cgen: fix struct ref field with no ref structinit (#21932)

vlib/net/http/download_progress.v

@@ -23,7 +23,7 @@ mut:
 pub struct DownloaderParams {
 	FetchConfig
 pub mut:
-	downloader &Downloader = TerminalStreamingDownloader{}
+	downloader &Downloader = &TerminalStreamingDownloader{}
 }
 
 // download_file_with_progress will save the URL `url` to the filepath `path` .

vlib/v/checker/struct.v

@@ -111,6 +111,11 @@ fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 			if field.has_default_expr {
 				c.expected_type = field.typ
 				field.default_expr_typ = c.expr(mut field.default_expr)
+				if field.typ.is_ptr() && !field.default_expr_typ.is_ptr()
+					&& field.default_expr is ast.StructInit {
+					c.error('reference field must be initialized with reference', field.default_expr.pos())
+				}
+
 				// disallow map `mut a = b`
 				field_sym := c.table.sym(field.typ)
 				expr_sym := c.table.sym(field.default_expr_typ)

vlib/v/checker/tests/struct_ref_field_no_ptr_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/struct_ref_field_no_ptr_err.vv:2:13: error: reference field must be initialized with reference
+    1 | struct Foo {
+    2 |     bar &Bar = Bar{}
+      |                ~~~~~
+    3 | }
+    4 |

vlib/v/checker/tests/struct_ref_field_no_ptr_err.vv

@@ -0,0 +1,7 @@
+struct Foo {
+	bar &Bar = Bar{}
+}
+
+struct Bar {
+	field int
+}