Macro assert_let 

macro_rules! assert_let {
    ( $pat:pat, $e:expr ) => { ... };
    ( $pat:pat, $e:expr, $($arg:tt)* ) => { ... };
}

Asserts that a pattern matches a given expression.

Generally speaking, assert_let(pattern, expr) is roughly equivalent to:

let pattern = expr else { panic!("some panic message with {} {}", pattern, expr)};