An unstable feature in const contexts was used.
Erroneous code example:
trait T {}
impl T for () {}
const fn foo() -> impl T { // error: `impl Trait` in const fn is unstable
()
}
To enable this feature on a nightly version of rustc, add the const_fn
feature flag:
#![feature(const_fn)]
trait T {}
impl T for () {}
const fn foo() -> impl T {
()
}