Rollup merge of rust-lang#151046 - semiopaque-eii-fix, r=jdonszelmann

compiler: Make Externally Implementable Item (eii) macros "semiopaque"

Otherwise eiis defined by std will produce large amounts of `missing stability attribute` errors. This problem is not eii specific, as can be seen in rust-lang#146993 and which is demonstrated in rust-lang#151022.

As can be seen with

```console
$ git grep rustc_macro_transparency
compiler/rustc_arena/src/lib.rs:#[rustc_macro_transparency = "semiopaque"]
[...]
```

it is very common for macros to use `"semiopaque"`.

r? @jdonszelmann

Tracking issue: rust-lang#125418

Needed for: rust-lang#150588

Author: JonathanBrouwer

compiler/rustc_builtin_macros/src/eii.rs

@@ -335,6 +335,9 @@ fn generate_attribute_macro_to_implement(
     // errors for eii's in std.
     macro_attrs.extend_from_slice(attrs_from_decl);
 
+    // Avoid "missing stability attribute" errors for eiis in std. See #146993.
+    macro_attrs.push(ecx.attr_name_value_str(sym::rustc_macro_transparency, sym::semiopaque, span));
+
     // #[builtin_macro(eii_shared_macro)]
     macro_attrs.push(ecx.attr_nested_word(sym::rustc_builtin_macro, sym::eii_shared_macro, span));