fix: add --coverage-target-only, to use rustflags only for target

[haraldh]

Use CARGO_TARGET_{target}_RUSTFLAGS rather than RUSTFLAGS, if --target <TRIPLE> and --coverage-target-only is specified.

This is important, if the project uses multiple targets via the cargo bindeps feature, and not all targets can use instrument-coverage, e.g. a microkernel, or an embedded binary.

Signed-off-by: Harald Hoyer harald@profian.com

[taiki-e]

Thanks for the PR. My concern is how this will interact with rustflags's priorities.

The cargo reference says:

There are three mutually exclusive sources of extra flags. They are checked in order, with the first one being used:

1. RUSTFLAGS environment variable.
2. All matching target..rustflags and target..rustflags config entries joined together.
3. build.rustflags config value.

I would expect that if an unstable feature like bindeps is not enabled, simply removing the RUSTFLAGS environment variable would be enough (note: I have not tested), but I feel that approach would break when bindeps becomes stable.

I'm not sure how that would work if bindeps (or multideps in the future) is enabled.
cargo-llvm-cov just want to emulate the behavior of cargo, so we need to understand how cargo handles this when artifact-dependencies is enabled.

[haraldh]

Do you want me to introduce a new option like --only-target?

[taiki-e]

introduce a new option like --only-target

Introducing a new option seems to make sense to me.

[haraldh]

introduce a new option like --only-target

Introducing a new option seems to make sense to me.

done

[haraldh]

Moved to #167 ... sorry for the inconvenience, but I referenced the my branch name in other locations.