Basically, anything that gets a .1 or .2 type release.
Also, shout out to the Rust community for maintaining and fixing issues quickly; not trying to pick on them, just trying to show some examples to show Rust is still young and very much a moving target.
To be fair, that affected a bunch of other languages as well so Rust is in (questionably) good company there.
I feel the need to keep your Rust install up to date might arguably be more ecosystem-dependent than anything to do with Rust itself. If stuff you're using requires a newer rustc then you don't have much choice but to upgrade, but if you're fine using something older and/or your dependencies keep support for older versions of rustc then you can probably get away with a lot fewer updates.
For those of us who may be out of the loop, could you elaborate on these points?