Fix codegen for add_static_aliases

Author: JonathanBrouwer

compiler/rustc_codegen_llvm/src/mono_item.rs

@@ -135,7 +135,8 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         let ty = self.get_type_of_global(aliasee);
 
         for (alias, linkage, visibility) in aliases {
-            let symbol_name = self.tcx.symbol_name(Instance::mono(self.tcx, *alias));
+            let instance = Instance::mono(self.tcx, *alias);
+            let symbol_name = self.tcx.symbol_name(instance);
             tracing::debug!("STATIC ALIAS: {alias:?} {linkage:?} {visibility:?}");
 
             let lldecl = llvm::add_alias(
@@ -145,6 +146,13 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
                 aliasee,
                 &CString::new(symbol_name.name).unwrap(),
             );
+            // Add the alias name to the set of cached items, so there is no duplicate
+            // instance added to it during the normal `external static` codegen
+            let prev_entry = self.instances.borrow_mut().insert(instance, lldecl);
+
+            // If there already was a previous entry, then `add_static_aliases` was called multiple times for the same `alias`
+            // which would result in incorrect codegen
+            assert!(prev_entry.is_none(), "An instance was already present for {instance:?}");
 
             llvm::set_visibility(lldecl, base::visibility_to_llvm(*visibility));
             llvm::set_linkage(lldecl, base::linkage_to_llvm(*linkage));