Ignore `cfg!`-gated code when disabled

[stouset]

I have some code that's gated by an

if cfg!(config_setting) {
    // do a thing
} else {
    // do a different thing
}

Given that only one of these branches can be hit statically, I'd appreciate being able to disable coverage for the lines that are excluded. Not sure how possible this is.

[taiki-e]

Using #[cfg] instead of cfg! will work as you expect (it is also consistent with how the compiler evaluates the code). For example:

https://github.com/taiki-e/cargo-llvm-cov/blob/a405503bbe508ceb3e9ab7246579eb72bcb49c51/tests/fixtures/coverage-reports/cargo_config/cargo_config.hide-instantiations.txt#L11-L14

Appling the same behavior to cfg! is not always good, considering that coverages generated under different test conditions can be merged. -- For now, I think it is reasonable to leave it up to the way the compiler evaluates the code for #[cfg] and cfg!.

[stouset]

I tried to merge two runs using the cfg flag on and off, and the coverage report would only ever show one or the other was hit. I followed the instructions pretty closely. This may be buggy.

Unfortunately while #[cfg(…)] works in the general case, it can also cause a ton of warnings as the Rust compiler no longer sees the other branch so might not see mut variables being mutated, imported modules being used, etc.

[stouset]

I don't have a minimal test-case for the first paragraph above yet. I'm neck-deep in some stuff, will try to get you one.