Rust errors/warnings in editor - `#![feature] may not be used on the stable release channel`

[MadLittleMods]

In my editor (VSCode), the following errors/warnings are ever present and annoying to look at. Is there anyway to resolve these to not complain by default?

error[E0554]: `#![feature]` may not be used on the stable release channel
  --> rust/benches/glob.rs:15:12
   |
15 | #![feature(test)]
   |            ^^^^

I feel like the warning is a more new thing since I only remember the errors being there before.

unused import: `std::collections::BTreeSet`
`#[warn(unused_imports)]` on by default

[MadLittleMods]

It looks like we get around this in CI by just using a nightly version. Should we suggest doing the same in the contributing guide?

synapse/.github/workflows/tests.yml

Lines 177 to 179 in 1404f68

	# We also lint against a nightly rustc so that we can lint the benchmark
	# suite, which requires a nightly compiler.
	lint-clippy-nightly: