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

[phansch]

As pointed out by @flip1995 in #3824 (comment), 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:

rust-clippy/.travis.yml

Line 24 in 8dfabdf

rustup component add rustfmt || cargo install --git https://github.com./rust-lang/rustfmt/ --force

rust-clippy/ci/base-tests.sh

Line 26 in 8dfabdf

cargo +nightly fmt --all -- --check

rust-clippy/ci/base-tests.sh

Lines 53 to 78 in 8dfabdf

	# make sure tests are formatted
	# some lints are sensitive to formatting, exclude some files
	tests_need_reformatting="false"
	# switch to nightly
	rustup override set nightly
	# avoid loop spam and allow cmds with exit status != 0
	set +ex
	for file in `find tests -not -path "tests/ui/doc.rs" -not -path "tests/ui/unused_unit.rs" | grep "\.rs$"` ; do
	rustfmt ${file} --check
	if [ $? -ne 0 ]; then
	echo "${file} needs reformatting!"
	tests_need_reformatting="true"
	fi
	done
	set -ex # reset
	if [ "${tests_need_reformatting}" == "true" ] ; then
	echo "Tests need reformatting!"
	exit 2
	fi
	# switch back to master
	rustup override set master

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 cargo fmt doesn't format files in sub-directories in tests rustfmt#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: rust-lang/rust-analyzer#163.

[ghost]

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?

[ghost]

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.

[ghost]

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 🎊