E0744.md

An unsupported expression was used inside a const context.

Erroneous code example:

const _: i32 = {
    let mut x = 0;

    for i in 0..4 { // error!
        x += i;
    }
};

At the moment, for loops, .await, and the Try operator (?) are forbidden inside a const, static, or const fn.

This may be allowed at some point in the future, but the implementation is not yet complete. See the tracking issues for async and ? in const fn, and (to support for loops in const fn) the tracking issues for impl const Trait for Ty and &mut T in const fn.