Cargo doesn't allow cyclic features

[retep998]

If a crate has two features foo and bar which depend on each other, Cargo will error.

[features]
foo = ["bar"]
bar = ["foo"]

error: Cyclic feature dependency: feature `foo` depends on itself

However, I ended up needing this in winapi. In order to allow people to compile only the bits they need to save time, winapi has a feature for each header. Since headers depend on each other, the features in turn had to depend on each other to ensure that winapi would compile correctly. Unfortunately sometimes those headers would have cyclic dependencies on each other, causing Cargo to complain. There were several workarounds I tried such as having a feature that is common to a set of headers which depend on each other, but I eventually settled on doing feature dependency resolution myself via a build script. https://github.com/retep998/winapi-rs/blob/dev/build.rs#L175-L191

[aturon]

@alexcrichton I presume the cycle dependency is largely just a debugging aid, right?

It seems probably not too hard to determine whether the cycle went through any intermediate nodes on the way.

@alexcrichton, if you agree, would you be open to taking a PR from @retep998 to this effect?

[alexcrichton]

Yes IIRC non-cyclic features was just matching non-cyclic dependencies. Cyclic dependencies fundamentally can't be compiled, but cyclic feature dependencies should always be compileable (e.g. just pass a bunch of --cfg flags)

In that sense should be good for a PR!

[antonlarin]

Can I take this on if @retep998 doesn't mind?

[retep998]

@antonlarin Feel free.