Make MutexGuard's Debug implementation more useful.
Make MutexGuard's Debug implementation more useful.

Fixes #57702.

graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust