D7579: tests: add test for Rust formatting

[Alphare (Raphaël Gomès)]

Alphare added a comment.


  While I am not a huge fan of the use of a nightly toolchain, I like the fact that we get the latest features and behavior as `rustfmt` is still pre-1.0 and subject to change. So far, it has been nice to us and has not broken anything I can recall, I vote for keeping it that way until it reaches 1.0.

REPOSITORY
  rHG Mercurial

CHANGES SINCE LAST ACTION
  https://phab.mercurial-scm.org/D7579/new/

REVISION DETAIL
  https://phab.mercurial-scm.org/D7579

To: indygreg, #hg-reviewers
Cc: Alphare, mercurial-devel