Ensure rustfmt is invoked without edition parameter

[apiraino]

Fixes #570

rustfmt should probably be invoked without defaulting to any --edition parameter as it collides with a user configuration in a rustfmt.toml or with the rustfmt default (2015 edition at the time of this patch).

This patch sets rust-rustfmt-switches to nil so in a sense, I think this could be a breaking change?

I've added a test to ensure that it stays so. The test is a bit flaky, it doesn't use a rutfmt.toml so it's not completely isolated. But I'm an Emacs newbie, so 🤷

Happy to receive feedback. Thanks for the review!

r? @psibi

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[psibi]

LGTM apart from minor comments, I like the tests!

[psibi]

@apiraino Missed this, one more change: Can you also update the changelog: https://github.com/rust-lang/rust-mode/blob/master/Changelog.md ?

[apiraino]

@apiraino Missed this, one more change: Can you also update the changelog: master/Changelog.md ?

One more thing about this change. As I mentioned in the opening comment, do you think this qualifies as a breaking change for users? I mean, people could start seeing their code being formatted differently. If I understand how MELPA publishing works, this commit will be automatically picked up by them once merged, correct?

Do you do some tagging/releases for other package repositories that don't stay on top of trunk commits - like Elpa?

[psibi]

@apiraino

do you think this qualifies as a breaking change for users?

Yes

If I understand how MELPA publishing works, this commit will be automatically picked up by them once merged, correct?

Yes. Although Melpa stable won't. Although very few people actually use MELPA stable.

Do you do some tagging/releases for other package repositories that don't stay on top of trunk commits - like Elpa?

We do it. Although we wait for quite a bit and for lots of change to accumulate. The last release was done a year ago because we want to release support with tree-sitter: https://github.com/rust-lang/rust-mode/releases/tag/1.0.6