parser: fix attributes with optional or result types (#16401)

vlib/v/parser/parse_type.v

@@ -381,15 +381,23 @@ pub fn (mut p Parser) parse_type() ast.Type {
 		p.next()
 		is_result = true
 	}
-	if (is_optional || is_result) && (p.tok.line_nr > line_nr || p.tok.kind in [.comma, .rpar]) {
-		mut typ := ast.void_type
-		if is_optional {
-			typ = typ.set_flag(.optional)
-		} else if is_result {
-			typ = typ.set_flag(.result)
+
+	if is_optional || is_result {
+		// maybe the '[' is the start of the field attribute
+		is_required_field := p.inside_struct_field_decl && p.tok.kind == .lsbr
+			&& p.peek_tok.kind == .name && p.peek_tok.lit == 'required'
+
+		if p.tok.line_nr > line_nr || p.tok.kind in [.comma, .rpar] || is_required_field {
+			mut typ := ast.void_type
+			if is_optional {
+				typ = typ.set_flag(.optional)
+			} else if is_result {
+				typ = typ.set_flag(.result)
+			}
+			return typ
 		}
-		return typ
 	}
+
 	is_shared := p.tok.kind == .key_shared
 	is_atomic := p.tok.kind == .key_atomic
 	if is_shared {

vlib/v/parser/tests/struct_field_required_fn_optional_type.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/struct_field_required_fn_optional_type.vv:6:7: error: field `Foo.foo` must be initialized
+    4 |
+    5 | fn main() {
+    6 |     f := Foo{}
+      |          ~~~~~
+    7 |     println(f)
+    8 | }

vlib/v/parser/tests/struct_field_required_fn_optional_type.vv

@@ -0,0 +1,8 @@
+struct Foo {
+	foo fn() ? [required]
+}
+
+fn main() {
+	f := Foo{}
+	println(f)
+}

vlib/v/parser/tests/struct_field_required_fn_result_type.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/struct_field_required_fn_result_type.vv:6:7: error: field `Foo.foo` must be initialized
+    4 |
+    5 | fn main() {
+    6 |     f := Foo{}
+      |          ~~~~~
+    7 |     println(f)
+    8 | }

vlib/v/parser/tests/struct_field_required_fn_result_type.vv

@@ -0,0 +1,8 @@
+struct Foo {
+	foo fn() ! [required]
+}
+
+fn main() {
+	f := Foo{}
+	println(f)
+}