Mark #[rustc_std_internal_symbol] as extern indicator

It currently implies #[no_mangle] which is alread an extern indicator,
but this will change in a future commit.

Author: bjorn3

compiler/rustc_middle/src/middle/codegen_fn_attrs.rs

@@ -166,6 +166,7 @@ impl CodegenFnAttrs {
     /// * `#[linkage]` is present
     pub fn contains_extern_indicator(&self) -> bool {
         self.flags.contains(CodegenFnAttrFlags::NO_MANGLE)
+            || self.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL)
             || self.export_name.is_some()
             || match self.linkage {
                 // These are private, so make sure we don't try to consider

compiler/rustc_passes/src/reachable.rs

@@ -184,9 +184,7 @@ impl<'tcx> ReachableContext<'tcx> {
                 CodegenFnAttrs::EMPTY
             };
             let is_extern = codegen_attrs.contains_extern_indicator();
-            let std_internal =
-                codegen_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL);
-            if is_extern || std_internal {
+            if is_extern {
                 self.reachable_symbols.insert(search_item);
             }
         } else {
@@ -426,7 +424,6 @@ fn has_custom_linkage(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
     }
     let codegen_attrs = tcx.codegen_fn_attrs(def_id);
     codegen_attrs.contains_extern_indicator()
-        || codegen_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_STD_INTERNAL_SYMBOL)
         // FIXME(nbdd0121): `#[used]` are marked as reachable here so it's picked up by
         // `linked_symbols` in cg_ssa. They won't be exported in binary or cdylib due to their
         // `SymbolExportLevel::Rust` export level but may end up being exported in dylibs.