`clippy::mut_mutex_lock`

Use get_mut when the mutex is already exclusively borrowed; locking it again can deadlock.

What it reads

Clippy, with the compiler behind it. The only pass that compiles.

Ordinary defect.

The tier bounds how severe a rule may be; its category bounds the tier.

Turn it off, or make it blocking, without touching the code it flags:

rust-doctor --rule clippy::mut_mutex_lock=off .
rust-doctor --rule clippy::mut_mutex_lock=error .