Fix unsafe_op_in_unsafe_fn warning for unsafe operations in generated module

[xinchengxx]

Fix unsafe_op_in_unsafe_fn warning for unsafe operations in generated module

This PR adds #[allow(unsafe_op_in_unsafe_fn)] attributes to autocxx-generated unsafe functions to suppress warnings that would appear when using Rust 2024 edition.

Background

Rust 2024 edition introduces the stabilized unsafe_op_in_unsafe_fn lint (Edition Guide), which requires unsafe operations inside unsafe functions to be explicitly wrapped in unsafe blocks.

While this is a valuable safety improvement for hand-written code (as described in RFC #2585), it creates unnecessary noise for generated FFI bindings like those produced by autocxx.

Example of warnings

Without this fix, code generated by autocxx produces warnings like:

warning[E0133]: call to unsafe function `std::pin::Pin::<&'a mut T>::get_unchecked_mut` is unsafe and requires unsafe block
 --> target/debug/build/utils-3c769df202ee350f/out/autocxx-build-dir/rs/autocxx-ffi-gen.rs:1:18417
  |
1 | ...b769b983bff6673a_autocxx_wrapper_0xb769b983bff6673a (this . get_unchecked_mut () . as_mut_ptr () , other) } } impl Drop for output :: PlainString { # [doc = ...
  |                                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^ call to unsafe function
  |
  = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/unsafe-op-in-unsafe-fn.html>
  = note: consult the function's documentation for information on how to avoid undefined behavior

Changes

This PR modifies the code generation templates to add the #[allow(unsafe_op_in_unsafe_fn)] attribute to generated unsafe functions. I think this is appropriate because:

• The generated bindings already have clear unsafe markers at the API boundary
• Internal implementation details are not directly visible to users
• The safety concerns are properly addressed at the function signature level

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[xinchengxx]

Note: According to RFC #2585, this warning may become a compilation error in a future. If that happens, we'll need to modify our code generator implementation to wrap unsafe operations in explicit unsafe blocks rather than using allow attributes.

[adetaylor]

Thanks. Yep. I'd be very pleased to receive a PR to do this "properly" i.e ensure we have an unsafe block everywhere it's needed.