LoanPath should not track Box at all

[zwarich]

The changes in #16102 cause the borrow checker to track Box with decreased precision. They should be removed when LoanPath doesn't track Box values at all.

[zwarich]

This will require a DerefMove trait or something like it.

[steveklabnik]

@zwarich is this issue still relevant?

[zwarich]

Yes. It will need to wait for after 1.0. It's also related to #14270.

[steveklabnik]

Triage: this issue is still relevant today, though I'm not sure keeping this issue open here is the right spot for it.

[Mark-Simulacrum]

I think it makes sense to close this at this point -- we're not able to make movement here before DerefMove or the equivalent is implemented. This issue is also misleading, because while it says this is cleanup it's more of a feature expansion.