This follows through on the comments on the __TestExhaustive variants
promising that their purpose will be substituted with a deny(reachable)
rustc lint once one is available.

That lint is now landing as non_exhaustive_omitted_patterns in Rust 1.57.
The correct way to use it for checking exhaustivity of a match is:

    match expr {
        Expr::Array(e) => {...}
        Expr::Assign(e) => {...}
        ...
        Expr::Yield(e) => {...}

        #[cfg_attr(test, deny(non_exhaustive_omitted_patterns))]
        _ => { /* some sane fallback */ }
    }

Switch non exhaustive syntax tree enums to use #[non_exhaustive]