Add a way to disable cfg to if defined() guards generation

[AlexanderSchuetz97]

I have a problem with a crate that provides a optional ffi api that cbindgen should generate headers for. The entire ffi api is optional in that crate as the provided functionality can be used directly as a pure rust dependency also.

This means that every export function (and struct) has this cfg feature above it.
#[cfg(feature = "ffi")]

Cbindgen is not really suited to handling this use case directly.
If I do nothing then it just does not generate the exports for any functions and the header is blank.
If I enable the feature in cbindgen.tomls parse.expand section
features = ["ffi"]
Then the generated header is correct but I get 100s of outputs
WARN: Missing [defines] entry for feature = "ffi" in cbindgen config.
This is concerning as it would drown out actual warnings that I do actually care about.

I would expect this warning to only occur once and there does not appear to be a way of telling cbindgen that this feature is always enabled in all situations where the header is used.

I have tried to example add
"feature = ffi" = ""
to the defines block.
This gets rid of the warning but the generated header is not quite correct.
It outputs stuff like this in the header:

#if defined()
bool export_function_name_here();
#endif

I would like a way to get rid of this warning and not emit any #if defined() block in the header.

[flaviojs]

In the current cbindgen (0.27.0) you can get around the issue by assigning a custom macro to the feature and always defining it in after_includes or header.

cbindgen.toml:

after_includes = """
#define WITH_FEATURE_FFI /* always expose #[cfg(feature = "ffi")] */
"""
[defines]
"feature = ffi" = "WITH_FEATURE_FFI"

[AlexanderSchuetz97]

This generates a header that works but contains 3 more lines per definition than necessary. Not ideal.
Thank you for providing this info. I will use this as a work around.