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;

tests/ui/feature-gates/issue-43106-gating-of-builtin-attrs-error.rs

@@ -164,4 +164,28 @@ mod repr {
     //~| NOTE not a struct, enum, or union
 }
 
+
+#[repr(Rust)]
+//~^ ERROR: attribute should be applied to a struct, enum, or union
+mod repr_rust {
+//~^ NOTE not a struct, enum, or union
+    mod inner { #![repr(Rust)] }
+    //~^ ERROR: attribute should be applied to a struct, enum, or union
+    //~| NOTE not a struct, enum, or union
+
+    #[repr(Rust)] fn f() { }
+    //~^ ERROR: attribute should be applied to a struct, enum, or union
+    //~| NOTE not a struct, enum, or union
+
+    struct S;
+
+    #[repr(Rust)] type T = S;
+    //~^ ERROR: attribute should be applied to a struct, enum, or union
+    //~| NOTE not a struct, enum, or union
+
+    #[repr(Rust)] impl S { }
+    //~^ ERROR: attribute should be applied to a struct, enum, or union
+    //~| NOTE not a struct, enum, or union
+}
+
 fn main() {}

tests/ui/feature-gates/issue-43106-gating-of-builtin-attrs-error.stderr

@@ -107,6 +107,21 @@ LL | |
 LL | | }
    | |_- not a struct, enum, or union
 
+error[E0517]: attribute should be applied to a struct, enum, or union
+  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:168:8
+   |
+LL |   #[repr(Rust)]
+   |          ^^^^
+LL |
+LL | / mod repr_rust {
+LL | |
+LL | |     mod inner { #![repr(Rust)] }
+LL | |
+...  |
+LL | |
+LL | | }
+   | |_- not a struct, enum, or union
+
 error: attribute should be applied to an `extern crate` item
   --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:26:1
    |
@@ -329,7 +344,31 @@ error[E0517]: attribute should be applied to a struct, enum, or union
 LL |     #[repr(C)] impl S { }
    |            ^   ---------- not a struct, enum, or union
 
-error: aborting due to 39 previous errors
+error[E0517]: attribute should be applied to a struct, enum, or union
+  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:172:25
+   |
+LL |     mod inner { #![repr(Rust)] }
+   |     --------------------^^^^---- not a struct, enum, or union
+
+error[E0517]: attribute should be applied to a struct, enum, or union
+  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:176:12
+   |
+LL |     #[repr(Rust)] fn f() { }
+   |            ^^^^   ---------- not a struct, enum, or union
+
+error[E0517]: attribute should be applied to a struct, enum, or union
+  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:182:12
+   |
+LL |     #[repr(Rust)] type T = S;
+   |            ^^^^   ----------- not a struct, enum, or union
+
+error[E0517]: attribute should be applied to a struct, enum, or union
+  --> $DIR/issue-43106-gating-of-builtin-attrs-error.rs:186:12
+   |
+LL |     #[repr(Rust)] impl S { }
+   |            ^^^^   ---------- not a struct, enum, or union
+
+error: aborting due to 44 previous errors
 
 Some errors have detailed explanations: E0517, E0518, E0658.
 For more information about an error, try `rustc --explain E0517`.