Replace rust_macro test to work on nightly

The current rust_test uses `stringify!()`. The documentation gives us
the warning:

> Note that the expanded results of the input tokens may change in the
> future. You should be careful if you rely on the output.

In the current nightly rust the result was indeed changed, so the test
not fails.

This PR replaces the test with another macro, that does not depend on
`stringify!()`.

Closes issue #504.

testing/templates/rust-macro-args.html

@@ -1,3 +1,3 @@
-{{ call_a_or_b_on_tail!((a: compute_len, b: zero), call b: only the terminal rules care.) }}
-{{ call_a_or_b_on_tail!((a: compute_len, b: zero), call a: some ninety one!) }}
-{{ call_a_or_b_on_tail!((a: compute_len, b: zero), call a: some ninety "(\"()"nine!) }}
+{{ call_a_or_b_on_tail!((a: year, b: month, c: day), call a: 2021, "July", (0+2)) }}
+{{ call_a_or_b_on_tail!((a: year, b: month, c: day), call b: 2021, "July", (0+2)) }}
+{{ call_a_or_b_on_tail!((a: year, b: day, c: month), call b: 2021, "July", (0+2)) }}

testing/tests/rust_macro.rs

@@ -17,25 +17,29 @@ fn main() {
 }
 
 macro_rules! call_a_or_b_on_tail {
-    ((a: $a:expr, b: $b:expr), call a: $($tail:tt)*) => {
-        $a(stringify!($($tail)*))
+    ((a: $a:expr, b: $b:expr, c: $c:expr), call a: $($tail:expr),*) => {
+        ($a)($($tail),*)
     };
 
-    ((a: $a:expr, b: $b:expr), call b: $($tail:tt)*) => {
-        $b(stringify!($($tail)*))
+    ((a: $a:expr, b: $b:expr, c: $c:expr), call b: $($tail:expr),*) => {
+        ($b)($($tail),*)
     };
 
-    ($ab:tt, $_skip:tt $($tail:tt)*) => {
-        call_a_or_b_on_tail!($ab, $($tail)*)
+    ((a: $a:expr, b: $b:expr, c: $c:expr), call c: $($tail:expr),*) => {
+        ($c)($($tail),*)
     };
 }
 
-fn compute_len(s: &str) -> usize {
-    s.len()
+fn year(y: u16, _: &str, _: u8) -> u16 {
+    y
 }
 
-fn zero(_s: &str) -> usize {
-    0
+fn month(_: u16, m: &str, _: u8) -> &str {
+    m
+}
+
+fn day(_: u16, _: &str, d: u8) -> u8 {
+    d
 }
 
 #[derive(Template)]
@@ -45,5 +49,5 @@ struct RustMacrosArgTemplate {}
 #[test]
 fn args() {
     let template = RustMacrosArgTemplate {};
-    assert_eq!("0\n17\n25", template.render().unwrap());
+    assert_eq!("2021\nJuly\n2", template.render().unwrap());
 }