Add map method to MutexGuard, RwLockReadGuard, RwLockWriteGuard

[sunjay]

Thanks for such an excellent crate! I'm really enjoying using it and it's very well documented. 😄

Version

tokio v0.2.20

Description

The parking_lot crate provides a map method on MutexGuard, RwLockReadGuard, RwLockWriteGuard that is very convenient when you need to return some part of a locked value. For example, this can be used to implement a method that return a reference to a single field of a value that you previously locked.

• MutexGuard::map
• RwLockReadGuard::map
• RwLockWriteGuard::map

Would it be possible for tokio to add these methods to its lock guards? Are there any major complications that could be encountered due to async or because of the mutex implementation?

I'd be willing to work on this if no one else is available. (Though I would not object if someone else is willing to take this on.)

[hawkw]

You may be happy to learn that there's already a PR open to add map methods to the RwLock guards!

[sunjay]

@hawkw This is great! I'm going to get started on a PR for adding map to MutexGuard. 🎉