rust: add UnsafePinned type

`UnsafePinned<T>` is useful for cases where a value might be shared with
C code but not directly used by it. In particular this is added for
storing additional data in the `MiscDeviceRegistration` which will be
shared between `fops->open` and the containing struct.

Similar to `Opaque` but guarantees that the value is always initialized
and that the inner value is dropped when `UnsafePinned` is dropped.

This was originally proposed for the IRQ abstractions [0] and is also
useful for other where the inner data may be aliased, but is always
valid and automatic `Drop` is desired.

Since then the `UnsafePinned` type was added to upstream Rust [1] by Sky
as a unstable feature, therefore this patch implements the subset of the
upstream API for the `UnsafePinned` type required for additional data in
`MiscDeviceRegistration` and in the implementation of the `Opaque` type.

Some differences to the upstream type definition are required in the
kernel implementation, because upstream type uses some compiler changes
to opt out of certain optimizations, this is documented in the
documentation and a comment on the `UnsafePinned` type.

The documentation on is based on the upstream rust documentation with
minor modifications for the kernel implementation.

Link: https://lore.kernel.org/rust-for-linux/CAH5fLgiOASgjoYKFz6kWwzLaH07DqP2ph+3YyCDh2+gYqGpABA@mail.gmail.com [0]
Link: rust-lang/rust#137043 [1]
Suggested-by: Alice Ryhl <[email protected]>
Co-developed-by: Sky <[email protected]>
Signed-off-by: Sky <[email protected]>
Signed-off-by: Christian Schrefl <[email protected]>

Author: onestacked

init/Kconfig

@@ -140,6 +140,9 @@ config LD_CAN_USE_KEEP_IN_OVERLAY
 config RUSTC_HAS_COERCE_POINTEE
 	def_bool RUSTC_VERSION >= 108400
 
+config RUSTC_HAS_UNSAFE_PINNED
+	def_bool RUSTC_VERSION >= 108800
+
 config PAHOLE_VERSION
 	int
 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))

rust/kernel/lib.rs

@@ -17,6 +17,7 @@
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(coerce_unsized))]
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(dispatch_from_dyn))]
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(unsize))]
+#![cfg_attr(CONFIG_RUSTC_HAS_UNSAFE_PINNED, feature(unsafe_pinned))]
 #![feature(inline_const)]
 #![feature(lint_reasons)]
 // Stable in Rust 1.82