Don't force recompilation when setting/removing the --error-format compiler flag

[randomPoison]

Over in AtomBuild/atom-build-cargo#65 I'm running into the issue that building my project from the editor includes RUSTFLAGS=--error-format=json, so switching between building in the editor and building from my terminal causes my entire project, including all dependencies, to be recompiled. Would it be possible to not force recompilation on all crates if the error format flag changes?

I suspect that this would piggy-back off #2982, but if it could be implemented without waiting on direct support for a --error-format cargo flag or be made to work with RUSTFLAGS, then I would appreciate that greatly.

[alexcrichton]

Thanks for the report! I think this is indeed a dupe of #2982 which is being fixed in #3000. In general though this is expected behavior as RUSTFLAGS is opaque to Cargo so it must assume the worst and bust all caches if it changes. With #3000 though it has specific knowledge that this doesn't break the cache!

[randomPoison]

Excellent, thanks for the info!