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

Otherwise eiis defined by std will produce large amounts of `missing
stability attribute` errors.

Author: Enselic

compiler/rustc_builtin_macros/src/eii.rs

@@ -425,6 +425,9 @@ 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));