[PATCH v3 0/7] ForLt/CovariantForLt split, auxiliary closure API and DevresLt

[Danilo Krummrich <dakr@kernel.org>]

The ForLt trait currently guarantees covariance, which allows safe
lifetime shortening via cast_ref(). However, some types (e.g. those
containing Mutex<&'bound T>) are invariant over their lifetime parameter
and cannot safely use cast_ref().

This series splits ForLt into two traits:

  - ForLt: base trait for all lifetime-parameterized types, providing
    only the Of<'a> GAT.

  - CovariantForLt: unsafe subtrait that guarantees covariance,
    providing a safe cast_ref() method.

For invariant types, a closure-based API (registration_data_with()) is
added to the auxiliary subsystem. The closure's HRTB prevents the caller
from choosing a concrete lifetime, which would be unsound for invariant
types.

On top of that, this series adds DevresLt<F: ForLt>, a thin wrapper
around Devres<F::Of<'static>> that shortens the stored 'static lifetime
back to the caller's borrow scope. DevresLt provides both closure-based
access (access_with/try_access_with for ForLt types) and direct
reference access (access/try_access for CovariantForLt types).

Also implement ForLt and CovariantForLt for Bar, IoMem and
ExclusiveIoMem, and update their into_devres() methods to return
DevresLt. Provide convenience type aliases DevresBar, DevresIoMem and
DevresExclusiveIoMem.

Changes in v3:
- Keep UnsafeForLtImpl as the shared helper for both ForLt! and
  CovariantForLt!, distinguished by const generic N
- Remove cast_ref_unchecked() from ForLt; lifetime shortening is
  handled by borrowing with the target lifetime directly or by
  decoupling the HRTB from the outer reference lifetime

Changes in v2:
- Fold the ForLt -> CovariantForLt rename and the new ForLt base trait
  into this series
- Add closure-based registration_data_with() for auxiliary ForLt types
- Add auxiliary sample demonstrating ForLt with an invariant Mutex type
- DevresLt: add closure-based access_with()/try_access_with() for ForLt
  types alongside direct access()/try_access() for CovariantForLt types
- Make DevresLt::new() unsafe; callers must guarantee the data outlives
  the device binding
- Implement both ForLt and CovariantForLt (previously just ForLt) for
  Bar, IoMem, ExclusiveIoMem
- Various safety comment and documentation improvements

Danilo Krummrich (7):
  rust: types: rename ForLt to CovariantForLt
  rust: types: introduce ForLt base trait for CovariantForLt
  rust: auxiliary: add registration_data_with() for ForLt types
  rust: auxiliary: sample: demonstrate ForLt with invariant Mutex type
  rust: devres: add DevresLt for ForLt-aware device resource access
  rust: pci: return DevresLt from Bar::into_devres()
  rust: io: mem: return DevresLt from
    IoMem/ExclusiveIoMem::into_devres()

 drivers/gpu/nova-core/driver.rs       |   4 +-
 drivers/pwm/pwm_th1520.rs             |   5 +-
 rust/kernel/auxiliary.rs              |  76 +++++++++++++++-----
 rust/kernel/devres.rs                 | 100 ++++++++++++++++++++++++++
 rust/kernel/io/mem.rs                 |  65 +++++++++++------
 rust/kernel/pci.rs                    |   1 +
 rust/kernel/pci/io.rs                 |  37 +++++++---
 rust/kernel/types.rs                  |   1 +
 rust/kernel/types/for_lt.rs           |  95 +++++++++++++++++-------
 rust/macros/for_lt.rs                 |  53 +++++++++++---
 rust/macros/lib.rs                    |  18 ++++-
 samples/rust/rust_driver_auxiliary.rs |  96 ++++++++++++++++++-------
 12 files changed, 432 insertions(+), 119 deletions(-)


base-commit: 83f1454877cc292b88baf13c829c16ce6937d120
-- 
2.54.0