checker,json2: relax checking of x.enum = integer at comptime; refactor json2 to clean it up (#16926)

vlib/v/checker/assign.v

@@ -593,7 +593,16 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 							node.pos)
 					}
 				} else {
-					c.error('cannot assign to `${left}`: ${err.msg()}', right.pos())
+					// allow `t.$(field.name) = 0` where `t.$(field.name)` is a enum
+					if c.inside_comptime_for_field && left is ast.ComptimeSelector {
+						field_sym := c.table.sym(c.unwrap_generic(c.comptime_fields_default_type))
+
+						if field_sym.kind == .enum_ && !right_type.is_int() {
+							c.error('enums can only be assigned `int` values', right.pos())
+						}
+					} else {
+						c.error('cannot assign to `${left}`: ${err.msg()}', right.pos())
+					}
 				}
 			}
 		}

vlib/v/checker/tests/assign_enum_at_comptime.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/assign_enum_at_comptime.vv:13:21: error: enums can only be assigned `int` values
+   11 |
+   12 |     $for field in TestStruct.fields {
+   13 |         t.$(field.name) = '1'
+      |                           ~~~
+   14 |     }
+   15 | }

vlib/v/checker/tests/assign_enum_at_comptime.vv

@@ -0,0 +1,15 @@
+enum TestEnum {
+	one = 1
+}
+
+struct TestStruct {
+	test TestEnum
+}
+
+fn main() {
+	mut t := TestStruct{}
+
+	$for field in TestStruct.fields {
+		t.$(field.name) = '1'
+	}
+}

vlib/v/tests/comptime_field_selector_test.v

@@ -5,6 +5,14 @@ mut:
 	name string
 }
 
+enum TestEnum {
+	one = 1
+}
+
+struct TestStruct {
+	test TestEnum
+}
+
 fn comptime_field_selector_read[T]() []string {
 	mut t := T{}
 	t.name = '2'
@@ -42,6 +50,15 @@ fn test_comptime_field_selector_write() {
 	assert res.name == '1'
 }
 
+fn test_comptime_field_selector_write_enum() {
+	mut t := TestStruct{}
+
+	$for field in TestStruct.fields {
+		t.$(field.name) = 1
+	}
+	assert t.test == .one
+}
+
 struct Foo2 {
 	f Foo
 }