Macro assert_matches

macro_rules! assert_matches {
    ($expression:expr, $pattern:pat $(if $condition:expr)? $(,)?) => { ... };
    ($expression:expr, $pattern:pat $(if $condition:expr)? => $code:expr $(,)?) => { ... };
    ($expression:expr, $pattern:pat $(if $condition:expr)?, $($arg:tt)+) => { ... };
    ($expression:expr, $pattern:pat $(if $condition:expr)? => $code:expr, $($arg:tt)+) => { ... };
}

Attempts to be a replacement for assert!(matches!(...)) but with better error messages, and allowing further code on success.

Matches the assert_eq! syntax for error messages.

let x = Some(42);
assert_matches!(x, Some(_));

assert_matches!(x, Some(x) => { assert_eq!(x, 42); });

assert_matches!(x, None, "Expected None, got {:?}", x);

assert_matches!(x, Some(x) => { assert_eq!(x, 41); }, "Expected Some(41), got {:?}", x);

Alternatives

We can’t use the std assert_matches! macro, as it looks like it is doomed to be stuck unstabilized for the foreseeable future.

This takes some inspiration regarding the => {} syntax from the assert_matches crate but the code had bugs with trailing commas and its error messages weren’t ideal.