Experiment: lint resetting mut bindings

compiler/rustc_hir_typeck/messages.ftl

@@ -46,6 +46,10 @@ hir_typeck_ctor_is_private = tuple struct constructor `{$def}` is private
 
 hir_typeck_deref_is_empty = this expression `Deref`s to `{$deref_ty}` which implements `is_empty`
 
+hir_typeck_dereferencing_mut_binding = dereferencing `mut` binding
+    .label = `mut` dereferences the type of this binding
+    .help = this will change in edition 2024
+
 hir_typeck_expected_default_return_type = expected `()` because of default return type
 
 hir_typeck_expected_return_type = expected `{$expected}` because of return type

compiler/rustc_hir_typeck/src/errors.rs

@@ -630,3 +630,11 @@ pub struct SuggestConvertViaMethod<'tcx> {
     pub expected: Ty<'tcx>,
     pub found: Ty<'tcx>,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(hir_typeck_dereferencing_mut_binding)]
+pub struct DereferencingMutBinding {
+    #[label]
+    #[help]
+    pub span: Span,
+}

compiler/rustc_hir_typeck/src/pat.rs

@@ -14,7 +14,7 @@ use rustc_infer::infer;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_middle::mir::interpret::ErrorHandled;
 use rustc_middle::ty::{self, Adt, BindingMode, Ty, TypeVisitableExt};
-use rustc_session::lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS;
+use rustc_session::lint;
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::hygiene::DesugaringKind;
 use rustc_span::source_map::Spanned;
@@ -600,8 +600,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         // Determine the binding mode...
         let bm = match ba {
-            hir::BindingAnnotation::NONE => def_bm,
-            _ => BindingMode::convert(ba),
+            hir::BindingAnnotation(ast::ByRef::No, hir::Mutability::Not) => def_bm,
+            hir::BindingAnnotation(ast::ByRef::No, mutbl @ hir::Mutability::Mut) => {
+                if let BindingMode::BindByReference(_) = def_bm {
+                    // `mut x` resets the binding mode.
+                    self.tcx.emit_spanned_lint(
+                        lint::builtin::DEREFERENCING_MUT_BINDING,
+                        pat.hir_id,
+                        pat.span,
+                        errors::DereferencingMutBinding { span: pat.span },
+                    );
+                }
+                BindingMode::BindByValue(mutbl)
+            }
+            hir::BindingAnnotation(ast::ByRef::Yes, mutbl) => BindingMode::BindByReference(mutbl),
         };
         // ...and store it in a side table:
         self.inh.typeck_results.borrow_mut().pat_binding_modes_mut().insert(pat.hir_id, bm);
@@ -1838,7 +1850,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             &unmentioned_fields.iter().map(|(_, i)| i).collect::<Vec<_>>(),
         );
 
-        self.tcx.struct_span_lint_hir(NON_EXHAUSTIVE_OMITTED_PATTERNS, pat.hir_id, pat.span, "some fields are not explicitly listed", |lint| {
+        self.tcx.struct_span_lint_hir(lint::builtin::NON_EXHAUSTIVE_OMITTED_PATTERNS, pat.hir_id, pat.span, "some fields are not explicitly listed", |lint| {
         lint.span_label(pat.span, format!("field{} {} not listed", rustc_errors::pluralize!(unmentioned_fields.len()), joined_patterns));
         lint.help(
             "ensure that all fields are mentioned explicitly by adding the suggested fields",

compiler/rustc_lint_defs/src/builtin.rs

@@ -35,6 +35,7 @@ declare_lint_pass! {
         DEPRECATED_CFG_ATTR_CRATE_TYPE_NAME,
         DEPRECATED_IN_FUTURE,
         DEPRECATED_WHERE_CLAUSE_LOCATION,
+        DEREFERENCING_MUT_BINDING,
         DUPLICATE_MACRO_ATTRIBUTES,
         ELIDED_LIFETIMES_IN_ASSOCIATED_CONSTANT,
         ELIDED_LIFETIMES_IN_PATHS,
@@ -1578,6 +1579,36 @@ declare_lint! {
     "detect mut variables which don't need to be mutable"
 }
 
+declare_lint! {
+    /// The `dereferencing_mut_binding` lint detects a `mut x` pattern that resets the binding mode,
+    /// as this behavior will change in rust 2024.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// # #![warn(dereferencing_mut_binding)]
+    /// let x = Some(123u32);
+    /// let _y = match &x {
+    ///     Some(mut x) => {
+    ///         x += 1;
+    ///         x
+    ///     }
+    ///     None => 0,
+    /// };
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Without the `mut`, `x` would have type `&u32`. Pre-2024, adding `mut` makes `x` have type
+    /// `u32`, which was deeped surprising. After edition 2024, adding `mut` will not change the
+    /// type of `x`. This lint warns users of editions before 2024 to update their code.
+    pub DEREFERENCING_MUT_BINDING,
+    Allow,
+    "detects `mut x` bindings that change the type of `x`"
+}
+
 declare_lint! {
     /// The `unconditional_recursion` lint detects functions that cannot
     /// return without calling themselves.

tests/ui/or-patterns/or-patterns-default-binding-modes.rs

@@ -5,6 +5,7 @@
 #![allow(irrefutable_let_patterns)]
 #![allow(dropping_copy_types)]
 #![allow(dropping_references)]
+#![allow(dereferencing_mut_binding)]
 
 fn main() {
     // A regression test for a mistake we made at one point:

tests/ui/pattern/move-ref-patterns/move-ref-patterns-default-binding-modes-fixable.fixed

@@ -1,5 +1,6 @@
 // run-rustfix
 #![allow(unused_variables)]
+#![warn(dereferencing_mut_binding)]
 fn main() {
     struct U;
 
@@ -9,4 +10,5 @@ fn main() {
     let mut p = (U, U);
     let (a, ref mut b) = &mut p;
     //~^ ERROR cannot move out of a mutable reference
+    //~| WARN dereferencing `mut`
 }

tests/ui/pattern/move-ref-patterns/move-ref-patterns-default-binding-modes-fixable.rs

@@ -1,5 +1,6 @@
 // run-rustfix
 #![allow(unused_variables)]
+#![warn(dereferencing_mut_binding)]
 fn main() {
     struct U;
 
@@ -9,4 +10,5 @@ fn main() {
     let mut p = (U, U);
     let (a, mut b) = &mut p;
     //~^ ERROR cannot move out of a mutable reference
+    //~| WARN dereferencing `mut`
 }

tests/ui/pattern/move-ref-patterns/move-ref-patterns-default-binding-modes-fixable.stderr

@@ -1,5 +1,22 @@
+warning: dereferencing `mut` binding
+  --> $DIR/move-ref-patterns-default-binding-modes-fixable.rs:11:13
+   |
+LL |     let (a, mut b) = &mut p;
+   |             ^^^^^ `mut` dereferences the type of this binding
+   |
+help: this will change in edition 2024
+  --> $DIR/move-ref-patterns-default-binding-modes-fixable.rs:11:13
+   |
+LL |     let (a, mut b) = &mut p;
+   |             ^^^^^
+note: the lint level is defined here
+  --> $DIR/move-ref-patterns-default-binding-modes-fixable.rs:3:9
+   |
+LL | #![warn(dereferencing_mut_binding)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0507]: cannot move out of a mutable reference
-  --> $DIR/move-ref-patterns-default-binding-modes-fixable.rs:10:22
+  --> $DIR/move-ref-patterns-default-binding-modes-fixable.rs:11:22
    |
 LL |     let (a, mut b) = &mut p;
    |             -----    ^^^^^^
@@ -12,6 +29,6 @@ help: consider borrowing the pattern binding
 LL |     let (a, ref mut b) = &mut p;
    |             +++
 
-error: aborting due to 1 previous error
+error: aborting due to 1 previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0507`.

tests/ui/pattern/move-ref-patterns/move-ref-patterns-default-binding-modes.rs

@@ -1,3 +1,4 @@
+#![warn(dereferencing_mut_binding)]
 fn main() {
     struct U;
 
@@ -7,4 +8,5 @@ fn main() {
     let p = (U, U);
     let (a, mut b) = &p;
     //~^ ERROR cannot move out of a shared reference
+    //~| WARN dereferencing `mut`
 }

tests/ui/pattern/move-ref-patterns/move-ref-patterns-default-binding-modes.stderr

@@ -1,5 +1,22 @@
+warning: dereferencing `mut` binding
+  --> $DIR/move-ref-patterns-default-binding-modes.rs:9:13
+   |
+LL |     let (a, mut b) = &p;
+   |             ^^^^^ `mut` dereferences the type of this binding
+   |
+help: this will change in edition 2024
+  --> $DIR/move-ref-patterns-default-binding-modes.rs:9:13
+   |
+LL |     let (a, mut b) = &p;
+   |             ^^^^^
+note: the lint level is defined here
+  --> $DIR/move-ref-patterns-default-binding-modes.rs:1:9
+   |
+LL | #![warn(dereferencing_mut_binding)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0507]: cannot move out of a shared reference
-  --> $DIR/move-ref-patterns-default-binding-modes.rs:8:22
+  --> $DIR/move-ref-patterns-default-binding-modes.rs:9:22
    |
 LL |     let (a, mut b) = &p;
    |             -----    ^^
@@ -12,6 +29,6 @@ help: consider borrowing the pattern binding
 LL |     let (a, ref mut b) = &p;
    |             +++
 
-error: aborting due to 1 previous error
+error: aborting due to 1 previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0507`.

tests/ui/rfcs/rfc-2005-default-binding-mode/for.rs

@@ -1,9 +1,11 @@
+#![warn(dereferencing_mut_binding)]
 struct Foo {}
 
 pub fn main() {
     let mut tups = vec![(Foo {}, Foo {})];
     // The below desugars to &(ref n, mut m).
     for (n, mut m) in &tups {
         //~^ ERROR cannot move out of a shared reference
+        //~| WARN dereferencing `mut`
     }
 }

tests/ui/rfcs/rfc-2005-default-binding-mode/for.stderr

@@ -1,5 +1,22 @@
+warning: dereferencing `mut` binding
+  --> $DIR/for.rs:7:13
+   |
+LL |     for (n, mut m) in &tups {
+   |             ^^^^^ `mut` dereferences the type of this binding
+   |
+help: this will change in edition 2024
+  --> $DIR/for.rs:7:13
+   |
+LL |     for (n, mut m) in &tups {
+   |             ^^^^^
+note: the lint level is defined here
+  --> $DIR/for.rs:1:9
+   |
+LL | #![warn(dereferencing_mut_binding)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0507]: cannot move out of a shared reference
-  --> $DIR/for.rs:6:23
+  --> $DIR/for.rs:7:23
    |
 LL |     for (n, mut m) in &tups {
    |             -----     ^^^^^
@@ -12,6 +29,6 @@ help: consider borrowing the pattern binding
 LL |     for (n, ref mut m) in &tups {
    |             +++
 
-error: aborting due to 1 previous error
+error: aborting due to 1 previous error; 1 warning emitted
 
 For more information about this error, try `rustc --explain E0507`.

tests/ui/rfcs/rfc-2005-default-binding-mode/general.rs

@@ -1,5 +1,6 @@
 // run-pass
 #![allow(unused_variables)]
+#![allow(dereferencing_mut_binding)]
 fn some_or_wildcard(r: &Option<i32>, b: &i32) {
     let _: &i32 = match r {
         Some(a) => a,

tests/ui/rfcs/rfc-2005-default-binding-mode/resetting-mut.rs

@@ -0,0 +1,30 @@
+// `mut` resets the binding mode.
+#![deny(dereferencing_mut_binding)]
+
+fn main() {
+    let (x, mut y) = &(0, 0);
+    //~^ ERROR dereferencing `mut`
+    let _: &u32 = x;
+    let _: u32 = y;
+
+    match &Some(5i32) {
+        Some(mut n) => {
+            //~^ ERROR dereferencing `mut`
+            n += 1;
+            let _ = n;
+        }
+        None => {}
+    };
+    if let Some(mut n) = &Some(5i32) {
+        //~^ ERROR dereferencing `mut`
+        n += 1;
+        let _ = n;
+    };
+    match &Some(5i32) {
+        &Some(mut n) => {
+            n += 1;
+            let _ = n;
+        }
+        None => {}
+    };
+}

tests/ui/rfcs/rfc-2005-default-binding-mode/resetting-mut.stderr

@@ -0,0 +1,43 @@
+error: dereferencing `mut` binding
+  --> $DIR/resetting-mut.rs:5:13
+   |
+LL |     let (x, mut y) = &(0, 0);
+   |             ^^^^^ `mut` dereferences the type of this binding
+   |
+help: this will change in edition 2024
+  --> $DIR/resetting-mut.rs:5:13
+   |
+LL |     let (x, mut y) = &(0, 0);
+   |             ^^^^^
+note: the lint level is defined here
+  --> $DIR/resetting-mut.rs:2:9
+   |
+LL | #![deny(dereferencing_mut_binding)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: dereferencing `mut` binding
+  --> $DIR/resetting-mut.rs:11:14
+   |
+LL |         Some(mut n) => {
+   |              ^^^^^ `mut` dereferences the type of this binding
+   |
+help: this will change in edition 2024
+  --> $DIR/resetting-mut.rs:11:14
+   |
+LL |         Some(mut n) => {
+   |              ^^^^^
+
+error: dereferencing `mut` binding
+  --> $DIR/resetting-mut.rs:18:17
+   |
+LL |     if let Some(mut n) = &Some(5i32) {
+   |                 ^^^^^ `mut` dereferences the type of this binding
+   |
+help: this will change in edition 2024
+  --> $DIR/resetting-mut.rs:18:17
+   |
+LL |     if let Some(mut n) = &Some(5i32) {
+   |                 ^^^^^
+
+error: aborting due to 3 previous errors
+