[MIR] Loops ignore their destination lvalue

[nagisa]

fn mir() -> () {
    let x = while 1 == 2 {};
    x
}

results in

which ends up never setting Var0 and thus returning a value that has never been defined, even if the value is zero-sized.

[nagisa]

This is pretty closely related to #30636. We could assign destination variable inside the body, but that sounds wasteful and the body doesn’t always have a “return” value.

[nagisa]

This is still not fixed yet. While we do hack around the issue by assigning loop’s destination with the “return” value of its body (i.e. unit, on every iteration), there’s cases where loop’s “return” value is not a unit. Namely, let x: T = loop {} is valid ∀T, but its body is required to have type of () nevertheless.

As commented #30636 (comment) by niko, we might want some sort of non-terminating kind for rvalues.

[nagisa]

I consider this to be fixed by #30945