Support single-expression closures in templates

[jannik4]

Fixes #284 (and #398?)

Example

use askama::Template;

#[derive(Debug, Clone)]
struct User { name: String }

impl User {
    fn ferris() -> Self { Self { name: "Ferris".to_string() } }
}

#[derive(Template)]
#[template(
    source = r#"Hello {{ user_opt.map(|user| user.name.as_str()).unwrap_or("World") }}"#,
    ext = "txt"
)]
struct ClosureTemplate<'a> {
    user_opt: Option<&'a User>,
}

#[test]
fn test_closure() {
    let user = User::ferris();
    let t = ClosureTemplate { user_opt: Some(&user) };
    assert_eq!(t.render().unwrap(), "Hello Ferris");

    let t = ClosureTemplate { user_opt: None };
    assert_eq!(t.render().unwrap(), "Hello World");
}

Drawbacks

I think the only inconvenience of the current implementation is that Copy types can cause problems in combination with the automatic borrowing, see below. This should always be fixable by a clone, in this case data.flag.clone(). (Edit: Or by using copied on the Option as mentioned by @vallentin)

The alternative would be to not automatically borrow the closure expression. But then non Copy types would not work anymore.

struct Data {
    flag: bool,
}

#[derive(Template)]
#[template(
    source = r#"{{ data_opt.map(|data| data.flag).unwrap_or(false) }}"#,
    ext = "txt"
)]
struct DataTemplate {
    data_opt: Option<Data>,
}

// error[E0308]: mismatched types
//   --> testing\tests\closures.rs:85:10
//    |
// 85 | #[derive(Template)]
//    |          ^^^^^^^^ expected `&bool`, found `bool`
//    |

[vallentin]

I understand why FALSE is needed, but it's still possible to do the following, right?

Suggested change

	source = r#"{{ user_opt.map(|user| user.flag).unwrap_or(FALSE) }}"#,
	source = r#"{{ user_opt.map(|user| user.flag).copied().unwrap_or(false) }}"#,

[jannik4]

Yes, that should work too. I used FALSE to statically ensure that there really is a reference. But since copied only works on references, that's probably better here.

[Kijewski]

Please have a look at jannik4#1. You don't need to add explicit lifetimes in all these places.