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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: Any way to use const member of non const value's type in a const context?

Been pulling my hair trying to figure this one out for some time now.

Say I have this:

trait Foo {
    const MEMBER: usize;
    fn get_member(&self) -> usize { Self::MEMBER }
}

Given x: TX with TX: Foo, I can get MEMBER either by doing TX::MEMBER or x.get_member(). This is good for non-const contexts.

Say I have a macro (or, really, any context where I can only get an opaque value as-is) that takes a value of a type implementing Foo. I want to perform a static check on the value of MEMBER:

macro_rules! mac {
    ($e:expr) => {
        const CHECK: () = assert!($e::MEMBER != 0); // sic
    }
}

The above doesn’t work, because :: must be used on a type. I can’t use get_member (even if it were a const fn) either because then I’d get error[E0435]: attempt to use a non-constant value in a constant.

I’ve thought about inference tricks but they all end up “using” $e in a way that counts as E0435.

If Rust had C’s typeof (which is const even for non-const operands, since they’re unevaluated) I could do typeof($e)::MEMBER, but it doesn’t.

Is there any way, in Rust (assume latest nightly, any unstable feature you want) to do this? To recap: getting a const member of the type of a non-const value in a const context.

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