{MutexGuard, RwLockReadGuard, RwLockWriteGuard}::map

[QuietMisdreavus]

Currently, the RefCell borrow guards have a function called map which allows you to transform a borrow of a whole struct to a borrow of one of its fields (or of some related reference with the same lifetime). Would it be possible to add these methods to Mutex and RwLock?

[sfackler]

It's come up before, but is unsound: #30834 (comment).

[QuietMisdreavus]

Ah, curses. Thanks for the link; i'll go ahead and close this out then.

[jonas-schievink]

The linked comment only says that MutexGuard::map would be unsound, not the RwLock*::map methods (which I could use right now) - can you reopen, @QuietMisdreavus?

[jonas-schievink]

Ah I see this was already discussed in #27746 (comment), so nevermind

EDIT: That said, map could simply return a MappedMutexGuard or MappedRwLock*Guard to fix the soundness problems.

[xmo-odoo]

Ah I see this was already discussed in #27746 (comment), so nevermind

EDIT: That said, map could simply return a MappedMutexGuard or MappedRwLock*Guard to fix the soundness problems.

For any reader stumbling upon this issue, that seems to be the option parking_lot uses: its rw guards are mappable and return a dedicated MappedGuard structure.