Linux 6.9 To Upgrade Rust Toolchain - Making More Features Stable

All of the Rust feature patches have already been submitted in a pull request to Linus Torvalds ahead of the upcoming Linux 6.9 merge window.

While Linux 6.8 saw an upgrade to the Rust 1.74.1 toolchain, for Linux 6.9 it's an upgrade to the Rust 1.76 toolchain. The kernel and Rust schedules aligned to upgrade two versions this time around rather than just hitting Rust 1.75. With the upgrade now to Rust 1.76, two more previously unstable features used by the Rust Linux kernel code are no longer unstable: const_maybe_uninit_zeroed and ptr_metadata.

The Rust code for Linux 6.9 also now marks the rustc compiler and other invocations as recursive, which fixes a warning plus prepares for a possible future of leveraging the Make jobserver.

There are also various updates to the Rust for Linux kernel and macros crates. Documentation improvements to the Rust for Linux kernel code are also part of this pull request for Linux 6.9. More details via this pull request.