[PATCH] rust: add C FFI types to the prelude

[Miguel Ojeda <ojeda@kernel.org>]

Rust kernel code is supposed to use the custom mapping of C FFI types,
i.e. those from the `ffi` crate, rather than the ones coming from `core`.

Thus, to minimize mistakes and to simplify the code everywhere, just
provide them in the `kernel` prelude and ask in the Coding Guidelines
to use them directly, i.e. as a single segment path.

After this lands, we can start cleaning up the existing users.

Ideally, we would use something like Clippy's `disallowed-types` to
prevent the use of the `core` ones, but that one sees through aliases.

Link: https://lore.kernel.org/rust-for-linux/CANiq72kc4gzfieD-FjuWfELRDXXD2vLgPv4wqk3nt4pjdPQ=qg@mail.gmail.com/
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 Documentation/rust/coding-guidelines.rst | 17 +++++++++++++++++
 rust/kernel/prelude.rs                   |  5 +++++
 2 files changed, 22 insertions(+)

diff --git a/Documentation/rust/coding-guidelines.rst b/Documentation/rust/coding-guidelines.rst
index 27f2a7bb5a4a..d0bf0b3a058a 100644
--- a/Documentation/rust/coding-guidelines.rst
+++ b/Documentation/rust/coding-guidelines.rst
@@ -191,6 +191,23 @@ or:
 	/// [`struct mutex`]: srctree/include/linux/mutex.h
 
 
+C FFI types
+-----------
+
+Rust kernel code does not use the C FFI types (such as ``c_char``) from
+``core::ffi::*``. Instead, a custom mapping that matches properly the C types
+used in the kernel is provided in the prelude, i.e. ``kernel::prelude::*``.
+
+These types (aliases) should generally be referred directly by their identifier,
+i.e. as a single segment path. For instance:
+
+.. code-block:: rust
+
+	fn f(p: *const c_char) -> c_int {
+	    // ...
+	}
+
+
 Naming
 ------
 
diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
index baa774a351ce..f869b02f1f25 100644
--- a/rust/kernel/prelude.rs
+++ b/rust/kernel/prelude.rs
@@ -14,6 +14,11 @@
 #[doc(no_inline)]
 pub use core::pin::Pin;
 
+pub use ::ffi::{
+    c_char, c_int, c_long, c_longlong, c_schar, c_short, c_uchar, c_uint, c_ulong, c_ulonglong,
+    c_ushort, c_void,
+};
+
 pub use crate::alloc::{flags::*, Box, KBox, KVBox, KVVec, KVec, VBox, VVec, Vec};
 
 #[doc(no_inline)]

base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
-- 
2.49.0