cgen: add comptime field.name checking (#17318)

vlib/v/gen/c/comptime.v

@@ -536,11 +536,13 @@ fn (mut g Gen) comptime_if_cond(cond ast.Expr, pkg_exist bool) (bool, bool) {
 				}
 				.eq, .ne {
 					// TODO Implement `$if method.args.len == 1`
-					if cond.left is ast.SelectorExpr && g.comptime_for_method.len > 0
+					if cond.left is ast.SelectorExpr
+						&& (g.comptime_for_field_var.len > 0 || g.comptime_for_method.len > 0)
 						&& cond.right is ast.StringLiteral {
 						selector := cond.left as ast.SelectorExpr
-						if selector.expr is ast.Ident
+						if selector.expr is ast.Ident && selector.field_name == 'name' {
-							&& (selector.expr as ast.Ident).name == g.comptime_for_method_var && selector.field_name == 'name' {
+							if g.comptime_for_method_var.len > 0
+								&& (selector.expr as ast.Ident).name == g.comptime_for_method_var {
 								is_equal := g.comptime_for_method == cond.right.val
 								if is_equal {
 									g.write('1')
@@ -548,6 +550,16 @@ fn (mut g Gen) comptime_if_cond(cond ast.Expr, pkg_exist bool) (bool, bool) {
 									g.write('0')
 								}
 								return is_equal, true
+							} else if g.comptime_for_field_var.len > 0
+								&& (selector.expr as ast.Ident).name == g.comptime_for_field_var {
+								is_equal := g.comptime_for_field_value.name == cond.right.val
+								if is_equal {
+									g.write('1')
+								} else {
+									g.write('0')
+								}
+								return is_equal, true
+							}
 						}
 					}
 					if cond.left is ast.SelectorExpr || cond.right is ast.SelectorExpr {

vlib/v/tests/comptime_name_check_test.v

@@ -0,0 +1,30 @@
+struct User {
+	name string
+	age  int
+}
+
+fn (u User) a() {}
+
+fn (u User) b() {}
+
+fn (u User) c() {}
+
+fn test_method_name() {
+	mut out := []string{}
+	$for field in User.methods {
+		$if field.name == 'b' {
+			out << 'ok'
+		}
+	}
+	assert out.len == 1
+}
+
+fn test_field_name() {
+	mut out := []string{}
+	$for field in User.fields {
+		$if field.name == 'name' {
+			out << 'ok'
+		}
+	}
+	assert out.len == 1
+}