An feature unstable 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 { () }