CLIPPY_FLAGS to pass options to clippy/ enable disable lints via env vars

[matthiaskrgr]

Right now, there is no way to globally enable/disable clippy lints on a system.
If we want to disable certain lint, we have to either modify the code and add #[allow(clippy::foo)] which is tedious, or we have to add -- -Aclippy::foo to EVERY clippy invocation which is also tedious and not always easily done, especially if clippy is invoked by another tool.

IMO the biggest problem is that we cannot use RUSTFLAGS for some reason:
RUSTFLAGS="-Aclippy::needless_return" cargo clippy will just throw errors:

error[E0602]: unknown lint: `clippy::needless_return`
  |
  = note: requested on the command line with `-A clippy::needless_return`

I would love to have something like
export CLIPPY_FLAGS="-A collapsible_if that I just throw into my zshrc and stop worrying about that lint forever.

[Manishearth]

I'd say we should wait for rust-lang/cargo#7533 first

[ebroto]

I can't reproduce the RUSTFLAGS problem anymore. Could you test it @matthiaskrgr?

This would be great news as it could help to solve the problem when Clippy does not re-run if flags passed after -- change.

[matthiaskrgr]

Yeah, looks like RUSTFLAGS="-Aclippy::needless_return" cargo clippy no longer throws errors! 🎉

[flip1995]

Can we close this issue now that RUSTFLAGS works? Or should we document this somewhere first?

[ebroto]

I'm preparing a PR for the "no re-run" problem and I hit that error in the dogfood. I still have to debug it, could be a misuse on my side, but in the meantime I would keep this open.