Cargo should detect changes of `RUSTC_LOG`

[oriongonza]

Problem

Cargo does a really good job at detecting whether to rebuild or not to rebuild, but one of the things that are missing is the internal RUSTC_LOG env variable.

Proposed Solution

No response

Notes

I can try to drive this forward as my first contribution, looks easy enough.

[epage]

Could you describe your proposed solution?

If its to rebuild if that env var changes, then that only helps you when changing the value, I assume.

We could always rebuild when its set so its guaranteed to show the log.

However, for both cases, this runs into intent. Different users can be setting this and have different expectations of what it should do (see logs for everything, see logs for only changed).

This is with me assuming we don't record and replay the log like we do warnings. If we do, then this might make more sense. If we don't, then doing so is another possibility.

[ehuss]

For my workflow, it usually involves building dependencies without RUSTC_LOG, because that would be too noisy or slow. Then building the crate of interest with RUSTC_LOG. If cargo rebuilt the dependencies, that would make it almost unusable in some situations.

I would agree that the reverse is a little annoying when you want to unset RUSTC_LOG, but cargo caches all the output and repeatedly displays it. However, it isn't too difficult to clear that out of the cache in the rare cases where that happens.