Function rustc_typeck::check::coercion::coerce_mutbls

pub(in check::coercion) fn coerce_mutbls<'tcx>(
    from_mutbl: Mutability, 
    to_mutbl: Mutability
) -> RelateResult<'tcx, ()>

Coercing a mutable reference to an immutable works, while coercing &T to &mut T should be forbidden.