Suppress unused_unsafe warnings within the generated code.

[anforowicz]

Sometimes Rust functions declared inside the #[cxx::bridge] have to be marked as unsafe (e.g. if they need to use an explicit lifetime). When the actual, wrapped function is not unsafe, then the generated code will result in an unused_unsafe warning. For example:

   Compiling cxx-test-suite v0.0.0 (/usr/local/google/home/lukasza/src/github/cxx/tests/ffi)
error: unnecessary `unsafe` block
   --> tests/ffi/lib.rs:276:19
    |
276 |         unsafe fn r_return_str_via_out_param<'a>(shared: &'a Shared, out_param: &mut &'a str);
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^ unnecessary `unsafe` block
    |
note: the lint level is defined here
   --> tests/ffi/lib.rs:20:9
    |
20  | #![deny(warnings)]  // Check that expansion of `cxx::bridge` doesn't trigger warnings.
    |         ^^^^^^^^
    = note: `#[deny(unused_unsafe)]` implied by `#[deny(warnings)]`

The warning above comes from the following expansion:

unsafe fn __r_return_str_via_out_param<'a>(...) {
    // The `unsafe` block below is unnecessary if the actual, wrapped
    // function is *not* `unsafe`.
    unsafe { super::r_return_str_via_out_param(...) }
}

This commit avoids the warning by including an explicit #[allow(unused_unsafe)] in the generated code.

[anforowicz]

This PR is motivated by the desire to address the TODO at https://skia-review.googlesource.com/c/skia/+/928799/4/experimental/rust_png/ffi/FFI.rs

[anforowicz]

Hmmm... when re-reading the commit / PR description, the 2nd part seems out-of-place (since there is no foo in the commit - this expansion comes from a separate, ad-hoc, library crate that I used to understand better what is going on).

Let me do a quick force-push to update the commit descriptions... sorry...

[dtolnay]

Thanks!

[dtolnay]

Published in 1.0.135.