^^^^^^^^^^^^^^^^^^ expected `size_of::<T>()`, found `size_of::<Foo<T>>()`
    |
-   = note: expected type `size_of::<T>()`
-              found type `size_of::<Foo<T>>()`
+   = note: expected constant `size_of::<T>()`
+              found constant `size_of::<Foo<T>>()`
 
 error: unconstrained generic constant
   --> $DIR/different-fn.rs:10:9
diff --git a/src/test/ui/const-generics/generic_const_exprs/issue-62504.full.stderr b/src/test/ui/const-generics/generic_const_exprs/issue-62504.full.stderr
index f2ae361dc81d9..0742db398c9c4 100644
--- a/src/test/ui/const-generics/generic_const_exprs/issue-62504.full.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/issue-62504.full.stderr
@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |         ArrayHolder([0; Self::SIZE])
    |                     ^^^^^^^^^^^^^^^ expected `X`, found `Self::SIZE`
    |
-   = note: expected type `X`
-              found type `Self::SIZE`
+   = note: expected constant `X`
+              found constant `Self::SIZE`
 
 error: unconstrained generic constant
   --> $DIR/issue-62504.rs:18:25
diff --git a/src/test/ui/const-generics/generic_const_exprs/issue-72819-generic-in-const-eval.full.stderr b/src/test/ui/const-generics/generic_const_exprs/issue-72819-generic-in-const-eval.full.stderr
index d536f6fd1d557..f2fddfbfbb52a 100644
--- a/src/test/ui/const-generics/generic_const_exprs/issue-72819-generic-in-const-eval.full.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/issue-72819-generic-in-const-eval.full.stderr
@@ -4,8 +4,15 @@ error[E0308]: mismatched types
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |            ^^^^^^^^^^^^^^^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-72819-generic-in-const-eval.rs:8:39
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                       ^^^^^^ required by this bound in `Arr`
 
 error[E0308]: mismatched types
   --> $DIR/issue-72819-generic-in-const-eval.rs:20:32
@@ -13,8 +20,15 @@ error[E0308]: mismatched types
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |                                ^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-72819-generic-in-const-eval.rs:8:39
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                       ^^^^^^ required by this bound in `Arr`
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/const-generics/generic_const_exprs/issue-83765.stderr b/src/test/ui/const-generics/generic_const_exprs/issue-83765.stderr
index 0332e82fe0727..b693023f125a4 100644
--- a/src/test/ui/const-generics/generic_const_exprs/issue-83765.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/issue-83765.stderr
@@ -4,8 +4,8 @@ error[E0308]: method not compatible with trait
 LL |     fn size(&self) -> [usize; DIM] {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `Self::DIM`, found `DIM`
    |
-   = note: expected type `Self::DIM`
-              found type `DIM`
+   = note: expected constant `Self::DIM`
+              found constant `DIM`
 
 error: unconstrained generic constant
   --> $DIR/issue-83765.rs:32:24
@@ -26,8 +26,8 @@ error[E0308]: mismatched types
 LL |         self.reference.size()
    |         ^^^^^^^^^^^^^^^^^^^^^ expected `DIM`, found `Self::DIM`
    |
-   = note: expected type `DIM`
-              found type `Self::DIM`
+   = note: expected constant `DIM`
+              found constant `Self::DIM`
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/const-generics/generic_const_exprs/issue-85848.stderr b/src/test/ui/const-generics/generic_const_exprs/issue-85848.stderr
index 808b305c680f3..09bcb0860b71b 100644
--- a/src/test/ui/const-generics/generic_const_exprs/issue-85848.stderr
+++ b/src/test/ui/const-generics/generic_const_exprs/issue-85848.stderr
@@ -54,8 +54,8 @@ error[E0308]: mismatched types
 LL |     writes_to_specific_path(&cap);
    |     ^^^^^^^^^^^^^^^^^^^^^^^ expected `true`, found `{ contains::<T, U>() }`
    |
-   = note: expected type `true`
-              found type `{ contains::<T, U>() }`
+   = note: expected constant `true`
+              found constant `{ contains::<T, U>() }`
 
 error: aborting due to 3 previous errors
 
diff --git a/src/test/ui/const-generics/generic_const_exprs/obligation-cause.rs b/src/test/ui/const-generics/generic_const_exprs/obligation-cause.rs
new file mode 100644
index 0000000000000..e7c8e4f667d05
--- /dev/null
+++ b/src/test/ui/const-generics/generic_const_exprs/obligation-cause.rs
@@ -0,0 +1,24 @@
+#![allow(incomplete_features)]
+#![feature(generic_const_exprs)]
+
+trait True {}
+
+struct Is<const V: bool>;
+
+impl True for Is<true> {}
+
+fn g<T>()
+//~^ NOTE required by a bound in this
+where
+    Is<{ std::mem::size_of::<T>() == 0 }>: True,
+    //~^ NOTE required by a bound in `g`
+    //~| NOTE required by this bound in `g`
+{
+}
+
+fn main() {
+    g::<usize>();
+    //~^ ERROR mismatched types
+    //~| NOTE expected `false`, found `true`
+    //~| NOTE expected constant `false`
+}
diff --git a/src/test/ui/const-generics/generic_const_exprs/obligation-cause.stderr b/src/test/ui/const-generics/generic_const_exprs/obligation-cause.stderr
new file mode 100644
index 0000000000000..a253ec676f716
--- /dev/null
+++ b/src/test/ui/const-generics/generic_const_exprs/obligation-cause.stderr
@@ -0,0 +1,20 @@
+error[E0308]: mismatched types
+  --> $DIR/obligation-cause.rs:20:5
+   |
+LL |     g::<usize>();
+   |     ^^^^^^^^^^ expected `false`, found `true`
+   |
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `g`
+  --> $DIR/obligation-cause.rs:13:44
+   |
+LL | fn g<T>()
+   |    - required by a bound in this
+...
+LL |     Is<{ std::mem::size_of::<T>() == 0 }>: True,
+   |                                            ^^^^ required by this bound in `g`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/const-generics/issues/issue-73260.stderr b/src/test/ui/const-generics/issues/issue-73260.stderr
index f1fc50e6e5914..7670032e5b758 100644
--- a/src/test/ui/const-generics/issues/issue-73260.stderr
+++ b/src/test/ui/const-generics/issues/issue-73260.stderr
@@ -4,8 +4,16 @@ error[E0308]: mismatched types
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |            ^^^^^^^^^^^^^^^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-73260.rs:6:37
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where
+LL |     Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                     ^^^^^^ required by this bound in `Arr`
 
 error[E0308]: mismatched types
   --> $DIR/issue-73260.rs:16:32
@@ -13,8 +21,16 @@ error[E0308]: mismatched types
 LL |     let x: Arr<{usize::MAX}> = Arr {};
    |                                ^^^ expected `false`, found `true`
    |
-   = note: expected type `false`
-              found type `true`
+   = note: expected constant `false`
+              found constant `true`
+note: required by a bound in `Arr`
+  --> $DIR/issue-73260.rs:6:37
+   |
+LL | struct Arr<const N: usize>
+   |        --- required by a bound in this
+LL | where
+LL |     Assert::<{N < usize::MAX / 2}>: IsTrue,
+   |                                     ^^^^^^ required by this bound in `Arr`
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/const-generics/issues/issue-79674.stderr b/src/test/ui/const-generics/issues/issue-79674.stderr
index 8c029289cbb0d..02b48b55f8b38 100644
--- a/src/test/ui/const-generics/issues/issue-79674.stderr
+++ b/src/test/ui/const-generics/issues/issue-79674.stderr
@@ -4,8 +4,16 @@ error[E0308]: mismatched types
 LL |     requires_distinct("str", 12);
    |     ^^^^^^^^^^^^^^^^^ expected `true`, found `false`
    |
-   = note: expected type `true`
-              found type `false`
+   = note: expected constant `true`
+              found constant `false`
+note: required by a bound in `requires_distinct`
+  --> $DIR/issue-79674.rs:23:37
+   |
+LL | fn requires_distinct<A, B>(_a: A, _b: B) where
+   |    ----------------- required by a bound in this
+LL |     A: MiniTypeId, B: MiniTypeId,
+LL |     Lift<{is_same_type::<A, B>()}>: IsFalse {}
+   |                                     ^^^^^^^ required by this bound in `requires_distinct`
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/const-generics/types-mismatch-const-args.full.stderr b/src/test/ui/const-generics/types-mismatch-const-args.full.stderr
index 486506239ddfd..b6a22df74369a 100644
--- a/src/test/ui/const-generics/types-mismatch-const-args.full.stderr
+++ b/src/test/ui/const-generics/types-mismatch-const-args.full.stderr
@@ -4,8 +4,8 @@ error[E0308]: mismatched types
 LL |     let _: A<'a, u32, {2u32}, {3u32}> = A::<'a, u32, {2u32 + 2u32}, {3u32}> { data: PhantomData };
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `2`, found `4`
    |
-   = note: expected type `2`
-              found type `4`
+   = note: expected constant `2`
+              found constant `4`
 
 error[E0308]: mismatched types
   --> $DIR/types-mismatch-const-args.rs:16:41
diff --git a/src/test/ui/consts/const-points-to-static.32bit.stderr b/src/test/ui/consts/const-points-to-static.32bit.stderr
index 97825dd0eb517..c7a435a1ee3fc 100644
--- a/src/test/ui/consts/const-points-to-static.32bit.stderr
+++ b/src/test/ui/consts/const-points-to-static.32bit.stderr
@@ -2,7 +2,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/const-points-to-static.rs:6:1
    |
 LL | const TEST: &u8 = &MY_STATIC;
-   | ^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable
+   | ^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable in a constant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 4, align: 4) {
diff --git a/src/test/ui/consts/const-points-to-static.64bit.stderr b/src/test/ui/consts/const-points-to-static.64bit.stderr
index 0d4a5a8ce4f33..4d5b8eac541df 100644
--- a/src/test/ui/consts/const-points-to-static.64bit.stderr
+++ b/src/test/ui/consts/const-points-to-static.64bit.stderr
@@ -2,7 +2,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/const-points-to-static.rs:6:1
    |
 LL | const TEST: &u8 = &MY_STATIC;
-   | ^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable
+   | ^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable in a constant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 8, align: 8) {
diff --git a/src/test/ui/consts/miri_unleashed/const_refers_to_static2.32bit.stderr b/src/test/ui/consts/miri_unleashed/const_refers_to_static2.32bit.stderr
index b3ad81e49bc16..14173ac9f69b4 100644
--- a/src/test/ui/consts/miri_unleashed/const_refers_to_static2.32bit.stderr
+++ b/src/test/ui/consts/miri_unleashed/const_refers_to_static2.32bit.stderr
@@ -2,7 +2,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/const_refers_to_static2.rs:11:1
    |
 LL | const REF_INTERIOR_MUT: &usize = {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable in a constant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 4, align: 4) {
@@ -13,7 +13,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/const_refers_to_static2.rs:18:1
    |
 LL | const READ_IMMUT: &usize = {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable
+   | ^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable in a constant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 4, align: 4) {
diff --git a/src/test/ui/consts/miri_unleashed/const_refers_to_static2.64bit.stderr b/src/test/ui/consts/miri_unleashed/const_refers_to_static2.64bit.stderr
index 24bd070928265..e7e51a41856e2 100644
--- a/src/test/ui/consts/miri_unleashed/const_refers_to_static2.64bit.stderr
+++ b/src/test/ui/consts/miri_unleashed/const_refers_to_static2.64bit.stderr
@@ -2,7 +2,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/const_refers_to_static2.rs:11:1
    |
 LL | const REF_INTERIOR_MUT: &usize = {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable in a constant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 8, align: 8) {
@@ -13,7 +13,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/const_refers_to_static2.rs:18:1
    |
 LL | const READ_IMMUT: &usize = {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable
+   | ^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable in a constant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 8, align: 8) {
diff --git a/src/test/ui/consts/miri_unleashed/const_refers_to_static_cross_crate.32bit.stderr b/src/test/ui/consts/miri_unleashed/const_refers_to_static_cross_crate.32bit.stderr
index 20a96b57f29b9..3a22b06891629 100644
--- a/src/test/ui/consts/miri_unleashed/const_refers_to_static_cross_crate.32bit.stderr
+++ b/src/test/ui/consts/miri_unleashed/const_refers_to_static_cross_crate.32bit.stderr
@@ -2,7 +2,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/const_refers_to_static_cross_crate.rs:12:1
    |
 LL | const SLICE_MUT: &[u8; 1] = {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable in a constant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 4, align: 4) {
@@ -19,7 +19,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/const_refers_to_static_cross_crate.rs:17:1
    |
 LL | const U8_MUT: &u8 = {
-   | ^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable
+   | ^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable in a constant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 4, align: 4) {
diff --git a/src/test/ui/consts/miri_unleashed/const_refers_to_static_cross_crate.64bit.stderr b/src/test/ui/consts/miri_unleashed/const_refers_to_static_cross_crate.64bit.stderr
index dfa0f76baa186..39c874d6498b2 100644
--- a/src/test/ui/consts/miri_unleashed/const_refers_to_static_cross_crate.64bit.stderr
+++ b/src/test/ui/consts/miri_unleashed/const_refers_to_static_cross_crate.64bit.stderr
@@ -2,7 +2,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/const_refers_to_static_cross_crate.rs:12:1
    |
 LL | const SLICE_MUT: &[u8; 1] = {
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable in a constant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 8, align: 8) {
@@ -19,7 +19,7 @@ error[E0080]: it is undefined behavior to use this value
   --> $DIR/const_refers_to_static_cross_crate.rs:17:1
    |
 LL | const U8_MUT: &u8 = {
-   | ^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable
+   | ^^^^^^^^^^^^^^^^^ constructing invalid value: encountered a reference pointing to a static variable in a constant
    |
    = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
    = note: the raw bytes of the constant (size: 8, align: 8) {
diff --git a/src/test/ui/drop/dropck_legal_cycles.rs b/src/test/ui/drop/dropck_legal_cycles.rs
index 27a599315dc1c..6a0fe7784fbcc 100644
--- a/src/test/ui/drop/dropck_legal_cycles.rs
+++ b/src/test/ui/drop/dropck_legal_cycles.rs
@@ -1017,7 +1017,7 @@ impl<'a> Children<'a> for HM<'a> {
         where C: Context + PrePost<Self>, Self: Sized
     {
         if let Some(ref hm) = self.contents.get() {
-            for (k, v) in hm.iter().nth(index / 2) {
+            if let Some((k, v)) = hm.iter().nth(index / 2) {
                 [k, v][index % 2].descend_into_self(context);
             }
         }
@@ -1032,7 +1032,7 @@ impl<'a> Children<'a> for VD<'a> {
         where C: Context + PrePost<Self>, Self: Sized
     {
         if let Some(ref vd) = self.contents.get() {
-            for r in vd.iter().nth(index) {
+            if let Some(r) = vd.iter().nth(index) {
                 r.descend_into_self(context);
             }
         }
@@ -1047,7 +1047,7 @@ impl<'a> Children<'a> for VM<'a> {
         where C: Context + PrePost<VM<'a>>
     {
         if let Some(ref vd) = self.contents.get() {
-            for (_idx, r) in vd.iter().nth(index) {
+            if let Some((_idx, r)) = vd.iter().nth(index) {
                 r.descend_into_self(context);
             }
         }
@@ -1062,7 +1062,7 @@ impl<'a> Children<'a> for LL<'a> {
         where C: Context + PrePost<LL<'a>>
     {
         if let Some(ref ll) = self.contents.get() {
-            for r in ll.iter().nth(index) {
+            if let Some(r) = ll.iter().nth(index) {
                 r.descend_into_self(context);
             }
         }
@@ -1077,7 +1077,7 @@ impl<'a> Children<'a> for BH<'a> {
         where C: Context + PrePost<BH<'a>>
     {
         if let Some(ref bh) = self.contents.get() {
-            for r in bh.iter().nth(index) {
+            if let Some(r) = bh.iter().nth(index) {
                 r.descend_into_self(context);
             }
         }
@@ -1092,7 +1092,7 @@ impl<'a> Children<'a> for BTM<'a> {
         where C: Context + PrePost<BTM<'a>>
     {
         if let Some(ref bh) = self.contents.get() {
-            for (k, v) in bh.iter().nth(index / 2) {
+            if let Some((k, v)) = bh.iter().nth(index / 2) {
                 [k, v][index % 2].descend_into_self(context);
             }
         }
@@ -1107,7 +1107,7 @@ impl<'a> Children<'a> for BTS<'a> {
         where C: Context + PrePost<BTS<'a>>
     {
         if let Some(ref bh) = self.contents.get() {
-            for r in bh.iter().nth(index) {
+            if let Some(r) = bh.iter().nth(index) {
                 r.descend_into_self(context);
             }
         }
diff --git a/src/test/ui/issues/issue-30371.rs b/src/test/ui/issues/issue-30371.rs
index a1ae9a36bc1d7..880558eb5b697 100644
--- a/src/test/ui/issues/issue-30371.rs
+++ b/src/test/ui/issues/issue-30371.rs
@@ -1,5 +1,6 @@
 // run-pass
 #![allow(unreachable_code)]
+#![allow(for_loop_over_fallibles)]
 #![deny(unused_variables)]
 
 fn main() {
diff --git a/src/test/ui/lint/for_loop_over_fallibles.rs b/src/test/ui/lint/for_loop_over_fallibles.rs
new file mode 100644
index 0000000000000..43d71c2e808a9
--- /dev/null
+++ b/src/test/ui/lint/for_loop_over_fallibles.rs
@@ -0,0 +1,43 @@
+// check-pass
+
+fn main() {
+    // Common
+    for _ in Some(1) {}
+    //~^ WARN for loop over an `Option`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider using `if let` to clear intent
+    for _ in Ok::<_, ()>(1) {}
+    //~^ WARN for loop over a `Result`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider using `if let` to clear intent
+
+    // `Iterator::next` specific
+    for _ in [0; 0].iter().next() {}
+    //~^ WARN for loop over an `Option`. This is more readably written as an `if let` statement
+    //~| HELP to iterate over `[0; 0].iter()` remove the call to `next`
+    //~| HELP consider using `if let` to clear intent
+
+    // `Result<impl Iterator, _>`, but function doesn't return `Result`
+    for _ in Ok::<_, ()>([0; 0].iter()) {}
+    //~^ WARN for loop over a `Result`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider using `if let` to clear intent
+}
+
+fn _returns_result() -> Result<(), ()> {
+    // `Result<impl Iterator, _>`
+    for _ in Ok::<_, ()>([0; 0].iter()) {}
+    //~^ WARN for loop over a `Result`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider unwrapping the `Result` with `?` to iterate over its contents
+    //~| HELP consider using `if let` to clear intent
+
+    // `Result<impl IntoIterator>`
+    for _ in Ok::<_, ()>([0; 0]) {}
+    //~^ WARN for loop over a `Result`. This is more readably written as an `if let` statement
+    //~| HELP to check pattern in a loop use `while let`
+    //~| HELP consider unwrapping the `Result` with `?` to iterate over its contents
+    //~| HELP consider using `if let` to clear intent
+
+    Ok(())
+}
diff --git a/src/test/ui/lint/for_loop_over_fallibles.stderr b/src/test/ui/lint/for_loop_over_fallibles.stderr
new file mode 100644
index 0000000000000..56e3126dc09a4
--- /dev/null
+++ b/src/test/ui/lint/for_loop_over_fallibles.stderr
@@ -0,0 +1,101 @@
+warning: for loop over an `Option`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:5:14
+   |
+LL |     for _ in Some(1) {}
+   |              ^^^^^^^
+   |
+   = note: `#[warn(for_loop_over_fallibles)]` on by default
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Some(_) = Some(1) {}
+   |     ~~~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Some(_) = Some(1) {}
+   |     ~~~~~~~~~~~~ ~~~
+
+warning: for loop over a `Result`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:9:14
+   |
+LL |     for _ in Ok::<_, ()>(1) {}
+   |              ^^^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Ok(_) = Ok::<_, ()>(1) {}
+   |     ~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Ok(_) = Ok::<_, ()>(1) {}
+   |     ~~~~~~~~~~ ~~~
+
+warning: for loop over an `Option`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:15:14
+   |
+LL |     for _ in [0; 0].iter().next() {}
+   |              ^^^^^^^^^^^^^^^^^^^^
+   |
+help: to iterate over `[0; 0].iter()` remove the call to `next`
+   |
+LL |     for _ in [0; 0].iter().by_ref() {}
+   |                           ~~~~~~~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Some(_) = [0; 0].iter().next() {}
+   |     ~~~~~~~~~~~~ ~~~
+
+warning: for loop over a `Result`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:21:14
+   |
+LL |     for _ in Ok::<_, ()>([0; 0].iter()) {}
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Ok(_) = Ok::<_, ()>([0; 0].iter()) {}
+   |     ~~~~~~~~~~~~~ ~~~
+help: consider using `if let` to clear intent
+   |
+LL |     if let Ok(_) = Ok::<_, ()>([0; 0].iter()) {}
+   |     ~~~~~~~~~~ ~~~
+
+warning: for loop over a `Result`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:29:14
+   |
+LL |     for _ in Ok::<_, ()>([0; 0].iter()) {}
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Ok(_) = Ok::<_, ()>([0; 0].iter()) {}
+   |     ~~~~~~~~~~~~~ ~~~
+help: consider unwrapping the `Result` with `?` to iterate over its contents
+   |
+LL |     for _ in Ok::<_, ()>([0; 0].iter())? {}
+   |                                        +
+help: consider using `if let` to clear intent
+   |
+LL |     if let Ok(_) = Ok::<_, ()>([0; 0].iter()) {}
+   |     ~~~~~~~~~~ ~~~
+
+warning: for loop over a `Result`. This is more readably written as an `if let` statement
+  --> $DIR/for_loop_over_fallibles.rs:36:14
+   |
+LL |     for _ in Ok::<_, ()>([0; 0]) {}
+   |              ^^^^^^^^^^^^^^^^^^^
+   |
+help: to check pattern in a loop use `while let`
+   |
+LL |     while let Ok(_) = Ok::<_, ()>([0; 0]) {}
+   |     ~~~~~~~~~~~~~ ~~~
+help: consider unwrapping the `Result` with `?` to iterate over its contents
+   |
+LL |     for _ in Ok::<_, ()>([0; 0])? {}
+   |                                 +
+help: consider using `if let` to clear intent
+   |
+LL |     if let Ok(_) = Ok::<_, ()>([0; 0]) {}
+   |     ~~~~~~~~~~ ~~~
+
+warning: 6 warnings emitted
+
diff --git a/src/tools/clippy/tests/ui/for_loop_unfixable.rs b/src/tools/clippy/tests/ui/for_loop_unfixable.rs
index efcaffce24ea4..203656fa4d6c2 100644
--- a/src/tools/clippy/tests/ui/for_loop_unfixable.rs
+++ b/src/tools/clippy/tests/ui/for_loop_unfixable.rs
@@ -8,6 +8,7 @@
     clippy::for_kv_map
 )]
 #[allow(clippy::linkedlist, clippy::unnecessary_mut_passed, clippy::similar_names)]
+#[allow(for_loop_over_fallibles)]
 fn main() {
     let vec = vec![1, 2, 3, 4];
 
diff --git a/src/tools/clippy/tests/ui/for_loop_unfixable.stderr b/src/tools/clippy/tests/ui/for_loop_unfixable.stderr
index f769b4bdc9411..50a86eaa68f7d 100644
--- a/src/tools/clippy/tests/ui/for_loop_unfixable.stderr
+++ b/src/tools/clippy/tests/ui/for_loop_unfixable.stderr
@@ -1,5 +1,5 @@
 error: you are iterating over `Iterator::next()` which is an Option; this will compile but is probably not what you want
-  --> $DIR/for_loop_unfixable.rs:14:15
+  --> $DIR/for_loop_unfixable.rs:15:15
    |
 LL |     for _v in vec.iter().next() {}
    |               ^^^^^^^^^^^^^^^^^
diff --git a/src/tools/clippy/tests/ui/for_loops_over_fallibles.rs b/src/tools/clippy/tests/ui/for_loops_over_fallibles.rs
index 3390111d0a8fe..3c7733e665356 100644
--- a/src/tools/clippy/tests/ui/for_loops_over_fallibles.rs
+++ b/src/tools/clippy/tests/ui/for_loops_over_fallibles.rs
@@ -1,4 +1,5 @@
 #![warn(clippy::for_loops_over_fallibles)]
+#![allow(for_loop_over_fallibles)]
 
 fn for_loops_over_fallibles() {
     let option = Some(1);
diff --git a/src/tools/clippy/tests/ui/for_loops_over_fallibles.stderr b/src/tools/clippy/tests/ui/for_loops_over_fallibles.stderr
index 8c8c022243aeb..5f5a81d24cfde 100644
--- a/src/tools/clippy/tests/ui/for_loops_over_fallibles.stderr
+++ b/src/tools/clippy/tests/ui/for_loops_over_fallibles.stderr
@@ -1,5 +1,5 @@
 error: for loop over `option`, which is an `Option`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:9:14
+  --> $DIR/for_loops_over_fallibles.rs:10:14
    |
 LL |     for x in option {
    |              ^^^^^^
@@ -8,7 +8,7 @@ LL |     for x in option {
    = help: consider replacing `for x in option` with `if let Some(x) = option`
 
 error: for loop over `option`, which is an `Option`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:14:14
+  --> $DIR/for_loops_over_fallibles.rs:15:14
    |
 LL |     for x in option.iter() {
    |              ^^^^^^
@@ -16,7 +16,7 @@ LL |     for x in option.iter() {
    = help: consider replacing `for x in option.iter()` with `if let Some(x) = option`
 
 error: for loop over `result`, which is a `Result`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:19:14
+  --> $DIR/for_loops_over_fallibles.rs:20:14
    |
 LL |     for x in result {
    |              ^^^^^^
@@ -24,7 +24,7 @@ LL |     for x in result {
    = help: consider replacing `for x in result` with `if let Ok(x) = result`
 
 error: for loop over `result`, which is a `Result`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:24:14
+  --> $DIR/for_loops_over_fallibles.rs:25:14
    |
 LL |     for x in result.iter_mut() {
    |              ^^^^^^
@@ -32,7 +32,7 @@ LL |     for x in result.iter_mut() {
    = help: consider replacing `for x in result.iter_mut()` with `if let Ok(x) = result`
 
 error: for loop over `result`, which is a `Result`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:29:14
+  --> $DIR/for_loops_over_fallibles.rs:30:14
    |
 LL |     for x in result.into_iter() {
    |              ^^^^^^
@@ -40,7 +40,7 @@ LL |     for x in result.into_iter() {
    = help: consider replacing `for x in result.into_iter()` with `if let Ok(x) = result`
 
 error: for loop over `option.ok_or("x not found")`, which is a `Result`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:33:14
+  --> $DIR/for_loops_over_fallibles.rs:34:14
    |
 LL |     for x in option.ok_or("x not found") {
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -48,7 +48,7 @@ LL |     for x in option.ok_or("x not found") {
    = help: consider replacing `for x in option.ok_or("x not found")` with `if let Ok(x) = option.ok_or("x not found")`
 
 error: you are iterating over `Iterator::next()` which is an Option; this will compile but is probably not what you want
-  --> $DIR/for_loops_over_fallibles.rs:39:14
+  --> $DIR/for_loops_over_fallibles.rs:40:14
    |
 LL |     for x in v.iter().next() {
    |              ^^^^^^^^^^^^^^^
@@ -56,7 +56,7 @@ LL |     for x in v.iter().next() {
    = note: `#[deny(clippy::iter_next_loop)]` on by default
 
 error: for loop over `v.iter().next().and(Some(0))`, which is an `Option`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:44:14
+  --> $DIR/for_loops_over_fallibles.rs:45:14
    |
 LL |     for x in v.iter().next().and(Some(0)) {
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -64,7 +64,7 @@ LL |     for x in v.iter().next().and(Some(0)) {
    = help: consider replacing `for x in v.iter().next().and(Some(0))` with `if let Some(x) = v.iter().next().and(Some(0))`
 
 error: for loop over `v.iter().next().ok_or("x not found")`, which is a `Result`. This is more readably written as an `if let` statement
-  --> $DIR/for_loops_over_fallibles.rs:48:14
+  --> $DIR/for_loops_over_fallibles.rs:49:14
    |
 LL |     for x in v.iter().next().ok_or("x not found") {
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -72,7 +72,7 @@ LL |     for x in v.iter().next().ok_or("x not found") {
    = help: consider replacing `for x in v.iter().next().ok_or("x not found")` with `if let Ok(x) = v.iter().next().ok_or("x not found")`
 
 error: this loop never actually loops
-  --> $DIR/for_loops_over_fallibles.rs:60:5
+  --> $DIR/for_loops_over_fallibles.rs:61:5
    |
 LL | /     while let Some(x) = option {
 LL | |         println!("{}", x);
@@ -83,7 +83,7 @@ LL | |     }
    = note: `#[deny(clippy::never_loop)]` on by default
 
 error: this loop never actually loops
-  --> $DIR/for_loops_over_fallibles.rs:66:5
+  --> $DIR/for_loops_over_fallibles.rs:67:5
    |
 LL | /     while let Ok(x) = result {
 LL | |         println!("{}", x);