Cargo should use a variable other than RUST_LOG for env_logger.

[zachlute]

Suggestion: CARGO_LOG.

This came up during the Debugging session at Rust Belt Rust. Basically, as soon as people set up their project to use env_logger and set RUST_LOG=debug, they were super confused by the crazy amount of seemingly-random output suddenly on their screen, which appeared to include parse tokens of some sort? The same problem exists at the info level, though obviously to a lesser extent.

While it makes sense for most things to just use RUST_LOG, and it's true that you can solve this problem by specifying a module filter for env_logger, it seems like a common and confusing enough behavior, and a likely enough case that somebody would do RUST_LOG=debug cargo run and get a whole bunch of confusing output, that it likely makes sense to just use a different variable for Cargo.

In fact, while working on a potential PR for this, it happened again to the guy sitting next to me. "What the heck is all this crazy output?" He wasn't a plant or anything!

The only potential downside here is if people expect RUST_LOG to work and it doesn't, but I suspect the number of workflows this would break is vanishingly small and it's a simple fix, so maybe that's not a big deal?

Anyway, PR coming shortly for feedback.

[dwijnand]

Noting for the record (there's a backlink, but still): #6190 fixes this but was postponed to a later date.

[JohnTitor]

rust-lang/rust#60401 was merged

[ehuss]

Fixed via #6918.