Rollup merge of #129422 - compiler-errors:repr-rust, r=fmease

Gate `repr(Rust)` correctly on non-ADT items

#114201 added `repr(Rust)` but didn't add any attribute validation to it like `repr(C)` has, to only allow it on ADT items.

I consider this code to be nonsense, for example:
```
#[repr(Rust)]
fn foo() {}
```

Reminder that it's different from `extern "Rust"`, which *is* valid on function items. But also this now disallows `repr(Rust)` on modules, impls, traits, etc.

I'll crater it, if it looks bad then I'll add an FCW.

---

https://github.com/rust-lang/rust/labels/relnotes: Compatibility (minor breaking change).

compiler/rustc_passes/src/check_attr.rs

@@ -1779,6 +1779,15 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             match hint.name_or_empty() {
                 sym::Rust => {
                     is_explicit_rust = true;
+                    match target {
+                        Target::Struct | Target::Union | Target::Enum => continue,
+                        _ => {
+                            self.dcx().emit_err(errors::AttrApplication::StructEnumUnion {
+                                hint_span: hint.span(),
+                                span,
+                            });
+                        }
+                    }
                 }
                 sym::C => {
                     is_c = true;