Add a generic infrastructure for performing field and index projections on
raw pointers. This will form the basis of performing I/O projections.

Pointers manipulations are intentionally using the safe wrapping variants
instead of the unsafe variants, as the latter requires pointers to be
inside an allocation which is not necessarily true for I/O pointers.

This projection macro protects against rogue `Deref` implementation, which
can causes the projected pointer to be outside the bounds of starting
pointer. This is extremely unlikely and Rust has a lint to catch this, but
is unsoundness regardless. The protection works by inducing type inference
ambiguity when `Deref` is implemented.

This projection macro also stops projecting into unaligned fields (i.e.
fields of `#[repr(packed)]` structs), as misaligned pointers require
special handling. This is implemented by attempting to create reference to
projected field inside a `if false` block. Despite being unreachable, Rust
still checks that they're not unaligned fields.

The projection macro supports both fallible and infallible index
projections. These are described in detail inside the documentation.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://patch.msgid.link/20260302164239.284084-3-gary@kernel.org
[ * Add intro-doc links where possible,
  * Fix typos and slightly improve wording, e.g. "as documentation
    describes" -> "as the documentation of [`Self::proj`] describes",
  * Add an empty line between regular and safety comments, before
    examples, and between logically independent comments,
  * Capitalize various safety comments.

    - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Add a `KnownSize` trait which is used obtain a size from a raw pointer's
metadata. This makes it possible to obtain size information on a raw slice
pointer. This is similar to Rust `core::mem::size_of_val_raw` which is not
yet stable.

Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://patch.msgid.link/20260302164239.284084-2-gary@kernel.org
[ Fix wording in doc-comment. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Since `ALIGN` is a const parameter, this assertion can be done in const
context using the `assert!` macro.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20251216-ptr_assert-v1-1-d8b2d5c5741d@nvidia.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

We want folks to use `kernel::fmt` but this is only used for `derive` so
can be removed entirely.

This backslid in commit ea60cea07d8c ("rust: add `Alignment` type").

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://patch.msgid.link/20251018-cstr-core-v18-9-9378a54385f8@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Alignment operations are very common in the kernel. Since they are
always performed using a power-of-two value, enforcing this invariant
through a dedicated type leads to fewer bugs and can improve the
generated code.

Introduce the `Alignment` type, inspired by the nightly Rust type of the
same name and providing the same interface, and a new `Alignable` trait
allowing unsigned integers to be aligned up or down.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
[ Used `build_assert!`, added intra-doc link, `allow`ed
  `clippy::incompatible_msrv`, added `feature(const_option)`, capitalized
  safety comment. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>