parser: disallow using attributes on embedded structs (#15901)

vlib/v/parser/struct.v

@@ -224,6 +224,9 @@ fn (mut p Parser) struct_decl(is_anon bool) ast.StructDecl {
 					p.error_with_pos('duplicate field `$field_name`', type_pos)
 					return ast.StructDecl{}
 				}
+				if p.tok.kind == .lsbr {
+					p.error('cannot use attributes on embedded structs')
+				}
 				embed_field_names << field_name
 				embed_types << typ
 				embeds << ast.Embed{

vlib/v/parser/tests/embeded_struct_attribute_err.out

@@ -0,0 +1,7 @@
+vlib/v/parser/tests/embeded_struct_attribute_err.vv:8:7: error: cannot use attributes on embedded structs
+    6 |
+    7 | struct Button {
+    8 |     Size [json: size] // fmt removes [json: size]
+      |          ^
+    9 | }
+   10 |

vlib/v/parser/tests/embeded_struct_attribute_err.vv

@@ -0,0 +1,14 @@
+import json
+
+struct Size {
+	width int
+}
+
+struct Button {
+	Size [json: size] // fmt removes [json: size]
+}
+
+fn main() {
+	a := Button{}
+	println(json.encode(a)) // {"size":{"width":0}}
+}