Re: [PATCH v3 08/25] rust: types: implement `Unique`

Boqun Feng
2024-08-04T06:55:19Z

On Thu, Aug 01, 2024 at 02:02:07AM +0200, Danilo Krummrich wrote:
> Implement the `Unique` type as a prerequisite for `Box` and `Vec`
> introduced in subsequent patches.
>
> `Unique` serves as wrapper around a `NonNull`, but indicates that the
> possessor of this wrapper owns the referent.
>
> This type already exists in Rust's core library, but, unfortunately, is
> exposed as unstable API and hence shouldn't be used in the kernel.
>
> This implementation of `Unique` is almost identical, but mostly stripped
> down to the functionality we need for `Box` and `Vec`. Additionally, all
> unstable features are removed and / or replaced by stable ones.
>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

If I understand correctly, `Unique` does nothing unless the compiler
supports putting noalias at the corresponding IRs. If so, should this
depend on whether Rust compiler supports using `Unique` outside std
libraries?

Regards,
Boqun

> ---
> rust/kernel/types.rs | 183 +++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 183 insertions(+)
>
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index bd189d646adb..7cf89067b5fc 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -473,3 +473,186 @@ unsafe impl AsBytes for str {}
> // does not have any uninitialized portions either.
> unsafe impl<T: AsBytes> AsBytes for [T] {}
> unsafe impl<T: AsBytes, const N: usize> AsBytes for [T; N] {}
> +
> +/// A wrapper around a raw non-null `*mut T` that indicates that the possessor
> +/// of this wrapper owns the referent. Useful for building abstractions like
> +/// `Box<T>`, `Vec<T>`, `String`, and `HashMap<K, V>`.
> +///
> +/// Unlike `*mut T`, `Unique<T>` behaves "as if" it were an instance of `T`.
> +/// It implements `Send`/`Sync` if `T` is `Send`/`Sync`. It also implies
> +/// the kind of strong aliasing guarantees an instance of `T` can expect:
> +/// the referent of the pointer should not be modified without a unique path to
> +/// its owning Unique.
> +///
> +/// If you're uncertain of whether it's correct to use `Unique` for your purposes,
> +/// consider using `NonNull`, which has weaker semantics.
> +///
> +/// Unlike `*mut T`, the pointer must always be non-null, even if the pointer
> +/// is never dereferenced. This is so that enums may use this forbidden value
> +/// as a discriminant -- `Option<Unique<T>>` has the same size as `Unique<T>`.
> +/// However the pointer may still dangle if it isn't dereferenced.
> +///
> +/// Unlike `*mut T`, `Unique<T>` is covariant over `T`. This should always be correct
> +/// for any type which upholds Unique's aliasing requirements.
> +#[repr(transparent)]
> +pub struct Unique<T: ?Sized> {
> + pointer: NonNull<T>,
> + // NOTE: this marker has no consequences for variance, but is necessary
> + // for dropck to understand that we logically own a `T`.
> + //
> + // For details, see:
> + // https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-generic-drop.md#phantom-data
> + _marker: PhantomData<T>,
> +}
> +
> +/// `Unique` pointers are `Send` if `T` is `Send` because the data they
> +/// reference is unaliased. Note that this aliasing invariant is
> +/// unenforced by the type system; the abstraction using the
> +/// `Unique` must enforce it.
> +unsafe impl<T: Send + ?Sized> Send for Unique<T> {}
> +
> +/// `Unique` pointers are `Sync` if `T` is `Sync` because the data they
> +/// reference is unaliased. Note that this aliasing invariant is
> +/// unenforced by the type system; the abstraction using the
> +/// `Unique` must enforce it.
> +unsafe impl<T: Sync + ?Sized> Sync for Unique<T> {}
> +
> +impl<T: Sized> Unique<T> {
> + /// Creates a new `Unique` that is dangling, but well-aligned.
> + ///
> + /// This is useful for initializing types which lazily allocate, like
> + /// `Vec::new` does.
> + ///
> + /// Note that the pointer value may potentially represent a valid pointer to
> + /// a `T`, which means this must not be used as a "not yet initialized"
> + /// sentinel value. Types that lazily allocate must track initialization by
> + /// some other means.
> + #[must_use]
> + #[inline]
> + pub const fn dangling() -> Self {
> + Unique {
> + pointer: NonNull::dangling(),
> + _marker: PhantomData,
> + }
> + }
> +}
> +
> +impl<T: ?Sized> Unique<T> {
> + /// Creates a new `Unique`.
> + ///
> + /// # Safety
> + ///
> + /// `ptr` must be non-null.
> + #[inline]
> + pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
> + // SAFETY: the caller must guarantee that `ptr` is non-null.
> + unsafe {
> + Unique {
> + pointer: NonNull::new_unchecked(ptr),
> + _marker: PhantomData,
> + }
> + }
> + }
> +
> + /// Creates a new `Unique` if `ptr` is non-null.
> + #[allow(clippy::manual_map)]
> + #[inline]
> + pub fn new(ptr: *mut T) -> Option<Self> {
> + if let Some(pointer) = NonNull::new(ptr) {
> + Some(Unique {
> + pointer,
> + _marker: PhantomData,
> + })
> + } else {
> + None
> + }
> + }
> +
> + /// Acquires the underlying `*mut` pointer.
> + #[must_use = "`self` will be dropped if the result is not used"]
> + #[inline]
> + pub const fn as_ptr(self) -> *mut T {
> + self.pointer.as_ptr()
> + }
> +
> + /// Dereferences the content.
> + ///
> + /// The resulting lifetime is bound to self so this behaves "as if"
> + /// it were actually an instance of T that is getting borrowed. If a longer
> + /// (unbound) lifetime is needed, use `&*my_ptr.as_ptr()`.
> + ///
> + /// # Safety
> + ///
> + /// Safety requirements for this function are inherited from [NonNull::as_ref].
> + ///
> + #[must_use]
> + #[inline]
> + pub const unsafe fn as_ref(&self) -> &T {
> + // SAFETY: the caller must guarantee that `self` meets all the
> + // requirements for a reference.
> + unsafe { self.pointer.as_ref() }
> + }
> +
> + /// Mutably dereferences the content.
> + ///
> + /// The resulting lifetime is bound to self so this behaves "as if"
> + /// it were actually an instance of T that is getting borrowed. If a longer
> + /// (unbound) lifetime is needed, use `&mut *my_ptr.as_ptr()`.
> + ///
> + /// # Safety
> + ///
> + /// Safety requirements for this function are inherited from [NonNull::as_mut].
> + #[must_use]
> + #[inline]
> + pub unsafe fn as_mut(&mut self) -> &mut T {
> + // SAFETY: the caller must guarantee that `self` meets all the
> + // requirements for a mutable reference.
> + unsafe { self.pointer.as_mut() }
> + }
> +
> + /// Casts to a pointer of another type.
> + #[must_use = "`self` will be dropped if the result is not used"]
> + #[inline]
> + pub fn cast(self) -> Unique {
> + Unique::from(self.pointer.cast())
> + }
> +
> + /// Acquires the underlying `*mut` pointer.
> + #[must_use = "`self` will be dropped if the result is not used"]
> + #[inline]
> + pub const fn as_non_null(self) -> NonNull<T> {
> + self.pointer
> + }
> +}
> +
> +impl<T: ?Sized> Clone for Unique<T> {
> + #[inline]
> + fn clone(&self) -> Self {
> + *self
> + }
> +}
> +
> +impl<T: ?Sized> Copy for Unique<T> {}
> +
> +impl<T: ?Sized> From<&mut T> for Unique<T> {
> + /// Converts a `&mut T` to a `Unique<T>`.
> + ///
> + /// This conversion is infallible since references cannot be null.
> + #[inline]
> + fn from(reference: &mut T) -> Self {
> + Self::from(NonNull::from(reference))
> + }
> +}
> +
> +impl<T: ?Sized> From<NonNull<T>> for Unique<T> {
> + /// Converts a `NonNull<T>` to a `Unique<T>`.
> + ///
> + /// This conversion is infallible since `NonNull` cannot be null.
> + #[inline]
> + fn from(pointer: NonNull<T>) -> Self {
> + Unique {
> + pointer,
> + _marker: PhantomData,
> + }
> + }
> +}
> --
> 2.45.2
>