Document RUSTFMT environment variable

[MonliH]

This PR documents the RUSTFMT enviorment variable, as specified in rust-lang/rustfmt/4426 and rust-lang/rustfmt/4419.

[rust-highfive]

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[ehuss]

Thanks for the PR! I'm a little uncertain about how to document this, since cargo-fmt is an external tool. We generally don't document those in the Cargo book. I think an exception could probably be made here, but maybe the text could be tweaked to make it clearer that this is for a different tool? Perhaps instead of saying Cargo will execute, it could be something like [`cargo fmt`](https://github.com/rust-lang/rustfmt) will execute?

[MonliH]

I'm a little uncertain about how to document this, since cargo-fmt is an external tool. We generally don't document those in the Cargo book. I think an exception could probably be made here

Yeah, rustdoc has something similar (RUSTDOC env var), which is also documented here. I understand rustdoc actually lives in the rustc repo, but I think rustfmt is an agreed-upon standard tool that it's worth documenting here.

Perhaps instead of saying Cargo will execute, it could be something like cargo fmt will execute?

I agree with you here, it's more clear this way. Perhaps even "rustfmt will execute"?

[calebcartwright]

Perhaps even "rustfmt will execute"?

The var is controlling which rustfmt will be invoked by cargo fmt (rustfmt isn't running itself), so I'd echo @ehuss's suggested text using cargo fmt instead of Cargo

[ehuss]

Thanks!
@bors r+

[bors]

📌 Commit f6abaa6 has been approved by ehuss

[bors]

⌛ Testing commit f6abaa6 with merge 0e37d515bb440472c368117a67586c1633dff298...

[bors]

💔 Test failed - checks-actions

[ehuss]

@bors retry

E: Failed to fetch https://packages.microsoft.com/ubuntu/18.04/prod/dists/bionic/main/binary-amd64/Packages.bz2 File has unexpected size (139863 != 139695). Mirror sync in progress? [IP: 40.117.131.251 443]

[bors]

⌛ Testing commit f6abaa6 with merge 31e1f1f...

[bors]

☀️ Test successful - checks-actions
Approved by: ehuss
Pushing 31e1f1f to master...