Rust 1.99.0 Released, Stabilizing C-ABI Variadics and Raw Pointer Layout Info

C-ABI Variadics Stabilized

Rust 1.99.0 now supports defining C-ABI variadic functions using "C" and "C-unwind" ABIs. This allows Rust functions to accept a variable number of arguments, similar to C's `va_list` type. Previously, Rust could only call externally defined variadic functions, but now developers can implement them directly in Rust.

The `VaList` type, which is ABI-compatible with C's `va_list`, is used for these functions. The `VaArgSafe` trait guards which types can be read from a `VaList`. This release also stabilizes support for defining naked variadic functions with non-"C" ABIs via inline assembly.

Raw Pointer Layout Information

The release clarifies the safety requirements for retrieving size and alignment information from raw pointers for both `Sized` and non-`Sized` types. This is achieved by stabilizing three specific functions. This change provides more consistent and safer ways to handle memory layout when working with raw pointers.

Box::leak Documentation Update

While no language semantics changed for `Box::leak`, its documentation has been updated. The new guidance recommends against patterns that deallocate memory after using `Box::leak`. This recommendation addresses problematic interactions with current and future compiler optimizations, especially with the upcoming stabilization of custom allocators.

Developers are now advised to prefer `Box::into_non_null` over `Box::leak` for scenarios where deallocation might occur, as `Box::into_non_null` is designed for such use cases without the potential issues associated with `Box::leak`.