Force rustfmt to use 2018 Rust edition in pre-commit hook

[dominic-mulligan-arm]

There is a bug in rustfmt currently that ignores the edition set in Cargo.toml files and defaults to 2015 edition. This means it errors on any staged file containing the async keyword, and any other change introduced in rust 2018.
The solution is to force the Rust edition to use when calling rustfmt.

[egrimley-arm]

Could you add a comment (in githooks/pre-commit) with a link to the upstream bug?

I tried looking for it myself, but all I could find was rust-lang/rustfmt#4645, which was closed with someone saying the reporter was using the wrong version of rustfmt.

Could you also point to a file in the repo (I mean in the discussion here rather than in the code) that needs the --edition=2018 so I can check whether my rustfmt is affected by the bug?

[egrimley-arm]

To answer my own question, an example of something that needs --edition=2018 is: async fn foo() {}

It seems that rustfmt is supposed to ignore the Cargo.toml, while cargo fmt is incapable of looking at a single source file, so invoking rustfmt with --edition=2018 seems to be the right thing to do in our case.

[dominic-mulligan-arm]

+1+1=+1 reached, merging.

[dominic-mulligan-arm]

Oops, seems there was some issue with the signing of commits. I've resigned and pushed again. @ShaleXIONG, @gbryant-arm can you take another look and approve, ready for merging?

[dominic-mulligan-arm]

+1 reached, merging.