cgen: add interface case for gen_struct_equality_fn (#12094)

vlib/v/gen/c/auto_eq_methods.v

@@ -155,6 +155,9 @@ fn (mut g Gen) gen_struct_equality_fn(left_type ast.Type) string {
 				fn_builder.write_string('${eq_fn}_alias_eq(a.$field_name, b.$field_name)')
 			} else if field_type.sym.kind == .function {
 				fn_builder.write_string('*((voidptr*)(a.$field_name)) == *((voidptr*)(b.$field_name))')
+			} else if field_type.sym.kind == .interface_ {
+				eq_fn := g.gen_interface_equality_fn(field.typ)
+				fn_builder.write_string('${eq_fn}_interface_eq(a.$field_name, b.$field_name)')
 			} else {
 				fn_builder.write_string('a.$field_name == b.$field_name')
 			}

vlib/v/tests/struct_auto_eq_gen_interface_test.v

@@ -0,0 +1,23 @@
+interface IExample {
+	thing() bool
+}
+
+struct Foo {}
+
+fn (n Foo) thing() bool {
+	return true
+}
+
+struct Test {
+	a IExample
+}
+
+fn new() Test {
+	return Test{Foo{}}
+}
+
+fn test_struct_auto_eq_gen_interface_case() {
+	w1 := new()
+	w2 := new()
+	assert w1 == w2
+}