`rust-toolchain.toml`: support multiple channels, or rather support nightly AND stable version.

Problem

My team prefers cargo +nightly fmt for formatting over stable.

I have a different nightly installed locally than we have in CI, causing CI checks to fail. A solution would be to use rust-tooolchain.toml, alas we only want to use nightly for fmt.

Today rust-tooolchain.toml does not support this, but I would like it to be supported.

What do you think?

Proposed Solution

Bikeshedding syntax/overall idea:

[toolchain]
nightly = "2024-12-14"
stable = "1.87.0"

And then when I run cargo +nightly the "2024-12-14" version will be used. But for non-nightly commands "1.87.0" is used.

Notes

We can also omit either or, i.e. both:

[toolchain]
nightly = "2024-12-14"

and

[toolchain]
stable = "1.87.0"

are valid.