Rust-clippy: Provide an easy way for contributors to run cargo fmt like it's executed on CI

[phansch]

As pointed out by @flip1995 in https://github.com/rust-lang/rust-clippy/pull/3824#r261652006, we currently don't have an easy way to run rustfmt on the tests locally like we do on CI.

What we are doing on CI currently:

https://github.com/rust-lang/rust-clippy/blob/8dfabdf11c6cdaffd7c6e6552a6ead8d52c49b10/.travis.yml#L24

https://github.com/rust-lang/rust-clippy/blob/8dfabdf11c6cdaffd7c6e6552a6ead8d52c49b10/ci/base-tests.sh#L26

https://github.com/rust-lang/rust-clippy/blob/8dfabdf11c6cdaffd7c6e6552a6ead8d52c49b10/ci/base-tests.sh#L53-L78

It would be nice if we could extract that into a separate script so that people can use to format their tests locally, instead of waiting for CI.

Some things to consider when doing this:

• The script also needs to add the rustfmt component if it isn't present locally
• We use rustfmt instead of cargo fmt for the test formatting because of https://github.com/rust-lang/rustfmt/issues/1820
• To make it idempotent, the new script should automatically detect the 'current' toolchain and switch back to it after the formatting script has finished (instead of switching to master)
• Unlike a fresh VM on CI, We can't just use rustup override set nightly, because that doesn't update the locally installed nightly rust. We should probably pin to a specific nightly instead and update the version from time to time.
• Using a shell script will not work cross-platform but I think doing it in Rust might be too much work?

[phansch]

If we want to provide a cargo format subcommand instead of a shell script, then it might also be worth to look at how rust-analyzer did it: https://github.com/rust-analyzer/rust-analyzer/pull/163.

[mikerite]

I'm working on adding a clippy dev subcommand to do this.

I'm don't think the command should install/update toolchains/components by default (especially if we want to run it from a test) but I'll provide an option to do that and error out if the required toolchains/components aren't installed and up to date.

[phansch]

friendly ping @mikerite: did you make some progress on this?

[mikerite]

Thanks for the ping.

I didn't get very far and I haven't looked at this in weeks. It's fine by me if someone else wants to do this. Otherwise, I'll try to make some progress this weekend.

[mikerite]

I've got something ready but I'll wait until the build is fixed before submitting the PR.

[phansch]

Closing this - thanks to @mikerite we now have util/dev fmt which has been working great :confetti_ball: