Consider moving to bindgen for tensorflow-sys

[daschl]

Hi,

I've used bindgen with great success for other c-bindings I maintain (couchbase-sys) and I was wondering if it could make sense to migrate tensorflow-sys to it as well (https://crates.io/crates/bindgen). It would allow us with very little effort to pull in the c headers and just generate the low level binding, and stay up to date.

Here is basically all it takes to generate the API from a header file: https://github.com/couchbaselabs/couchbase-rs/blob/master/couchbase-sys/build.rs#L31

If that sounds like a good path to investigate I could take it on and see if I can make it work.

[adamcrume]

We were using bindgen originally, but it broke at one point, so we ripped it out because the amount of generated code was small anyway. When it broke, there was no working version of bindgen on crates.io for a couple of months (https://webcache.googleusercontent.com/search?q=cache:d-V7reDJfwYJ:https://github.com/Yamakaky/rust-bindgen/issues/251+&cd=1&hl=en&ct=clnk&gl=us&client=ubuntu) and we were concerned about having an unreliable dependency.

Since the size of the C API is increasing, it might be worth it to revisit bindgen if it is more actively maintained now.

There might be other complications, though, like finding the header file if we're trying to build with an existing TensorFlow installation. Not insurmountable, of course, but it adds complexity.

[daschl]

Makes sense - the good news is mozilla took it over and is now actively maintaining it again. Another option I've been considering for my crates is to just include a script for contributors to refresh the API based on a source code and then just check in the generated files for the C lib versions. Then, based on a compile time flag giving it the tag it could include the proper FFI code, making it possible to support more than one version at the same time. I'm not sure if this is perfect either, but it would also remove the dependencies needed to build the library, since you need clang and all kinds of other dependencies installed to make it work, which I don't really like but there is no other option around at the moment if you want it fully integrated.

[nbigaouette-eai]

I've attempted to use bindgen to generate the binding. See #67.