mention Arc::make_mut alongside Mutex

[lolbinarycat]

Location

https://doc.rust-lang.org/stable/std/sync/struct.Arc.html

Summary

The prose here mentions how Arc does not provide interior mutability, but in many cases you don't actually need shared interior mutability, just the ability to effeciently mutate something that happens to be in an Arc.

Seeing how many people who are new to rust gravitate towards Arc<Mutex<T>> to avoid dealing with references, it would probably be good to mention copy-on-write semantics as an alternative.

[xizheyin]

Shared interior mutability, i.e. Arc<Mutex<T>> can cause performance degradation, I can try this. I made this mistake once.

@rustbot claim