Auto merge of #114201 - Centri3:explicit-repr-rust, r=WaffleLapkin

Allow explicit `#[repr(Rust)]`

This is identical to no `repr()` at all. For `Rust, packed` and `Rust, align(x)`, it should be the same as no `Rust` at all (as, afaik, `#[repr(align(16))]` uses the Rust ABI.)

The main use case for this is being able to explicitly say "I want to use the Rust ABI" in very very rare circumstances where the first obvious choice would be the C ABI yet is undesirable, which is already possible with functions as `extern "Rust"`. This would be useful for silencing https://github.com/rust-lang/rust-clippy/pull/11253. It's also more consistent with `extern`.

The lack of this also tripped me up a bit when I was new to Rust, as I expected this to be possible.

7 files changed, 24 insertions, 8 deletions

diff --git a/compiler/rustc_attr/src/builtin.rs b/compiler/rustc_attr/src/builtin.rs
index 3592287b95c..ca4b3662a08 100644
--- a/compiler/rustc_attr/src/builtin.rs
+++ b/compiler/rustc_attr/src/builtin.rs
@@ -937,6 +937,7 @@ pub fn find_deprecation(
 #[derive(PartialEq, Debug, Encodable, Decodable, Copy, Clone)]
 pub enum ReprAttr {
     ReprInt(IntType),
+    ReprRust,
     ReprC,
     ReprPacked(u32),
     ReprSimd,
@@ -985,6 +986,7 @@ 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),
diff --git a/compiler/rustc_middle/src/ty/mod.rs b/compiler/rustc_middle/src/ty/mod.rs
index 1274f427e4f..8fedf4dca95 100644
--- a/compiler/rustc_middle/src/ty/mod.rs
+++ b/compiler/rustc_middle/src/ty/mod.rs
@@ -2150,6 +2150,7 @@ 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();
diff --git a/compiler/rustc_passes/messages.ftl b/compiler/rustc_passes/messages.ftl
index 6eacbebe75f..b2a4da885aa 100644
--- a/compiler/rustc_passes/messages.ftl
+++ b/compiler/rustc_passes/messages.ftl
@@ -721,7 +721,7 @@ 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
diff --git a/compiler/rustc_passes/src/check_attr.rs b/compiler/rustc_passes/src/check_attr.rs
index 197b335bdec..50a087c7847 100644
--- a/compiler/rustc_passes/src/check_attr.rs
+++ b/compiler/rustc_passes/src/check_attr.rs
@@ -1732,6 +1732,7 @@ impl CheckAttrVisitor<'_> {
             }
 
             match hint.name_or_empty() {
+                sym::Rust => {}
                 sym::C => {
                     is_c = true;
                     match target {
diff --git a/tests/ui/abi/explicit_repr_rust.rs b/tests/ui/abi/explicit_repr_rust.rs
new file mode 100644
index 00000000000..4f8cab3bf0e
--- /dev/null
+++ b/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() {}
diff --git a/tests/ui/issues/issue-43988.stderr b/tests/ui/issues/issue-43988.stderr
index 02c5dd5bfb7..7bbb8ed2ca9 100644
--- a/tests/ui/issues/issue-43988.stderr
+++ b/tests/ui/issues/issue-43988.stderr
@@ -32,7 +32,7 @@ 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
@@ -40,7 +40,7 @@ error[E0552]: unrecognized representation hint
 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
diff --git a/tests/ui/repr/invalid_repr_list_help.stderr b/tests/ui/repr/invalid_repr_list_help.stderr
index 48a6af3dd4c..7ffe1287eb3 100644
--- a/tests/ui/repr/invalid_repr_list_help.stderr
+++ b/tests/ui/repr/invalid_repr_list_help.stderr
@@ -4,7 +4,7 @@ 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[E0552]: unrecognized representation hint
   --> $DIR/invalid_repr_list_help.rs:6:8
@@ -12,7 +12,7 @@ error[E0552]: unrecognized representation hint
 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
@@ -20,7 +20,7 @@ error[E0552]: unrecognized representation hint
 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
@@ -28,7 +28,7 @@ error[E0552]: unrecognized representation hint
 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