Cleanup profraw files + --no-clean

[SamChou19815]

I really like the --no-clean flag since it makes incremental re-run of tests much faster. However, it eventually gets slower and slower since there are tons of profraw files generated.

In practice, I found that running rm -f target/llvm-cov-target/*.profraw before a llvm-cov run with --no-clean flag has the best of both world: the target/llvm-cov-target won't blow up in size, and we still get all the incremental runs' benefit.

Could this be added as a builtin flag so I don't have to write a custom script to do this?

[taiki-e]

Sounds reasonable to me.

[TroyKomodo]

Do incremental builds actually work?

$ cargo llvm-cov show-env
...
CARGO_INCREMENTAL="0"
...

Im curious why it has to be disabled?

[taiki-e]

@TroyKomodo
--no-clean is unrelated to the incremental compilation of rustc itself, but improves the performance of successive builds, since the cleanup to prevent false positives in several situations that occur before the build and the rebuilds triggered by the cleanup are no longer performed.

CARGO_INCREMENTAL=0 was needed at some point in the past, but may no longer be needed with the cleanup in 0.1.5.