Added a unsafe_ffi_drop_implementations lint.

[DanielKeep]

This detects cases where a struct or enum are annotated with #[repr(C)],
and do not have #[unsafe_no_drop_flag], whereby it warns the user that
the type may not have the expected size or layout.

The lint was set to "Warn" by default as I wasn't sure if "Deny" would be too strong.

A tangential change was to remove #[repr(C)] from Cleanup in src/libstd/sys/windows/backtrace.rs. I believe this to be justified in that Cleanup is only ever used exactly once as an RAII handle, and is never passed to any FFI function.

Two tests including to ensure the lint triggers when #[unsafe_no_drop_flag] is missing, and does not trigger if it is present.

Addresses #18380.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nikomatsakis (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[alexcrichton]

cc #21761

[nikomatsakis]

The code certainly looks good. The need for this lint is probably somewhat time-limited, but in the meantime it can't hurt.

[DanielKeep]

@nikomatsakis Added a struct and enum with #[repr(C)] and no Drop to the rpass test.

[kornelski]

I think I've ran into this problem and this lint would have saved me some hair-pulling :)

[alexcrichton]

Gah sorry for letting this fall by the wayside! This was actually recently reimplemented in #24935, so I'm going to close this. Thanks regardless though!