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

This issue has been migrated from [#15706](https://github.com/matrix-org/synapse/issues/15706).

---

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
```

Start by inspecting rust/benches/glob.rs, especially the reported feature declaration and unused import, then reproduce the diagnostics in VSCode. Determine the project-supported way to prevent these editor errors and warnings by default; done means the reported diagnostics no longer appear during normal editing without hiding unrelated problems.

Written by the indexing model from the issue text.