Auto merge of #118032 - RalfJung:char-u32, r=Mark-Simulacrum

guarantee that char and u32 are ABI-compatible

In https://github.com/rust-lang/rust/pull/116894 we added a guarantee that `char` has the same alignment as `u32`, but there is still one axis where these types could differ: function call ABI. So let's nail that down as well: in a function signature, `char` and `u32` are completely equivalent.

This is a new stable guarantee, so it will need t-lang approval.

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -384,10 +384,12 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
 
         // Compatible integer types (in particular, usize vs ptr-sized-u32/u64).
+        // `char` counts as `u32.`
         let int_ty = |ty: Ty<'tcx>| {
             Some(match ty.kind() {
                 ty::Int(ity) => (Integer::from_int_ty(&self.tcx, *ity), /* signed */ true),
                 ty::Uint(uty) => (Integer::from_uint_ty(&self.tcx, *uty), /* signed */ false),
+                ty::Char => (Integer::I32, /* signed */ false),
                 _ => return None,
             })
         };