We can count lines of code via 'cloc' (or latest tooling).
If learned alot from readintg the source code of a trait, please post a link below (with a short description of what you learned / how it leveld up your programming skills).
Main focus is on what you learned from te crate (rather than whether you would use the crate in production) -- so please feel free to list crates that are a "toy implementation" of FOO, but happens to be very well written and instructive.
That's a very broad request. Are there things you don't want? I feel like it wouldn't help you, if I dropped a link to my atomic cell code, next week, after finishing the prototype + documenting it. It features atomics and unsafe. It's going to be a small library, but also heavy to digest for beginners.
Also, although it is not really one crate, and it may be, in total, more lines than what you were asking for, I find that @anon15139276's blog post series from building your own async executor to its (comparatively) simple ::smolasync framework, to be really interesting.
For some reason, though, the blog posts are no longer available (the reason I am trying to ping their author within this very post).
That being said, Internet being what it is, we got snapshots :