cbe: support rendering union values that have no defined tag type

This was regressed in d657b6c, when
the comptime memory model for unions was changed to allow them to have
no defined tag type.

Author: kcbanner

src/codegen/c.zig

@@ -1499,56 +1499,84 @@ pub const DeclGen = struct {
                 else => unreachable,
             },
             .un => |un| {
-                if (!location.isInitializer()) {
-                    try writer.writeByte('(');
-                    try dg.renderType(writer, ty);
-                    try writer.writeByte(')');
-                }
-
                 const union_obj = mod.typeToUnion(ty).?;
-                const field_i = mod.unionTagFieldIndex(union_obj, un.tag.toValue()).?;
-                const field_ty = union_obj.field_types.get(ip)[field_i].toType();
-                const field_name = union_obj.field_names.get(ip)[field_i];
-                if (union_obj.getLayout(ip) == .Packed) {
-                    if (field_ty.hasRuntimeBits(mod)) {
-                        if (field_ty.isPtrAtRuntime(mod)) {
-                            try writer.writeByte('(');
-                            try dg.renderType(writer, ty);
-                            try writer.writeByte(')');
-                        } else if (field_ty.zigTypeTag(mod) == .Float) {
-                            try writer.writeByte('(');
+                if (un.tag == .none) {
+                    const backing_ty = try ty.unionBackingType(mod);
+                    switch (union_obj.getLayout(ip)) {
+                        .Packed => {
+                            if (!location.isInitializer()) {
+                                try writer.writeByte('(');
+                                try dg.renderType(writer, backing_ty);
+                                try writer.writeByte(')');
+                            }
+                            try dg.renderValue(writer, backing_ty, un.val.toValue(), initializer_type);
+                        },
+                        .Extern => {
+                            if (location == .StaticInitializer) {
+                                return dg.fail("TODO: C backend: implement extern union backing type rendering in static initializers", .{});
+                            }
+
+                            try writer.writeAll("*((");
                             try dg.renderType(writer, ty);
-                            try writer.writeByte(')');
+                            try writer.writeAll("*)(");
+                            try dg.renderType(writer, backing_ty);
+                            try writer.writeAll("){");
+                            try dg.renderValue(writer, backing_ty, un.val.toValue(), initializer_type);
+                            try writer.writeAll("})");
+                        },
+                        else => unreachable,
+                    }
+                } else {
+                    if (!location.isInitializer()) {
+                        try writer.writeByte('(');
+                        try dg.renderType(writer, ty);
+                        try writer.writeByte(')');
+                    }
+
+                    const field_i = mod.unionTagFieldIndex(union_obj, un.tag.toValue()).?;
+                    const field_ty = union_obj.field_types.get(ip)[field_i].toType();
+                    const field_name = union_obj.field_names.get(ip)[field_i];
+                    if (union_obj.getLayout(ip) == .Packed) {
+                        if (field_ty.hasRuntimeBits(mod)) {
+                            if (field_ty.isPtrAtRuntime(mod)) {
+                                try writer.writeByte('(');
+                                try dg.renderType(writer, ty);
+                                try writer.writeByte(')');
+                            } else if (field_ty.zigTypeTag(mod) == .Float) {
+                                try writer.writeByte('(');
+                                try dg.renderType(writer, ty);
+                                try writer.writeByte(')');
+                            }
+                            try dg.renderValue(writer, field_ty, un.val.toValue(), initializer_type);
+                        } else {
+                            try writer.writeAll("0");
                         }
-                        try dg.renderValue(writer, field_ty, un.val.toValue(), initializer_type);
-                    } else {
-                        try writer.writeAll("0");
+                        return;
                     }
-                    return;
-                }
 
-                try writer.writeByte('{');
-                if (ty.unionTagTypeSafety(mod)) |tag_ty| {
-                    const layout = mod.getUnionLayout(union_obj);
-                    if (layout.tag_size != 0) {
-                        try writer.writeAll(" .tag = ");
-                        try dg.renderValue(writer, tag_ty, un.tag.toValue(), initializer_type);
+                    try writer.writeByte('{');
+                    if (ty.unionTagTypeSafety(mod)) |tag_ty| {
+                        const layout = mod.getUnionLayout(union_obj);
+                        if (layout.tag_size != 0) {
+                            try writer.writeAll(" .tag = ");
+                            try dg.renderValue(writer, tag_ty, un.tag.toValue(), initializer_type);
+                        }
+                        if (ty.unionHasAllZeroBitFieldTypes(mod)) return try writer.writeByte('}');
+                        if (layout.tag_size != 0) try writer.writeByte(',');
+                        try writer.writeAll(" .payload = {");
                     }
-                    if (ty.unionHasAllZeroBitFieldTypes(mod)) return try writer.writeByte('}');
-                    if (layout.tag_size != 0) try writer.writeByte(',');
-                    try writer.writeAll(" .payload = {");
-                }
-                if (field_ty.hasRuntimeBits(mod)) {
-                    try writer.print(" .{ } = ", .{fmtIdent(ip.stringToSlice(field_name))});
-                    try dg.renderValue(writer, field_ty, un.val.toValue(), initializer_type);
-                    try writer.writeByte(' ');
-                } else for (union_obj.field_types.get(ip)) |this_field_ty| {
-                    if (!this_field_ty.toType().hasRuntimeBits(mod)) continue;
-                    try dg.renderValue(writer, this_field_ty.toType(), Value.undef, initializer_type);
-                    break;
+                    if (field_ty.hasRuntimeBits(mod)) {
+                        try writer.print(" .{ } = ", .{fmtIdent(ip.stringToSlice(field_name))});
+                        try dg.renderValue(writer, field_ty, un.val.toValue(), initializer_type);
+                        try writer.writeByte(' ');
+                    } else for (union_obj.field_types.get(ip)) |this_field_ty| {
+                        if (!this_field_ty.toType().hasRuntimeBits(mod)) continue;
+                        try dg.renderValue(writer, this_field_ty.toType(), Value.undef, initializer_type);
+                        break;
+                    }
+                    if (ty.unionTagTypeSafety(mod)) |_| try writer.writeByte('}');
+                    try writer.writeByte('}');
                 }
-                if (ty.unionTagTypeSafety(mod)) |_| try writer.writeByte('}');
-                try writer.writeByte('}');
             },
         }
     }