add a rust-toolchain.toml to pin the rust version?

[asukaminato0721]

I've checked the issues, no relevant issues.

I noticed that

fish-shell/.github/workflows/rust_checks.yml

Lines 32 to 34 in f7b5706

	# This used to have --deny=warnings, but that turns rust release day
	# into automatic CI failure day, so we don't do that.
	run: cargo clippy --workspace --all-targets

so manually pin a version will fix that. (But require manually bump the toolchain version, which is not so frequently).

something like

[toolchain]
# older versions may fail to compile; newer versions may fail the clippy tests
channel = "1.75"
components = ["rustfmt", "clippy"]

ref https://rust-lang.github.io/rustup/overrides.html

[krobelus]

to build the software we don't want to force users to install a specific version of Rust. Any Rust >= 1.66.0 should work.
Between normal CI builds and asan we already test 1.67.0 and nightly, and we clippy-check stable.

Clippy and rustfmt checks should either

• not be pinned but follow stable Rust. This means that we want to allow warnings (current state)
• be pinned, then they can fail on warnings.

The second would require developers use a pinned version, and use clippy.

See also the typical use case for rust-toolchain.toml which is not ours:

Some projects find themselves 'pinned' to a specific release of Rust and want this information reflected in their source repository. This is most often the case for nightly-only software that pins to a revision from the release archives.