Expand description
https://github.com/rust-analyzer/rowan/blob/v0.16.1/src/cow_mut.rs
This module provides a CowMut type, which is a mutable version of Cow.
Although it is strange that we can have a CowMut, because it should “copy
on write”, we also don’t love the Cow API and use Cow without even
touching its DerefMut feature.