The syntax is documented in the reference: https://doc.rust-lang.org/stable/reference/patterns.html#identifier-pattern, but afaict not the meaning.

Impact

Experienced users can still get tripped up by things like: https://stackoverflow.com/questions/69845063/why-does-a-level-of-indirection-allow-omitting-mut (I'm a Rust n00b, but posted the question for a Rust-experienced colleague).

How to fix:

If someone has a definition of mut handy, I can make PRs to update relevant parts of the docs. Apologies if mut is already defined and I missed it.

moved from rust-lang/rust#90605