Give global_asm mangled names

`global_asm!` themselves don't need symbol names; they currently only have
a symbol name during mono collecion to identify them to partitioning
algorithm.

However it will have shims generated under it which will need unique symbol
names. The name themselves ultimately doesn't matter, so they're generated
like other shim instances.

Author: nbdd0121

compiler/rustc_middle/src/mir/mono.rs

@@ -87,7 +87,7 @@ impl<'tcx> MonoItem<'tcx> {
             MonoItem::Fn(instance) => tcx.symbol_name(instance),
             MonoItem::Static(def_id) => tcx.symbol_name(Instance::mono(tcx, def_id)),
             MonoItem::GlobalAsm(item_id) => {
-                SymbolName::new(tcx, &format!("global_asm_{:?}", item_id.owner_id))
+                tcx.symbol_name(Instance::mono(tcx, item_id.owner_id.to_def_id()))
             }
         }
     }

compiler/rustc_symbol_mangling/src/legacy.rs

@@ -33,6 +33,11 @@ pub(super) fn mangle<'tcx>(
                 debug!(?instance_ty);
                 break;
             }
+            DefPathData::GlobalAsm => {
+                // `global_asm!` doesn't have a type.
+                instance_ty = tcx.types.unit;
+                break;
+            }
             _ => {
                 // if we're making a symbol for something, there ought
                 // to be a value or type-def or something in there

compiler/rustc_symbol_mangling/src/v0.rs

@@ -792,6 +792,16 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
             // are effectively living in their parent modules.
             DefPathData::ForeignMod => return print_prefix(self),
 
+            // Global asm are handled similar to shims.
+            DefPathData::GlobalAsm => {
+                return self.path_append_ns(
+                    print_prefix,
+                    'S',
+                    disambiguated_data.disambiguator as u64,
+                    "global_asm",
+                );
+            }
+
             // Uppercase categories are more stable than lowercase ones.
             DefPathData::TypeNs(_) => 't',
             DefPathData::ValueNs(_) => 'v',
@@ -803,7 +813,6 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
             // These should never show up as `path_append` arguments.
             DefPathData::CrateRoot
             | DefPathData::Use
-            | DefPathData::GlobalAsm
             | DefPathData::Impl
             | DefPathData::MacroNs(_)
             | DefPathData::LifetimeNs(_) => {