If expressions not evaluated for constants

[JinShil]

This works fine:

fn main() {
    let condition: bool = true;
    let result: u8 = if condition { 1 } else { 0 };
    println!("{}", result);
}

If assume, therefore, that this should work as well...

const CONDITION: bool = true;
const RESULT: u8 = if CONDITION { 1 } else { 0 };

fn main() {
    println!("{}", RESULT);
}

...but it fails with

error[E0019]: constant contains unimplemented expression type
 --> <anon>:2:20
  |
2 | const RESULT: u8 = if CONDITION { 1 } else { 0 };
  |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Stebalien]

Your first example is evaluated at runtime, the second is evaluated at compile time. Rust doesn't currently support evaluating conditionals at compile time (support for calling functions is in the works but that doesn't cover conditionals).

[JinShil]

Your first example is evaluated at runtime, the second is evaluated at compile time.

Just to clarify, I understand that one is evaluated at runtime, and the other at compile time. The reason for submitting this issue is to point out the inconsistency. That is, if if expressions are expressions, I think they should be usable just like any other expression. For example if the expression A + B in const NUM: i32 = A + B; evaluates fine at compile time, I think the if expressions should also.

[Stebalien]

@JinShil Got it. That kind of issue usually goes in the rust-lang/rfcs repo (see rust-lang/rfcs#322, specifically).

[est31]

miri will make this possible, and much more.

[steveklabnik]

Yes, @Stebalien is right here. Thanks @JinShil , but we should track this in the RFCs repo (and already are)