[PATCH v3 0/2] rust: add `Alignment` type

[Alexandre Courbot <acourbot@nvidia.com>]

Thanks to the feedback received on the last revision, I feel like we are
converging.

The ACP for `ffs`/`fls` equivalents having been accepted [1], I am
dropping any work related to them from this patchset which is now
exclusively focused on alignment features.

The first patch makes two additions:

- Bring an equivalent of the nightly upstream `Alignment` type [2] with
  an identical public interface,
- Add a new `Alignable` extension trait that provides a way to align
  values up or down according to an `Alignment`, and implement it on
  unsigned integer types.

The second patch makes use of these in the Nova driver.

[3] is recommended as a dependency, a small cleanup preventing an unwrap
upon applying this series.

[1] https://github.com/rust-lang/libs-team/issues/631
[2] https://doc.rust-lang.org/std/ptr/struct.Alignment.html
[3] https://lore.kernel.org/rust-for-linux/20250808-falcondma_256b-v1-1-15f911d89ffd@nvidia.com/

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
Changes in v3:
- Move `align_down` and `align_up` into their own `Alignable` trait.
  (thanks Miguel!)
- Fix `Alignment::mask` implementation to match upstream's.
- Add missing `SAFETY:` comments.
- Improve grammar on a few hard-to-parse comments.
- Link to v2: https://lore.kernel.org/r/20250804-num-v2-0-a96b9ca6eb02@nvidia.com

Changes in v2:
- Remove `last_checked_bit` (use Rust's `checked_ilog2` instead).
- Port Rust nightly `Alignment` type and extend it with `align_down` and
  `align_up` methods.
- Link to v1: https://lore.kernel.org/r/20250620-num-v1-0-7ec3d3fb06c9@nvidia.com

Changes since split from the nova-core series:
- Rename `fls` to `last_set_bit`,
- Generate per-type doctests,
- Add invariants section to `PowerOfTwo`.
- Do not use reference to `self` in `PowerOfTwo` methods since it
  implements `Copy`,
  - Use #[derive] where possible instead of implementing traits
    manually,
    - Remove `Deref` and `Borrow` implementations.

---
Alexandre Courbot (2):
      rust: add `Alignment` type
      gpu: nova-core: use Alignment for alignment-related operations

 Documentation/gpu/nova/core/todo.rst |   1 -
 drivers/gpu/nova-core/fb.rs          |   6 +-
 drivers/gpu/nova-core/vbios.rs       |   4 +-
 rust/kernel/lib.rs                   |   1 +
 rust/kernel/ptr.rs                   | 218 +++++++++++++++++++++++++++++++++++
 5 files changed, 224 insertions(+), 6 deletions(-)
---
base-commit: 062b3e4a1f880f104a8d4b90b767788786aa7b78
change-id: 20250620-num-9420281c02c7
prerequisite-message-id: <20250808-falcondma_256b-v1-1-15f911d89ffd@nvidia.com>
prerequisite-patch-id: 2439f5f9b560ee4867716f0018b5326dcd72cda3

Best regards,
-- 
Alexandre Courbot <acourbot@nvidia.com>