parser: make old [attr] syntax an error

cmd/tools/test_if_v_test_system_works.v

@@ -75,7 +75,7 @@ fn check_assert_continues_works() ! {
 	result.has('assert_continues_option_works_test.v:5: fn test_fail2')
 	result.has('> assert 2 == 4').has('> assert 2 == 1').has('> assert 2 == 0')
 	// Check if a test function, tagged with [assert_continues], has the same behaviour, without needing additional options
-	create_test('assert_continues_tag_works_test.v', '[assert_continues]fn test_fail1() { assert 2==4\nassert 2==1\nassert 2==0 }\nfn test_ok(){ assert true }\nfn test_fail2() { assert false\n assert false }')!
+	create_test('assert_continues_tag_works_test.v', '@[assert_continues]fn test_fail1() { assert 2==4\nassert 2==1\nassert 2==0 }\nfn test_ok(){ assert true }\nfn test_fail2() { assert false\n assert false }')!
 	tag_res := check_fail('${vexe} assert_continues_tag_works_test.v')
 	tag_res.has('assert_continues_tag_works_test.v:1: fn test_fail1')
 	tag_res.has('assert_continues_tag_works_test.v:2: fn test_fail1')

vlib/v/parser/parser.v

@@ -1937,12 +1937,15 @@ fn (mut p Parser) is_attributes() bool {
 	return true
 }
 
-// when is_top_stmt is true attrs are added to p.attrs
+// when is_top_stmt is true, attrs are added to p.attrs
 fn (mut p Parser) attributes() {
 	start_pos := p.tok.pos()
 	mut is_at := false
 	if p.tok.kind == .lsbr {
-		p.warn('`[attr]` has been deprecated, use `@[attr]` instead')
+		if p.pref.is_fmt {
+		} else {
+			p.error('`[attr]` has been deprecated, use `@[attr]` instead')
+		}
 		// [attr]
 		p.check(.lsbr)
 	} else if p.tok.kind == .at {