Need some way to retrieve expected enum discriminant for comparison #4017

There's currently no way to retrieve and store std::mem::Discriminant without explicitly construct an enum value first. However preparing such an enum value might be nontrivial or even have side effects.

I propose for variants defined as constructors (with parenthesis), we could support retrieving the discriminant somehow, directly.

Strawman proposal:

Add a macro called constructor_discriminant (or some better name).

Make this piece of code usable:

enum OutputType {
    StdOut(std::io::StdOut),
    StdErr(std::io::StdErr),
}

let v = OutputType::StdOut(std::io::stdout());
assert_eq!(std::mem::discriminant(v), constructor_discriminant!(OutputType::StdOut));