match should fail when there's no matching branch

[ghost]

This program incorrectly prints "second branch" even though the match expression should fail.

extern crate core;
use core::mem;

enum Value {
    Val1,
    Val2
}

fn main() {
    let mut value = Val2;
    unsafe { *mem::transmute::<&mut Value, &mut int>(&mut value) += 10; }

    match value {
        Val1 => println!("first branch"),
        Val2 => println!("second branch")
    }
}

[huonw]

Storing value in an enum type other than one of the predefined variants is undefined behaviour, that is, it's fundamentally incorrect and invalid to do it, even (especially!) in unsafe code. Hence this isn't a bug.

See the manual for the full list of UB, for this specific case:

• Invalid values in primitive types, even in private fields/locals:
  • A discriminant in an enum not included in the type definition

(Thanks for making the effort to file it!)