Cannot include structs with bitfields in linux-api

[stevenengler]

If you try to add a Linux struct containing bitfields to linux-api, it will cause cbindgen to fail.

For example if you try to add tcp_info which contains:

struct tcp_info {
    ...
    __u8 tcpi_snd_wscale : 4, tcpi_rcv_wscale : 4;

bindgen will generate rust bindings like:

#[repr(C)]
pub struct linux_tcp_info {
    ...
    pub l_bitfield_1: linux___BindgenBitfieldUnit<[u8; 2usize]>,
    ...
}
#[repr(C)]
pub struct linux___BindgenBitfieldUnit<Storage> {
    storage: Storage,
}

The linux-api build script then generates C bindings for this rust code, which fails with:

thread 'main' panicked at ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cbindgen-0.26.0/src/bindgen/mangle.rs:132:17:
  not implemented: Unable to mangle generic parameter Array(Primitive(Integer { zeroable: true, signed: false, kind: B8 }), Value("2")) for 'linux___BindgenBitfieldUnit'

So cbindgen can't create C bindings for this structure. We also cannot exclude this structure from cbindgen using the "exclude" option since it appears that this mangling occurs before types are excluded: https://github.com/mozilla/cbindgen/blob/6bfc2176187a6fc6fba6315323b0296112330294/src/bindgen/library.rs#L69-L72.

I'm not sure what the best option is. We could either wait until we no longer need to build C bindings for linux-api, but this would require moving more of the shim to rust. We could also split the rust bindings into two files, and only generate C bindings for one of the files, so this could be messy. We could also just manually write rust bindings for tcp_info if this is the only struct containing bitfields that we need to deal with.