mangling_v0: Skip extern blocks during mangling

compiler/rustc_symbol_mangling/src/v0.rs

@@ -771,9 +771,9 @@ impl<'tcx> Printer<'tcx> for &mut SymbolMangler<'tcx> {
         disambiguated_data: &DisambiguatedDefPathData,
     ) -> Result<Self::Path, Self::Error> {
         let ns = match disambiguated_data.data {
-            // FIXME: It shouldn't be necessary to add anything for extern block segments,
-            // but we add 't' for backward compatibility.
-            DefPathData::ForeignMod => 't',
+            // Extern block segments can be skipped, names from extern blocks
+            // are effectively living in their parent modules.
+            DefPathData::ForeignMod => return print_prefix(self),
 
             // Uppercase categories are more stable than lowercase ones.
             DefPathData::TypeNs(_) => 't',

src/test/ui/symbol-names/foreign-types.rs

@@ -1,5 +1,5 @@
 // build-fail
-// compile-flags: -Z symbol-mangling-version=v0
+// compile-flags: -C symbol-mangling-version=v0
 
 #![feature(extern_types)]
 #![feature(rustc_attrs)]

src/test/ui/symbol-names/foreign-types.stderr

@@ -1,4 +1,4 @@
-error: symbol-name(_RMCsCRATE_HASH_13foreign_typesINtB<REF>_5CheckNvNtB<REF>_011ForeignTypeE)
+error: symbol-name(_RMCsCRATE_HASH_13foreign_typesINtB<REF>_5CheckNvB<REF>_11ForeignTypeE)
   --> $DIR/foreign-types.rs:13:1
    |
 LL | #[rustc_symbol_name]