parser: check error for struct field type using 'mut' (#13847)

vlib/v/parser/parse_type.v

@@ -376,8 +376,12 @@ pub fn (mut p Parser) parse_type() ast.Type {
 		p.register_auto_import('sync')
 	}
 	mut nr_muls := 0
-	if p.inside_fn_return && p.tok.kind == .key_mut {
-		p.error_with_pos('cannot use `mut` on fn return type', p.tok.pos())
+	if p.tok.kind == .key_mut {
+		if p.inside_fn_return {
+			p.error_with_pos('cannot use `mut` on fn return type', p.tok.pos())
+		} else if p.inside_struct_field_decl {
+			p.error_with_pos('cannot use `mut` on struct field type', p.tok.pos())
+		}
 	}
 	if p.tok.kind == .key_mut || is_shared || is_atomic {
 		nr_muls++

vlib/v/parser/parser.v

@@ -54,8 +54,9 @@ mut:
 	inside_asm_template       bool
 	inside_asm                bool
 	inside_defer              bool
-	inside_generic_params     bool       // indicates if parsing between `<` and `>` of a method/function
-	inside_receiver_param     bool       // indicates if parsing the receiver parameter inside the first `(` and `)` of a method
+	inside_generic_params     bool // indicates if parsing between `<` and `>` of a method/function
+	inside_receiver_param     bool // indicates if parsing the receiver parameter inside the first `(` and `)` of a method
+	inside_struct_field_decl  bool
 	or_is_handled             bool       // ignore `or` in this expression
 	builtin_mod               bool       // are we in the `builtin` module?
 	mod                       string     // current module name

vlib/v/parser/struct.v

@@ -223,7 +223,9 @@ fn (mut p Parser) struct_decl() ast.StructDecl {
 						break
 					}
 				}
+				p.inside_struct_field_decl = true
 				typ = p.parse_type()
+				p.inside_struct_field_decl = false
 				if typ.idx() == 0 {
 					// error is set in parse_type
 					return ast.StructDecl{}

vlib/v/parser/tests/struct_field_mut_type_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/struct_field_mut_type_err.vv:3:6: error: cannot use `mut` on struct field type
+    1 | struct Foo {
+    2 | mut:
+    3 |     foo mut string
+      |         ~~~
+    4 | }
+    5 |

vlib/v/parser/tests/struct_field_mut_type_err.vv

@@ -0,0 +1,9 @@
+struct Foo {
+mut:
+	foo mut string
+}
+
+fn main() {
+	f := Foo{}
+	println(f)
+}