fix(.rustfmt.toml): specify Rust edition 2021 (#1722)

* fix(.rustfmt.toml): specify Rust edition 2021

Emacs' popular Rust plugin Rustic [1] uses `rustfmt` to format code. `rustfmt`
only picks up the Rust edition from `Cargo.toml` when executed through `cargo fmt` [2] [3].

This commit duplicates the Rust edition in the `.rustfmt.toml` file. Thus all
users of `rustfmt` (whether through `cargo fmt` or plain `rustfmt`) will get
formatting according to their Rust edition, e.g. helpful when interacting with
`async` code.

[1]: https://github.com/brotzeit/rustic
[2]: https://github.com/brotzeit/rustic?tab=readme-ov-file#edition-2018
[3]: https://github.com/rust-lang/rustfmt?tab=readme-ov-file#rusts-editions

* Document keeping edition statements in sync

.rustfmt.toml

@@ -1,3 +1,10 @@
+# Keep in sync with `Cargo.toml` `edition`.
+#
+# `rustfmt` envoked not through `cargo fmt` but directly does not pick up Rust
+# edition in `Cargo.toml`. Thus duplicate here. See
+# https://github.com/mozilla/neqo/pull/1722 for details.
+edition = "2021"
+
 comment_width=100
 wrap_comments=true
 

Cargo.toml

@@ -16,6 +16,7 @@ homepage = "https://github.com/mozilla/neqo/"
 repository = "https://github.com/mozilla/neqo/"
 authors = ["The Neqo Authors <necko@mozilla.com>"]
 version = "0.7.1"
+# Keep in sync with `.rustfmt.toml` `edition`.
 edition = "2021"
 license = "MIT OR Apache-2.0"
 # Don't increase beyond what Firefox is currently using: