Allow omitting "with" keyword in match blocks

Askama uses the syntax `{% when Variant with (parameters) %}` in
`{% match %}` blocks. This is done because Askama does not implement the
whole pattern matching of Rust's `match` statements.

This PR wants to bring Askama a step closer Rust's matching, so the
"with" keyword should not be needed anymore.

askama_shared/src/parser.rs

@@ -550,7 +550,7 @@ fn parameters(i: &[u8]) -> IResult<&[u8], Vec<&str>> {
 
 fn with_parameters(i: &[u8]) -> IResult<&[u8], MatchParameters<'_>> {
     let (i, (_, value)) = tuple((
-        tag("with"),
+        opt(tag("with")),
         alt((match_simple_parameters, match_named_parameters)),
     ))(i)?;
     Ok((i, value))

testing/tests/matches.rs

@@ -113,3 +113,20 @@ fn test_match_no_whitespace() {
     let s = MatchNoWhitespace { foo: Some(1) };
     assert_eq!(s.render().unwrap(), "1");
 }
+
+#[derive(Template)]
+#[template(
+    source = "{% match foo %}{% when Some(bar) %}{{ bar }}{% when None %}{% endmatch %}",
+    ext = "txt"
+)]
+struct MatchWithoutWithKeyword {
+    foo: Option<usize>,
+}
+
+#[test]
+fn test_match_without_with_keyword() {
+    let s = MatchWithoutWithKeyword { foo: Some(1) };
+    assert_eq!(s.render().unwrap(), "1");
+    let s = MatchWithoutWithKeyword { foo: None };
+    assert_eq!(s.render().unwrap(), "");
+}