The rust-toolchain file makes
it possible to share a basic
rustup configuration across the
project.
https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file
Important to note, this is the
current rustup hierarchy:
1. An explicit toolchain, e.g. cargo +beta,
2. The RUSTUP_TOOLCHAIN environment variable,
3. A directory override, ala rustup override set beta,
4. The rust-toolchain file,
5. The default toolchain,
source:
c2db7dac6b/README.md (toolchain-override-shorthand)
