[PATCH v6 0/4] rust: replace kernel::str::CStr w/ core::ffi::CStr

[Tamir Duberstein <tamird@gmail.com>]

This picks up from Michal Rostecki's work[0]. Per Michal's guidance I
have omitted Co-authored tags, as the end result is quite different.

Link: https://lore.kernel.org/rust-for-linux/20240819153656.28807-2-vadorovsky@protonmail.com/t/#u [0]
Closes: https://github.com/Rust-for-Linux/linux/issues/1075

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
Changes in v6:
- Split the work into several commits for ease of review.
- Restore `{from,as}_char_ptr` to allow building on ARM (see commit
  message).
- Add `CStrExt` to `kernel::prelude`. (Alice Ryhl)
- Remove `CStrExt::from_bytes_with_nul_unchecked_mut` and restore
  `DerefMut for CString`. (Alice Ryhl)
- Rename and hide `kernel::c_str!` to encourage use of C-String
  literals.
- Drop implementation and invocation changes in kunit.rs. (Trevor Gross)
- Drop docs on `Display` impl. (Trevor Gross)
- Rewrite docs in the style of the standard library.
- Restore the `test_cstr_debug` unit tests to demonstrate that the
  implementation has changed.

Changes in v5:
- Keep the `test_cstr_display*` unit tests.

Changes in v4:
- Provide the `CStrExt` trait with `display()` method, which returns a
   `CStrDisplay` wrapper with `Display` implementation. This addresses
   the lack of `Display` implementation for `core::ffi::CStr`.
- Provide `from_bytes_with_nul_unchecked_mut()` method in `CStrExt`,
   which might be useful and is going to prevent manual, unsafe casts.
- Fix a typo (s/preffered/prefered/).

Changes in v3:
- Fix the commit message.
- Remove redundant braces in `use`, when only one item is imported.

Changes in v2:
- Do not remove `c_str` macro. While it's preferred to use C-string
   literals, there are two cases where `c_str` is helpful:
   - When working with macros, which already return a Rust string literal
     (e.g. `stringify!`).
   - When building macros, where we want to take a Rust string literal as an
     argument (for caller's convenience), but still use it as a C-string
     internally.
- Use Rust literals as arguments in macros (`new_mutex`, `new_condvar`,
   `new_mutex`). Use the `c_str` macro to convert these literals to C-string
   literals.
- Use `c_str` in kunit.rs for converting the output of `stringify!` to a
   `CStr`.
- Remove `DerefMut` implementation for `CString`.

---
Tamir Duberstein (4):
      rust: move BStr,CStr Display impls behind method
      rust: replace `CStr` with `core::ffi::CStr`
      rust: replace `kernel::c_str!` with C-Strings
      rust: remove core::ffi::CStr reexport

 drivers/gpu/drm/drm_panic_qr.rs  |   2 +-
 drivers/net/phy/ax88796b_rust.rs |   7 +-
 drivers/net/phy/qt2025.rs        |   5 +-
 rust/kernel/device.rs            |   5 +-
 rust/kernel/error.rs             |  10 +-
 rust/kernel/firmware.rs          |   9 +-
 rust/kernel/kunit.rs             |  18 +-
 rust/kernel/miscdevice.rs        |   5 +-
 rust/kernel/net/phy.rs           |   8 +-
 rust/kernel/prelude.rs           |   2 +-
 rust/kernel/seq_file.rs          |   4 +-
 rust/kernel/str.rs               | 579 +++++++++++----------------------------
 rust/kernel/sync.rs              |   4 +-
 rust/kernel/sync/condvar.rs      |   4 +-
 rust/kernel/sync/lock.rs         |   5 +-
 rust/kernel/sync/lock/global.rs  |   7 +-
 rust/kernel/sync/poll.rs         |   1 +
 rust/kernel/workqueue.rs         |   3 +-
 18 files changed, 210 insertions(+), 468 deletions(-)
---
base-commit: bdf3d0e99c106932167de56141be5092762bab48
change-id: 20250201-cstr-core-d4b9b69120cf

Best regards,
-- 
Tamir Duberstein <tamird@gmail.com>