Document @ syntax in pattern matching

[andrew-3kb]

There is a really useful feature currently in the match expression:

match foo {
    ref mut a @ Something(_) => something_else(a),
    ...
}

if you don't know what it does it binds the entire thing matched to a variable, so in the above case something_else will be called with a mutable reference to foo if it meets the pattern Something(_).

The feature seems really useful but it's currently not documented anywhere, I guess someone should document it.

[huonw]

Thanks for the report.

You are correct that it is undocumented; in fact, this was already noted in #7173.