Rollup merge of #138285 - beetrees:repr128-stable, r=traviscross,bjorn3

Stabilize `repr128`

## Stabilisation report

The `repr128` feature ([tracking issue](https://github.com/rust-lang/rust/issues/56071)) allows the use of `#[repr(u128)]` and `#[repr(i128)]` on enums in the same way that other primitive representations such as `#[repr(u64)]` can be used. For example:

```rust
#[repr(u128)]
enum Foo {
    One = 1,
    Two,
    Big = u128::MAX,
}

#[repr(i128)]
enum Bar {
    HasThing(u16) = 42,
    HasSomethingElse(i64) = u64::MAX as i128 + 1,
    HasNothing,
}
```

This is the final part of adding 128-bit integers to Rust ([RFC 1504](https://rust-lang.github.io/rfcs/1504-int128.html)); all other parts of 128-bit integer support were stabilised in #49101 back in 2018.

From a design perspective, `#[repr(u128)]`/`#[repr(i128)]` function like `#[repr(u64)]`/`#[repr(i64)]` but for 128-bit integers instead of 64-bit integers. The only differences are:

- FFI safety: as `u128`/`i128` are not currently considered FFI safe, neither are `#[repr(u128)]`/`#[repr(i128)]` enums (I discovered this wasn't the case while drafting this stabilisation report, so I have submitted #138282 to fix this).
- Debug info: while none of the major debuggers currently support 128-bit integers, as of LLVM 20 `rustc` will emit valid debuginfo for both DWARF and PDB (PDB makes use of the same natvis that is also used for all enums with fields, whereas DWARF has native support).

Tests for `#[repr(u128)]`/`#[repr(i128)]` enums include:
- [ui/enum-discriminant/repr128.rs](https://github.com/rust-lang/rust/blob/385970f0c1fd0c09bac426b02f38300c0b1ba9a2/tests/ui/enum-discriminant/repr128.rs): checks that 128-bit enum discriminants have the correct values.
- [debuginfo/msvc-pretty-enums.rs](https://github.com/rust-lang/rust/blob/385970f0c1fd0c09bac426b02f38300c0b1ba9a2/tests/debuginfo/msvc-pretty-enums.rs): checks the PDB debuginfo is correct.
- [run-make/repr128-dwarf](https://github.com/rust-lang/rust/blob/385970f0c1fd0c09bac426b02f38300c0b1ba9a2/tests/run-make/repr128-dwarf/rmake.rs): checks the DWARF debuginfo is correct.

Stabilising this feature does not require any changes to the Rust Reference as [the documentation on primitive representations](https://doc.rust-lang.org/nightly/reference/type-layout.html#r-layout.repr.primitive.intro) already includes `u128` and `i128`.

Closes #56071
Closes https://github.com/rust-lang/reference/issues/1368

r? lang

```@rustbot``` label +I-lang-nominated +T-lang

4 files changed, 6 insertions, 25 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0658.md b/compiler/rustc_error_codes/src/error_codes/E0658.md
index 24245a38ae0..65c82e4fb6e 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0658.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0658.md
@@ -3,10 +3,7 @@ An unstable feature was used.
 Erroneous code example:
 
 ```compile_fail,E0658
-#[repr(u128)] // error: use of unstable library feature 'repr128'
-enum Foo {
-    Bar(u64),
-}
+use std::intrinsics; // error: use of unstable library feature `core_intrinsics`
 ```
 
 If you're using a stable or a beta version of rustc, you won't be able to use
@@ -17,12 +14,9 @@ If you're using a nightly version of rustc, just add the corresponding feature
 to be able to use it:
 
 ```
-#![feature(repr128)]
+#![feature(core_intrinsics)]
 
-#[repr(u128)] // ok!
-enum Foo {
-    Bar(u64),
-}
+use std::intrinsics; // ok!
 ```
 
 [rustup]: https://rust-lang.github.io/rustup/concepts/channels.html
diff --git a/compiler/rustc_feature/src/accepted.rs b/compiler/rustc_feature/src/accepted.rs
index 820af9ac84b..ffa6ffb40b6 100644
--- a/compiler/rustc_feature/src/accepted.rs
+++ b/compiler/rustc_feature/src/accepted.rs
@@ -360,6 +360,8 @@ declare_features! (
     (accepted, relaxed_adts, "1.19.0", Some(35626)),
     /// Lessens the requirements for structs to implement `Unsize`.
     (accepted, relaxed_struct_unsize, "1.58.0", Some(81793)),
+    /// Allows the `#[repr(i128)]` attribute for enums.
+    (accepted, repr128, "CURRENT_RUSTC_VERSION", Some(56071)),
     /// Allows `repr(align(16))` struct attribute (RFC 1358).
     (accepted, repr_align, "1.25.0", Some(33626)),
     /// Allows using `#[repr(align(X))]` on enums with equivalent semantics
diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index 3e408a03111..b46eac6d8a6 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/compiler/rustc_feature/src/unstable.rs
@@ -621,8 +621,6 @@ declare_features! (
     (incomplete, ref_pat_eat_one_layer_2024_structural, "1.81.0", Some(123076)),
     /// Allows using the `#[register_tool]` attribute.
     (unstable, register_tool, "1.41.0", Some(66079)),
-    /// Allows the `#[repr(i128)]` attribute for enums.
-    (incomplete, repr128, "1.16.0", Some(56071)),
     /// Allows `repr(simd)` and importing the various simd intrinsics.
     (unstable, repr_simd, "1.4.0", Some(27731)),
     /// Allows bounding the return type of AFIT/RPITIT.
diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index db7a5fe7897..846eacce9e1 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -18,7 +18,7 @@ use rustc_middle::middle::resolve_bound_vars::ResolvedArg;
 use rustc_middle::middle::stability::EvalResult;
 use rustc_middle::ty::error::TypeErrorToStringExt;
 use rustc_middle::ty::layout::{LayoutError, MAX_SIMD_LANES};
-use rustc_middle::ty::util::{Discr, IntTypeExt};
+use rustc_middle::ty::util::Discr;
 use rustc_middle::ty::{
     AdtDef, BottomUpFolder, GenericArgKind, RegionKind, TypeFoldable, TypeSuperVisitable,
     TypeVisitable, TypeVisitableExt, fold_regions,
@@ -1385,19 +1385,6 @@ fn check_enum(tcx: TyCtxt<'_>, def_id: LocalDefId) {
         );
     }
 
-    let repr_type_ty = def.repr().discr_type().to_ty(tcx);
-    if repr_type_ty == tcx.types.i128 || repr_type_ty == tcx.types.u128 {
-        if !tcx.features().repr128() {
-            feature_err(
-                &tcx.sess,
-                sym::repr128,
-                tcx.def_span(def_id),
-                "repr with 128-bit type is unstable",
-            )
-            .emit();
-        }
-    }
-
     for v in def.variants() {
         if let ty::VariantDiscr::Explicit(discr_def_id) = v.discr {
             tcx.ensure_ok().typeck(discr_def_id.expect_local());