Upstream reference: https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file

There's two forms I've seen in the wild, a simple string like stable or 1.64.0 or a TOML file that could optionally indicate what compilation target or components were required, as well as what version. Per upstream the former is considered legacy/deprecated.

From my POV, this would both:

• ease onboarding for projects that previously/currently assumed Rustup was in use
• allow projects that support or assume Riff to still indicate to new or Nix-averse contributors what toolchain is being targeted

Known complications:

• Nixpkgs provides a single rustc toolchain version per commit (as with most software)
• There is a great deal of personal taste involved in choosing between fenix vs oxalica/rust-overlay, and there is lingering OSS code and prose that recommends Mozilla's overlay to be found without searching very hard.

I myself am not a nightly user but I imagine this support would be attractive to such projects as well.