Tips and tricks for a nice Rust development experience

Using VSCode

Ensure you're using the rust-analyzer extension for VSCode, rather than earlier forms of Rust support.
Run gn with the --export-rust-project flag, such as: gn gen out/Release --export-rust-project.
ln -s out/Release/rust-project.json rust-project.json
When you run VSCode, or any other IDE that uses rust-analyzer it should detect the rust-project.json and use this to give you rich browsing, autocompletion, type annotations etc. for all the Rust within the Chromium codebase.
Point rust-analyzer to the rust toolchain in Chromium. Otherwise you will need to install Rustc in your system, and Chromium uses the nightly compiler, so you would need that to match. Add the following to .vscode/settings.json in the Chromium checkout (e.g. chromium/src/.vscode/settings.json; create the subdirectory and file if they do not already exist):
```
{
   // The rest of the settings...

   "rust-analyzer.cargo.extraEnv": {
     "PATH": "../../third_party/rust-toolchain/bin:$PATH",
   }
}
```
This assumes you are working with an output directory like out/Debug which has two levels; adjust the number of .. in the path according to your own setup.

Suppressing unused code warnings

We don't want to commit Rust code with unused variables, but work-in-progress changes often temporarily have unused code. The following options exist for dealing with unused code warnings:

Avoid warnings by prefixing unused variables, parameters, and other APIs with an underscore character (e.g. using _foo rather than foo as the name).
Temporarily insert #![allow(unused)] /* DO NOT SUBMIT */ at the top of the affected .rs files.
- See rustc -W help for a list of warnings covered by the unused group of warnings.
- You can also use #![warn(...)] to still see the warnings, but avoid treating them as errors.
Locally set treat_warnings_as_errors = false in your args.gn. This will affect all Rust and Clippy warnings (as well as C/C++ warnings).

Better Gerrit experience for 100-columns-wide code

Rust Style Guide allows up to 100-characters-wide lines (just like Java). If Gerrit is configured for 80-characters-wide lines, it can lead to a suboptimal user experience. This can be resolved by changing Gerrit settings (e.g. at https://chromium-review.googlesource.com/settings) as follows:

Diff Preferences => Diff width: 101
Edit Preferences => Columns: 100