cgen: keep interface typeof independent of builtin string

vlib/v/gen/c/cgen.v

@@ -1388,11 +1388,11 @@ pub fn (mut g Gen) write_typeof_functions() {
 			}
 			already_generated_ifaces[sym.cname] = true
 			impl_types := inter_info.implementor_types(true)
-			g.definitions.writeln('${g.static_non_parallel}string v_typeof_interface_${sym.cname}(u32 sidx);')
+			g.definitions.writeln('${g.static_non_parallel}char * v_typeof_interface_${sym.cname}(u32 sidx);')
 			if g.pref.parallel_cc {
-				g.extern_out.writeln('extern string v_typeof_interface_${sym.cname}(u32 sidx);')
+				g.extern_out.writeln('extern char * v_typeof_interface_${sym.cname}(u32 sidx);')
 			}
-			g.writeln('${g.static_non_parallel}string v_typeof_interface_${sym.cname}(u32 sidx) {')
+			g.writeln('${g.static_non_parallel}char * v_typeof_interface_${sym.cname}(u32 sidx) {')
 			for t in impl_types {
 				sub_sym := g.table.sym(ast.mktyp(t))
 				if sub_sym.kind == .interface {
@@ -1405,9 +1405,9 @@ pub fn (mut g Gen) write_typeof_functions() {
 					&& sub_sym.idx !in g.table.used_features.used_syms {
 					continue
 				}
-				g.writeln('\tif (sidx == _${sym.cname}_${sub_sym.cname}_index) return _S("${util.strip_main_name(sub_sym.name)}");')
+				g.writeln('\tif (sidx == _${sym.cname}_${sub_sym.cname}_index) return "${util.strip_main_name(sub_sym.name)}";')
 			}
-			g.writeln2('\treturn _S("unknown ${util.strip_main_name(sym.name)}");', '}')
+			g.writeln2('\treturn "unknown ${util.strip_main_name(sym.name)}";', '}')
 			// Avoid duplicate symbol '_v_typeof_interface_idx_IError' when using -usecache
 			if g.pref.build_mode != .build_module {
 				interface_idx_static_prefix := if g.pref.is_o { 'static ' } else { '' }
@@ -11664,7 +11664,7 @@ return ${cast_shared_struct_str};
 					conversion_functions.writeln('\tif (x._typ == _${interface_name}_${variant_sym.cname}_index) return I_${variant_sym.cname}_to_Interface_${vsym.cname}(x._${variant_sym.cname});')
 				}
 			}
-			pmessage := 'builtin__string__plus(builtin__string__plus(_S("`as_cast`: cannot convert "), v_typeof_interface_${interface_name}(x._typ)), _S(" to ${util.strip_main_name(vsym.name)}"))'
+			pmessage := 'builtin__string__plus(builtin__string__plus(_S("`as_cast`: cannot convert "), builtin__tos3(v_typeof_interface_${interface_name}(x._typ))), _S(" to ${util.strip_main_name(vsym.name)}"))'
 			if g.pref.is_debug {
 				// TODO: actually return a valid position here
 				conversion_functions.write_string2('\tbuiltin__panic_debug(1, builtin__tos3("builtin.v"), builtin__tos3("builtin"), builtin__tos3("__as_cast"), ',

vlib/v/gen/c/fn.v

@@ -4194,7 +4194,8 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 						')', node)
 					return
 				} else if left_sym.kind == .interface {
-					g.conversion_function_call('v_typeof_interface_${typ_sym.cname}', '', node)
+					g.conversion_function_call('builtin__charptr_vstring_literal(v_typeof_interface_${typ_sym.cname}',
+						')', node)
 					return
 				}
 			}

vlib/v/gen/c/testdata/interface_type_name.c.must_have

@@ -0,0 +1 @@
+builtin__charptr_vstring_literal(v_typeof_interface_main__Thing(

vlib/v/gen/c/testdata/interface_type_name.vv

@@ -0,0 +1,8 @@
+interface Thing {}
+
+struct Item {}
+
+fn main() {
+	t := Thing(Item{})
+	println(t.type_name())
+}

vlib/v/gen/c/testdata/no_builtin_interface_typeof.c.must_have

@@ -0,0 +1,3 @@
+static char * v_typeof_interface_main__Thing(u32 sidx) {
+if (sidx == _main__Thing_main__Item_index) return "Item";
+return "unknown Thing";

vlib/v/gen/c/testdata/no_builtin_interface_typeof.vv

@@ -0,0 +1,9 @@
+// vtest vflags: -no-builtin
+interface Thing {}
+
+struct Item {}
+
+fn main() {
+	h := Thing(Item{})
+	_ = h
+}