Allow explicit #[repr(Rust)]

compiler/rustc_attr/src/builtin.rs

@@ -940,6 +940,7 @@ where
 #[derive(PartialEq, Debug, Encodable, Decodable, Copy, Clone)]
 pub enum ReprAttr {
     ReprInt(IntType),
+    ReprRust,
     ReprC,
     ReprPacked(u32),
     ReprSimd,
@@ -988,6 +989,7 @@ pub fn parse_repr_attr(sess: &Session, attr: &Attribute) -> Vec<ReprAttr> {
             let mut recognised = false;
             if item.is_word() {
                 let hint = match item.name_or_empty() {
+                    sym::Rust => Some(ReprRust),
                     sym::C => Some(ReprC),
                     sym::packed => Some(ReprPacked(1)),
                     sym::simd => Some(ReprSimd),

compiler/rustc_middle/src/ty/mod.rs

@@ -2257,6 +2257,7 @@ impl<'tcx> TyCtxt<'tcx> {
         for attr in self.get_attrs(did, sym::repr) {
             for r in attr::parse_repr_attr(&self.sess, attr) {
                 flags.insert(match r {
+                    attr::ReprRust => ReprFlags::empty(),
                     attr::ReprC => ReprFlags::IS_C,
                     attr::ReprPacked(pack) => {
                         let pack = Align::from_bytes(pack as u64).unwrap();

compiler/rustc_passes/messages.ftl

@@ -706,7 +706,7 @@ passes_unrecognized_field =
 
 passes_unrecognized_repr_hint =
     unrecognized representation hint
-    .help = valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+    .help = valid reprs are `Rust` (default), `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
 
 passes_unused =
     unused attribute

compiler/rustc_passes/src/check_attr.rs

@@ -1683,6 +1683,7 @@ impl CheckAttrVisitor<'_> {
             }
 
             match hint.name_or_empty() {
+                sym::Rust => {}
                 sym::C => {
                     is_c = true;
                     match target {

tests/ui/abi/explicit_repr_rust.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+#[repr(Rust)]
+struct A;
+
+#[repr(Rust, align(16))]
+struct B;
+
+#[repr(Rust, packed)]
+struct C;
+
+fn main() {}

tests/ui/issues/issue-43988.stderr

@@ -32,15 +32,15 @@ error[E0552]: unrecognized representation hint
 LL |     #[repr(nothing)]
    |            ^^^^^^^
    |
-   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+   = help: valid reprs are `Rust` (default), `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
 
 error[E0552]: unrecognized representation hint
   --> $DIR/issue-43988.rs:18:12
    |
 LL |     #[repr(something_not_real)]
    |            ^^^^^^^^^^^^^^^^^^
    |
-   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+   = help: valid reprs are `Rust` (default), `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
 
 error[E0518]: attribute should be applied to function or closure
   --> $DIR/issue-43988.rs:30:5

tests/ui/repr/invalid_repr_list_help.stderr

@@ -4,31 +4,31 @@ error[E0552]: unrecognized representation hint
 LL | #[repr(uwu)]
    |        ^^^
    |
-   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+   = help: valid reprs are `Rust` (default), `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
 
 error[E0552]: unrecognized representation hint
   --> $DIR/invalid_repr_list_help.rs:6:8
    |
 LL | #[repr(uwu = "a")]
    |        ^^^^^^^^^
    |
-   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+   = help: valid reprs are `Rust` (default), `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
 
 error[E0552]: unrecognized representation hint
   --> $DIR/invalid_repr_list_help.rs:9:8
    |
 LL | #[repr(uwu(4))]
    |        ^^^^^^
    |
-   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+   = help: valid reprs are `Rust` (default), `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
 
 error[E0552]: unrecognized representation hint
   --> $DIR/invalid_repr_list_help.rs:14:8
    |
 LL | #[repr(uwu, u8)]
    |        ^^^
    |
-   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+   = help: valid reprs are `Rust` (default), `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
 
 warning: unknown `doc` attribute `owo`
   --> $DIR/invalid_repr_list_help.rs:20:7
@@ -46,7 +46,7 @@ error[E0552]: unrecognized representation hint
 LL | #[repr(uwu)]
    |        ^^^
    |
-   = help: valid reprs are `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
+   = help: valid reprs are `Rust` (default), `C`, `align`, `packed`, `transparent`, `simd`, `i8`, `u8`, `i16`, `u16`, `i32`, `u32`, `i64`, `u64`, `i128`, `u128`, `isize`, `usize`
 
 error: aborting due to 5 previous errors; 1 warning emitted