rust: use custom FFI integer types

commit d072acda4862f095ec9056979b654cc06a22cc68 upstream.

Currently FFI integer types are defined in libcore. This commit creates
the `ffi` crate and asks bindgen to use that crate for FFI integer types
instead of `core::ffi`.

This commit is preparatory and no type changes are made in this commit
yet.

Signed-off-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240913213041.395655-4-gary@garyguo.net
[ Added `rustdoc`, `rusttest` and KUnit tests support. Rebased on top of
  `rust-next` (e.g. migrated more `core::ffi` cases). Reworded crate
  docs slightly and formatted. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

rust/kernel/sync/arc.rs

@@ -332,11 +332,11 @@ impl<T: ?Sized> Arc<T> {
 impl<T: 'static> ForeignOwnable for Arc<T> {
     type Borrowed<'a> = ArcBorrow<'a, T>;
 
-    fn into_foreign(self) -> *const core::ffi::c_void {
+    fn into_foreign(self) -> *const crate::ffi::c_void {
         ManuallyDrop::new(self).ptr.as_ptr() as _
     }
 
-    unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
+    unsafe fn borrow<'a>(ptr: *const crate::ffi::c_void) -> ArcBorrow<'a, T> {
         // By the safety requirement of this function, we know that `ptr` came from
         // a previous call to `Arc::into_foreign`.
         let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();
@@ -346,7 +346,7 @@ impl<T: 'static> ForeignOwnable for Arc<T> {
         unsafe { ArcBorrow::new(inner) }
     }
 
-    unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
+    unsafe fn from_foreign(ptr: *const crate::ffi::c_void) -> Self {
         // SAFETY: By the safety requirement of this function, we know that `ptr` came from
         // a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
         // holds a reference count increment that is transferrable to us.

rust/kernel/sync/condvar.rs

@@ -7,6 +7,7 @@
 
 use super::{lock::Backend, lock::Guard, LockClassKey};
 use crate::{
+    ffi::{c_int, c_long},
     init::PinInit,
     pin_init,
     str::CStr,
@@ -14,7 +15,6 @@ use crate::{
     time::Jiffies,
     types::Opaque,
 };
-use core::ffi::{c_int, c_long};
 use core::marker::PhantomPinned;
 use core::ptr;
 use macros::pin_data;

rust/kernel/sync/lock.rs

@@ -46,7 +46,7 @@ pub unsafe trait Backend {
     /// remain valid for read indefinitely.
     unsafe fn init(
         ptr: *mut Self::State,
-        name: *const core::ffi::c_char,
+        name: *const crate::ffi::c_char,
         key: *mut bindings::lock_class_key,
     );
 

rust/kernel/sync/lock/mutex.rs

@@ -96,7 +96,7 @@ unsafe impl super::Backend for MutexBackend {
 
     unsafe fn init(
         ptr: *mut Self::State,
-        name: *const core::ffi::c_char,
+        name: *const crate::ffi::c_char,
         key: *mut bindings::lock_class_key,
     ) {
         // SAFETY: The safety requirements ensure that `ptr` is valid for writes, and `name` and

rust/kernel/sync/lock/spinlock.rs

@@ -95,7 +95,7 @@ unsafe impl super::Backend for SpinLockBackend {
 
     unsafe fn init(
         ptr: *mut Self::State,
-        name: *const core::ffi::c_char,
+        name: *const crate::ffi::c_char,
         key: *mut bindings::lock_class_key,
     ) {
         // SAFETY: The safety requirements ensure that `ptr` is valid for writes, and `name` and