Handle return type promotion in the high layer

[uweigand]

As described in #66, the libffi library implicitly extends small integer types used as return types, both in calls and closure callbacks. This is not currently addressed by the existing code base.

For the low and middle layers, the user can manually address this issue by using appropriate return types. However, for the high layer, the types are automatically determined, and are currently incorrect. This leads to problems (primarily) on big-endian platforms.

This PR addresses the return type extension in the high layer by storing the extended type used by libffi as an associated type RetType in the CType trait, and using that type when performing calls or constructing closure callbacks.

This implementation does not change the libffi-rs API, and should not add any significant run-time overhead.

[yorickpeterse]

Are these changes backwards compatible, or should they be accompanied by a major version bump?

[uweigand]

Are these changes backwards compatible, or should they be accompanied by a major version bump?

These changes should be backwards compatible. There should be no change in the public API, and the only changes in generated code actually fix the underlying bug (missing type promotion).

I noticed that tests were failing on Rust 1.48.0, because I was using the unwrap_unchecked function which isn't available yet in that release. I've replaced this now, which should remove those failures.

After re-running the tests I still see two failures that do not appear to be obviously related to my changes, not sure what's going on there.

[uweigand]

Ping? Any other questions or issues with this patch?

[uweigand]

Thank you! With those PRs merged, the test suite now fully passes on s390x.

In order to refer to the updated libffi-rs from other rust packages, it would be good to have a new version on cargo. Do you have an outlook when you're planning to publish the next version?

[yorickpeterse]

@uweigand All recent changes have just been pushed as new versions of libffi-sys and libffi 😃

[uweigand]

@uweigand All recent changes have just been pushed as new versions of libffi-sys and libffi smiley

Perfect! Thank you very much for your support :-)

[uweigand]

Unfortunately trying to pull the new release into upstream rust/miri failed:

  ../src/tramp.c:262:22: error: call to undeclared function 'open_temp_exec_file'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    tramp_globals.fd = open_temp_exec_file ();
                       ^
  1 error generated.

This looks like it has been fixed upstream in libffi here: libffi/libffi#764
(Not sure why I didn't see this when doing standalone build - they must be using different compiler options ...)

What would be the best way forward here? Just backport that fix?

[yorickpeterse]

@uweigand A new release of libffi would have to be published. Maintaining a set of patches isn't something we're interested in.