rust: init: add assert_pinned macro

Add a macro to statically check if a field of a struct is marked with
`#[pin]` ie that it is structurally pinned. This can be used when
`unsafe` code needs to rely on fields being structurally pinned.

The macro has a special "inline" mode for the case where the type
depends on generic parameters from the surrounding scope.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240814-linked-list-v5-1-f5f5e8075da0@google.com
[ Replaced `compile_fail` with `ignore` and a TODO note. Removed
  `pub` from example to clean `unreachable_pub` lint. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/init/__internal.rs

@@ -228,3 +228,32 @@ impl OnlyCallFromDrop {
         Self(())
     }
 }
+
+/// Initializer that always fails.
+///
+/// Used by [`assert_pinned!`].
+///
+/// [`assert_pinned!`]: crate::assert_pinned
+pub struct AlwaysFail<T: ?Sized> {
+    _t: PhantomData<T>,
+}
+
+impl<T: ?Sized> AlwaysFail<T> {
+    /// Creates a new initializer that always fails.
+    pub fn new() -> Self {
+        Self { _t: PhantomData }
+    }
+}
+
+impl<T: ?Sized> Default for AlwaysFail<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+// SAFETY: `__pinned_init` always fails, which is always okay.
+unsafe impl<T: ?Sized> PinInit<T, ()> for AlwaysFail<T> {
+    unsafe fn __pinned_init(self, _slot: *mut T) -> Result<(), ()> {
+        Err(())
+    }
+}