Auto merge of #81234 - repnop:fn-alignment, r=lcnr

Allow specifying alignment for functions

Fixes #75072

This allows the user to specify alignment for functions, which can be useful for low level work where functions need to necessarily be aligned to a specific value.

I believe the error cases not covered in the match are caught earlier based on my testing so I had them just return `None`.

Author: bors

compiler/rustc_attr/src/builtin.rs

@@ -862,18 +862,6 @@ pub fn find_repr_attrs(sess: &Session, attr: &Attribute) -> Vec<ReprAttr> {
         if let Some(items) = attr.meta_item_list() {
             sess.mark_attr_used(attr);
             for item in items {
-                if !item.is_meta_item() {
-                    handle_errors(
-                        &sess.parse_sess,
-                        item.span(),
-                        AttrError::UnsupportedLiteral(
-                            "meta item in `repr` must be an identifier",
-                            false,
-                        ),
-                    );
-                    continue;
-                }
-
                 let mut recognised = false;
                 if item.is_word() {
                     let hint = match item.name_or_empty() {
@@ -890,23 +878,6 @@ pub fn find_repr_attrs(sess: &Session, attr: &Attribute) -> Vec<ReprAttr> {
                         acc.push(h);
                     }
                 } else if let Some((name, value)) = item.name_value_literal() {
-                    let parse_alignment = |node: &ast::LitKind| -> Result<u32, &'static str> {
-                        if let ast::LitKind::Int(literal, ast::LitIntType::Unsuffixed) = node {
-                            if literal.is_power_of_two() {
-                                // rustc_middle::ty::layout::Align restricts align to <= 2^29
-                                if *literal <= 1 << 29 {
-                                    Ok(*literal as u32)
-                                } else {
-                                    Err("larger than 2^29")
-                                }
-                            } else {
-                                Err("not a power of two")
-                            }
-                        } else {
-                            Err("not an unsuffixed integer")
-                        }
-                    };
-
                     let mut literal_error = None;
                     if name == sym::align {
                         recognised = true;
@@ -966,13 +937,7 @@ pub fn find_repr_attrs(sess: &Session, attr: &Attribute) -> Vec<ReprAttr> {
                 }
                 if !recognised {
                     // Not a word we recognize
-                    struct_span_err!(
-                        diagnostic,
-                        item.span(),
-                        E0552,
-                        "unrecognized representation hint"
-                    )
-                    .emit();
+                    diagnostic.delay_span_bug(item.span(), "unrecognized representation hint");
                 }
             }
         }
@@ -1080,3 +1045,16 @@ fn allow_unstable<'a>(
         name
     })
 }
+
+pub fn parse_alignment(node: &ast::LitKind) -> Result<u32, &'static str> {
+    if let ast::LitKind::Int(literal, ast::LitIntType::Unsuffixed) = node {
+        if literal.is_power_of_two() {
+            // rustc_middle::ty::layout::Align restricts align to <= 2^29
+            if *literal <= 1 << 29 { Ok(*literal as u32) } else { Err("larger than 2^29") }
+        } else {
+            Err("not a power of two")
+        }
+    } else {
+        Err("not an unsuffixed integer")
+    }
+}

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -280,6 +280,9 @@ pub fn from_fn_attrs(cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value, instance: ty::
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::CMSE_NONSECURE_ENTRY) {
         llvm::AddFunctionAttrString(llfn, Function, cstr!("cmse_nonsecure_entry"));
     }
+    if let Some(align) = codegen_fn_attrs.alignment {
+        llvm::set_alignment(llfn, align as usize);
+    }
     sanitize(cx, codegen_fn_attrs.no_sanitize, llfn);
 
     // Always annotate functions with the target-cpu they are compiled for.

compiler/rustc_feature/src/active.rs

@@ -642,6 +642,9 @@ declare_features! (
     /// Allows `extern "C-unwind" fn` to enable unwinding across ABI boundaries.
     (active, c_unwind, "1.52.0", Some(74990), None),
 
+    /// Allows using `#[repr(align(...))]` on function items
+    (active, fn_align, "1.53.0", Some(82232), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_middle/src/middle/codegen_fn_attrs.rs

@@ -38,6 +38,9 @@ pub struct CodegenFnAttrs {
     /// be generated against a specific instruction set. Only usable on architectures which allow
     /// switching between multiple instruction sets.
     pub instruction_set: Option<InstructionSetAttr>,
+    /// The `#[repr(align(...))]` attribute. Indicates the value of which the function should be
+    /// aligned to.
+    pub alignment: Option<u32>,
 }
 
 bitflags! {
@@ -103,6 +106,7 @@ impl CodegenFnAttrs {
             link_section: None,
             no_sanitize: SanitizerSet::empty(),
             instruction_set: None,
+            alignment: None,
         }
     }
 

compiler/rustc_passes/src/check_attr.rs

@@ -1127,17 +1127,41 @@ impl CheckAttrVisitor<'tcx> {
         let mut is_transparent = false;
 
         for hint in &hints {
+            if !hint.is_meta_item() {
+                struct_span_err!(
+                    self.tcx.sess,
+                    hint.span(),
+                    E0565,
+                    "meta item in `repr` must be an identifier"
+                )
+                .emit();
+                continue;
+            }
+
             let (article, allowed_targets) = match hint.name_or_empty() {
-                _ if !matches!(target, Target::Struct | Target::Enum | Target::Union) => {
-                    ("a", "struct, enum, or union")
-                }
-                name @ sym::C | name @ sym::align => {
-                    is_c |= name == sym::C;
+                sym::C => {
+                    is_c = true;
                     match target {
                         Target::Struct | Target::Union | Target::Enum => continue,
                         _ => ("a", "struct, enum, or union"),
                     }
                 }
+                sym::align => {
+                    if let (Target::Fn, true) = (target, !self.tcx.features().fn_align) {
+                        feature_err(
+                            &self.tcx.sess.parse_sess,
+                            sym::fn_align,
+                            hint.span(),
+                            "`repr(align)` attributes on functions are unstable",
+                        )
+                        .emit();
+                    }
+
+                    match target {
+                        Target::Struct | Target::Union | Target::Enum | Target::Fn => continue,
+                        _ => ("a", "struct, enum, function, or union"),
+                    }
+                }
                 sym::packed => {
                     if target != Target::Struct && target != Target::Union {
                         ("a", "struct or union")
@@ -1194,7 +1218,17 @@ impl CheckAttrVisitor<'tcx> {
                         continue;
                     }
                 }
-                _ => continue,
+                _ => {
+                    struct_span_err!(
+                        self.tcx.sess,
+                        hint.span(),
+                        E0552,
+                        "unrecognized representation hint"
+                    )
+                    .emit();
+
+                    continue;
+                }
             };
 
             struct_span_err!(

compiler/rustc_span/src/symbol.rs

@@ -561,6 +561,7 @@ symbols! {
         fmt,
         fmt_internals,
         fmul_fast,
+        fn_align,
         fn_must_use,
         fn_mut,
         fn_once,

compiler/rustc_typeck/src/collect.rs

@@ -15,6 +15,8 @@
 //! At present, however, we do run collection across all items in the
 //! crate as a kind of pass. This should eventually be factored away.
 
+// ignore-tidy-filelength
+
 use crate::astconv::{AstConv, SizedByDefault};
 use crate::bounds::Bounds;
 use crate::check::intrinsic::intrinsic_operation_unsafety;
@@ -2889,6 +2891,36 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, id: DefId) -> CodegenFnAttrs {
                     None
                 }
             };
+        } else if tcx.sess.check_name(attr, sym::repr) {
+            codegen_fn_attrs.alignment = match attr.meta_item_list() {
+                Some(items) => match items.as_slice() {
+                    [item] => match item.name_value_literal() {
+                        Some((sym::align, literal)) => {
+                            let alignment = rustc_attr::parse_alignment(&literal.kind);
+
+                            match alignment {
+                                Ok(align) => Some(align),
+                                Err(msg) => {
+                                    struct_span_err!(
+                                        tcx.sess.diagnostic(),
+                                        attr.span,
+                                        E0589,
+                                        "invalid `repr(align)` attribute: {}",
+                                        msg
+                                    )
+                                    .emit();
+
+                                    None
+                                }
+                            }
+                        }
+                        _ => None,
+                    },
+                    [] => None,
+                    _ => None,
+                },
+                None => None,
+            };
         }
     }
 

src/test/codegen/align-fn.rs

@@ -0,0 +1,9 @@
+// compile-flags: -C no-prepopulate-passes -Z mir-opt-level=0
+
+#![crate_type = "lib"]
+#![feature(fn_align)]
+
+// CHECK: align 16
+#[no_mangle]
+#[repr(align(16))]
+pub fn fn_align() {}

src/test/ui/attributes/nonterminal-expansion.rs

@@ -3,7 +3,6 @@
 macro_rules! pass_nonterminal {
     ($n:expr) => {
         #[repr(align($n))] //~ ERROR expected unsuffixed literal or identifier, found `n!()`
-                           //~| ERROR unrecognized representation hint
         struct S;
     };
 }

src/test/ui/attributes/nonterminal-expansion.stderr

@@ -9,17 +9,5 @@ LL | pass_nonterminal!(n!());
    |
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error[E0552]: unrecognized representation hint
-  --> $DIR/nonterminal-expansion.rs:5:16
-   |
-LL |         #[repr(align($n))]
-   |                ^^^^^^^^^
-...
-LL | pass_nonterminal!(n!());
-   | ------------------------ in this macro invocation
-   |
-   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0552`.

src/test/ui/error-codes/E0565.rs

@@ -1,6 +1,5 @@
 // repr currently doesn't support literals
 #[repr("C")] //~ ERROR E0565
-             //~| ERROR E0565
-struct A {  }
+struct A {}
 
-fn main() {  }
+fn main() {}

src/test/ui/error-codes/E0565.stderr

@@ -4,12 +4,6 @@ error[E0565]: meta item in `repr` must be an identifier
 LL | #[repr("C")]
    |        ^^^
 
-error[E0565]: meta item in `repr` must be an identifier
-  --> $DIR/E0565.rs:2:8
-   |
-LL | #[repr("C")]
-   |        ^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0565`.

src/test/ui/feature-gates/feature-gate-fn_align.rs

@@ -0,0 +1,4 @@
+#![crate_type = "lib"]
+
+#[repr(align(16))] //~ ERROR `repr(align)` attributes on functions are unstable
+fn requires_alignment() {}

src/test/ui/feature-gates/feature-gate-fn_align.stderr

@@ -0,0 +1,12 @@
+error[E0658]: `repr(align)` attributes on functions are unstable
+  --> $DIR/feature-gate-fn_align.rs:3:8
+   |
+LL | #[repr(align(16))]
+   |        ^^^^^^^^^
+   |
+   = note: see issue #82232 <https://github.com/rust-lang/rust/issues/82232> for more information
+   = help: add `#![feature(fn_align)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/issues/issue-43988.rs

@@ -13,13 +13,13 @@ fn main() {
 
     #[repr(nothing)]
     let _x = 0;
-    //~^^ ERROR attribute should be applied to a struct, enum, or union
+    //~^^ ERROR E0552
 
     #[repr(something_not_real)]
     loop {
         ()
     };
-    //~^^^^ ERROR attribute should be applied to a struct, enum, or union
+    //~^^^^ ERROR E0552
 
     #[repr]
     let _y = "123";

src/test/ui/issues/issue-43988.stderr

@@ -26,23 +26,17 @@ LL |     #[inline(XYZ)]
 LL |     let _b = 4;
    |     ----------- not a function or closure
 
-error[E0517]: attribute should be applied to a struct, enum, or union
+error[E0552]: unrecognized representation hint
   --> $DIR/issue-43988.rs:14:12
    |
 LL |     #[repr(nothing)]
    |            ^^^^^^^
-LL |     let _x = 0;
-   |     ----------- not a struct, enum, or union
 
-error[E0517]: attribute should be applied to a struct, enum, or union
+error[E0552]: unrecognized representation hint
   --> $DIR/issue-43988.rs:18:12
    |
-LL |       #[repr(something_not_real)]
-   |              ^^^^^^^^^^^^^^^^^^
-LL | /     loop {
-LL | |         ()
-LL | |     };
-   | |_____- not a struct, enum, or union
+LL |     #[repr(something_not_real)]
+   |            ^^^^^^^^^^^^^^^^^^
 
 error[E0518]: attribute should be applied to function or closure
   --> $DIR/issue-43988.rs:30:5
@@ -54,5 +48,5 @@ LL |     foo();
 
 error: aborting due to 7 previous errors
 
-Some errors have detailed explanations: E0517, E0518.
-For more information about an error, try `rustc --explain E0517`.
+Some errors have detailed explanations: E0518, E0552.
+For more information about an error, try `rustc --explain E0518`.

src/test/ui/repr/repr-disallow-on-variant.rs

@@ -2,7 +2,7 @@ struct Test;
 
 enum Foo {
     #[repr(u8)]
-    //~^ ERROR attribute should be applied to a struct, enum, or union
+    //~^ ERROR attribute should be applied to an enum
     Variant,
 }
 

src/test/ui/repr/repr-disallow-on-variant.stderr

@@ -1,11 +1,11 @@
-error[E0517]: attribute should be applied to a struct, enum, or union
+error[E0517]: attribute should be applied to an enum
   --> $DIR/repr-disallow-on-variant.rs:4:12
    |
 LL |     #[repr(u8)]
    |            ^^
 LL |
 LL |     Variant,
-   |     ------- not a struct, enum, or union
+   |     ------- not an enum
 
 error: aborting due to previous error