There is, yes. You put a file in the root of your project with the specific version of Rust you want, and it’ll get picked up and used by the tooling.

You can do that! But you may run into a problem where one of the crates you use detects that you're using nightly, and then opts in to nightly-only features. Except it may be using features that are only available on the latest version of nightly, or an ancient version that you don't have, and there is no version that makes both your code and the dependent package happy.

It's much better to just use stable. Then you're guaranteed to have forward compatibility, and you only have one place where you need to deal with the nightly weirdness.

There is, but then you're pinning yourself to whatever bugs are in that nightly, and making the eventual upgrade that much worse.