Because specialization is not stable, I've needed the following function for ad-hoc specialization: Given a type T, is T == f64 or not?
With T: 'static this is easy to implement using the current TypeId. It would also be easy to implement with a more relaxed type id, and it would seemingly be easily sound (because one of the two types in the comparison is completely without lifetimes). However, there is no way with the current type system to constrain a function like is_equal<T, U> to only types where U is "lifetime-free".