v.c.gen: fix codegen of auto str methods for structs having &voidptr fields

vlib/v/ast/types.v

@@ -411,8 +411,18 @@ pub const (
 	error_type         = new_type(error_type_idx)
 	charptr_types      = [charptr_type, new_type(char_type_idx).set_nr_muls(1)]
 	byteptr_types      = [byteptr_type, new_type(byte_type_idx).set_nr_muls(1)]
+	voidptr_types      = [voidptr_type, new_type(voidptr_type_idx).set_nr_muls(1)]
+	cptr_types         = merge_types(voidptr_types, byteptr_types, charptr_types)
 )
 
+pub fn merge_types(params ...[]Type) []Type {
+	mut res := []Type{}
+	for types in params {
+		res << types
+	}
+	return res
+}
+
 pub const (
 	builtin_type_names = ['void', 'voidptr', 'charptr', 'byteptr', 'i8', 'i16', 'int', 'i64', 'u16',
 		'u32', 'u64', 'int_literal', 'f32', 'f64', 'float_literal', 'string', 'ustring', 'char',

vlib/v/gen/c/auto_str_methods.v

@@ -728,12 +728,12 @@ fn (g &Gen) type_to_fmt1(typ ast.Type) StrIntpType {
 	if typ == ast.char_type_idx {
 		return .si_c
 	}
-	if typ == ast.voidptr_type_idx || typ in ast.byteptr_types {
+	if typ in ast.voidptr_types || typ in ast.byteptr_types {
 		return .si_p
 	}
 	if typ in ast.charptr_types {
-		return .si_s
 		// return '%C\\000' // a C string
+		return .si_s
 	}
 	sym := g.table.get_type_symbol(typ)
 	if typ.is_ptr() && (typ.is_int_valptr() || typ.is_float_valptr()) {
@@ -831,10 +831,10 @@ fn (mut g Gen) gen_str_for_struct(info ast.Struct, styp string, str_fn_name stri
 		}
 
 		mut func := struct_auto_str_func1(sym, field.typ, field_styp_fn_name, field.name)
-
+		if field.typ in ast.cptr_types {
-		// manage reference types can be "nil"
+			func = '(voidptr) it.$field.name'
-		if field.typ.is_ptr() && !(field.typ in ast.charptr_types
+		} else if field.typ.is_ptr() {
-			|| field.typ in ast.byteptr_types || field.typ == ast.voidptr_type_idx) {
+			// reference types can be "nil"
 			fn_builder.write_string('isnil(it.${c_name(field.name)})')
 			fn_builder.write_string(' ? _SLIT("nil") : ')
 			// struct, floats and ints have a special case through the _str function

vlib/v/tests/interface_edge_cases/empty_interface_1_test.v

@@ -0,0 +1,16 @@
+interface IEmpty {}
+
+struct Abc {
+	x string
+}
+
+fn test_empty_interface_string_interpolation() {
+	a := IEmpty(u64(1))
+	b := IEmpty(f32(1))
+	c := IEmpty(Abc{'abc'})
+	assert '$a' == 'IEmpty(1)'
+	assert '$b' == 'IEmpty(1.)'
+	assert '$c'.starts_with('IEmpty(Abc{')
+	assert '$c'.contains("x: 'abc'")
+	assert '$c'.ends_with('})')
+}

vlib/v/tests/interface_edge_cases/empty_interface_println_test.v

@@ -0,0 +1,11 @@
+interface IEmpty {
+}
+
+struct Abc {
+	abc int
+}
+
+fn test_an_empty_interface_can_be_printed() {
+	println(IEmpty(Abc{123}))
+	assert true
+}

vlib/v/tests/structs_with_voidptr_fields_can_be_printed_test.v

@@ -0,0 +1,13 @@
+struct Abc {
+mut:
+	vptr    voidptr
+	vptrptr &voidptr = 0
+	bptr    &byte    = 0
+	cptr    &char    = c'abcdef'
+}
+
+fn test_structs_can_be_printed() {
+	a := Abc{}
+	println(a)
+	assert true
+}