Sema: fix @extern decls

Closes #18550

Author: jacobly0

src/Sema.zig

@@ -25675,6 +25675,7 @@ fn zirBuiltinExtern(
     extended: Zir.Inst.Extended.InstData,
 ) CompileError!Air.Inst.Ref {
     const mod = sema.mod;
+    const ip = &mod.intern_pool;
     const extra = sema.code.extraData(Zir.Inst.BinNode, extended.operand).data;
     const ty_src: LazySrcLoc = .{ .node_offset_builtin_call_arg0 = extra.node };
     const options_src: LazySrcLoc = .{ .node_offset_builtin_call_arg1 = extra.node };
@@ -25714,34 +25715,38 @@ fn zirBuiltinExtern(
     const new_decl = mod.declPtr(new_decl_index);
     new_decl.name = options.name;
 
-    {
-        const new_var = try mod.intern(.{ .variable = .{
-            .ty = ptr_info.child,
-            .init = .none,
-            .decl = sema.owner_decl_index,
-            .lib_name = options.library_name,
-            .is_extern = true,
-            .is_const = ptr_info.flags.is_const,
-            .is_threadlocal = options.is_thread_local,
-            .is_weak_linkage = options.linkage == .Weak,
-        } });
-
-        new_decl.src_line = sema.owner_decl.src_line;
-        // We only access this decl through the decl_ref with the correct type created
-        // below, so this type doesn't matter
-        new_decl.ty = Type.fromInterned(ptr_info.child);
-        new_decl.val = Value.fromInterned(new_var);
-        new_decl.alignment = .none;
-        new_decl.@"linksection" = .none;
-        new_decl.has_tv = true;
-        new_decl.analysis = .complete;
-        new_decl.generation = mod.generation;
-    }
+    new_decl.src_line = sema.owner_decl.src_line;
+    new_decl.ty = Type.fromInterned(ptr_info.child);
+    new_decl.val = Value.fromInterned(
+        if (Type.fromInterned(ptr_info.child).zigTypeTag(mod) == .Fn)
+            try ip.getExternFunc(sema.gpa, .{
+                .ty = ptr_info.child,
+                .decl = new_decl_index,
+                .lib_name = options.library_name,
+            })
+        else
+            try mod.intern(.{ .variable = .{
+                .ty = ptr_info.child,
+                .init = .none,
+                .decl = new_decl_index,
+                .lib_name = options.library_name,
+                .is_extern = true,
+                .is_const = ptr_info.flags.is_const,
+                .is_threadlocal = options.is_thread_local,
+                .is_weak_linkage = options.linkage == .Weak,
+            } }),
+    );
+    new_decl.alignment = .none;
+    new_decl.@"linksection" = .none;
+    new_decl.has_tv = true;
+    new_decl.owns_tv = true;
+    new_decl.analysis = .complete;
+    new_decl.generation = mod.generation;
 
     try sema.ensureDeclAnalyzed(new_decl_index);
 
     return Air.internedToRef((try mod.getCoerced(Value.fromInterned((try mod.intern(.{ .ptr = .{
-        .ty = switch (mod.intern_pool.indexToKey(ty.toIntern())) {
+        .ty = switch (ip.indexToKey(ty.toIntern())) {
             .ptr_type => ty.toIntern(),
             .opt_type => |child_type| child_type,
             else => unreachable,

src/codegen/llvm.zig

@@ -2941,17 +2941,20 @@ pub const Object = struct {
         const target = owner_mod.resolved_target.result;
         const sret = firstParamSRet(fn_info, zcu);
 
+        const is_extern = decl.isExtern(zcu);
         const function_index = try o.builder.addFunction(
             try o.lowerType(zig_fn_type),
-            try o.builder.string(ip.stringToSlice(try decl.getFullyQualifiedName(zcu))),
+            try o.builder.string(ip.stringToSlice(if (is_extern)
+                decl.name
+            else
+                try decl.getFullyQualifiedName(zcu))),
             toLlvmAddressSpace(decl.@"addrspace", target),
         );
         gop.value_ptr.* = function_index.ptrConst(&o.builder).global;
 
         var attributes: Builder.FunctionAttributes.Wip = .{};
         defer attributes.deinit(&o.builder);
 
-        const is_extern = decl.isExtern(zcu);
         if (!is_extern) {
             function_index.setLinkage(.internal, &o.builder);
             function_index.setUnnamedAddr(.unnamed_addr, &o.builder);

test/behavior/extern.zig

@@ -12,3 +12,16 @@ test "anyopaque extern symbol" {
 }
 
 export var a_mystery_symbol: i32 = 1234;
+
+test "function extern symbol" {
+    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_x86_64 and builtin.target.ofmt != .elf) return error.SkipZigTest;
+
+    const a = @extern(*const fn () callconv(.C) i32, .{ .name = "a_mystery_function" });
+    try expect(a() == 4567);
+}
+
+export fn a_mystery_function() i32 {
+    return 4567;
+}