Lints shouldn't trigger a full rebuild

[c410-f3r]

If duplicated or Rustc stuff, feel free to close to it.

When using environment variables, lints can be declared within RUSTFLAGS but anytime RUSTFLAGS changes, a full rebuild is triggered and this can lead to expensive and time consuming tasks/compilations. Forgive my ignorance since I am not a cargo expert but looks like that all lints should only affect the target crate instead of all dependencies.

[ehuss]

Changes to RUSTFLAGS currently requires a full rebuild because including the flags in the filename causes problems in some situations (see #8716 for tracking this issue). Giving higher-level controls over lints is tracked in #5034. The only workaround right now is to use cargo rustc -- -Dfoo or to use different target directories.

[c410-f3r]

@ehuss Thanks for the explanation. A different target directory looks like a reasonable alternative

I was thinking about adding something like RUSTLINTS to workaround rebuilds. Not sure is it is something feasible or if the cargo team is willing to accept such thing.

[ehuss]

I think it would be best to have a design for #5034 and/or #8424 for controlling lints and that would hopefully consider the one-off command-line or env-var options. Unfortunately it is not clear which direction that should go, and I don't have much time to help it along. But feel free to engage on those issues or the forums to discuss various use cases and options.

[ehuss]

Closing in preference for the linked issues.