[pointer] Rename Any -> Unknown, remove aliasing

Don't implement `Aliasing` for `Unknown` - every `Ptr` always has *some*
aliasing that is known at construction.

Author: joshlf

src/pointer/invariant.rs

@@ -81,24 +81,11 @@ pub trait Validity: Sealed {}
 /// Exclusive`.
 pub trait Reference: Aliasing + Sealed {}
 
-/// No requirement - any invariant is allowed.
-pub enum Any {}
-impl Aliasing for Any {
-    const IS_EXCLUSIVE: bool = false;
+/// It is unknown whether any invariant holds.
+pub enum Unknown {}
 
-    // SAFETY: Since we don't know what aliasing model this is, we have to be
-    // conservative. Invariance is strictly more restrictive than any other
-    // variance model, so this can never cause soundness issues.
-    //
-    // `fn() -> T` and `fn(T) -> ()` are covariant and contravariant in `T`,
-    // respectively. [1] Thus, `fn(T) -> T` is invariant in `T`. Thus, `fn(&'a
-    // T) -> &'a T` is invariant in `'a` and `T`.
-    //
-    // [1] https://doc.rust-lang.org/1.81.0/reference/subtyping.html#variance
-    type Variance<'a, T: 'a + ?Sized> = fn(&'a T) -> &'a T;
-}
-impl Alignment for Any {}
-impl Validity for Any {}
+impl Alignment for Unknown {}
+impl Validity for Unknown {}
 
 /// The `Ptr<'a, T>` adheres to the aliasing rules of a `&'a T`.
 ///
@@ -212,7 +199,7 @@ mod sealed {
 
     pub trait Sealed {}
 
-    impl Sealed for Any {}
+    impl Sealed for Unknown {}
 
     impl Sealed for Shared {}
     impl Sealed for Exclusive {}

src/pointer/mod.rs

@@ -24,14 +24,14 @@ use crate::Unaligned;
 /// to [`TryFromBytes::is_bit_valid`].
 ///
 /// [`TryFromBytes::is_bit_valid`]: crate::TryFromBytes::is_bit_valid
-pub type Maybe<'a, T, Aliasing = invariant::Shared, Alignment = invariant::Any> =
+pub type Maybe<'a, T, Aliasing = invariant::Shared, Alignment = invariant::Unknown> =
     Ptr<'a, T, (Aliasing, Alignment, invariant::Initialized)>;
 
 /// A semi-user-facing wrapper type representing a maybe-aligned reference, for
 /// use in [`TryFromBytes::is_bit_valid`].
 ///
 /// [`TryFromBytes::is_bit_valid`]: crate::TryFromBytes::is_bit_valid
-pub type MaybeAligned<'a, T, Aliasing = invariant::Shared, Alignment = invariant::Any> =
+pub type MaybeAligned<'a, T, Aliasing = invariant::Shared, Alignment = invariant::Unknown> =
     Ptr<'a, T, (Aliasing, Alignment, invariant::Valid)>;
 
 // These methods are defined on the type alias, `MaybeAligned`, so as to bring

src/pointer/ptr.rs

@@ -676,7 +676,7 @@ mod _transitions {
         #[doc(hidden)]
         #[must_use]
         #[inline]
-        pub const fn forget_aligned(self) -> Ptr<'a, T, I::WithAlignment<Any>> {
+        pub const fn forget_aligned(self) -> Ptr<'a, T, I::WithAlignment<Unknown>> {
             // SAFETY: `Any` is less restrictive than `Aligned`.
             unsafe { self.assume_invariants() }
         }
@@ -714,7 +714,7 @@ mod _casts {
         pub unsafe fn cast_unsized_unchecked<U: 'a + ?Sized, F: FnOnce(*mut T) -> *mut U>(
             self,
             cast: F,
-        ) -> Ptr<'a, U, (I::Aliasing, Any, Any)> {
+        ) -> Ptr<'a, U, (I::Aliasing, Unknown, Unknown)> {
             let ptr = cast(self.as_inner().as_non_null().as_ptr());
 
             // SAFETY: Caller promises that `cast` returns a pointer whose
@@ -784,7 +784,10 @@ mod _casts {
         /// - `u` has the same provenance as `p`
         #[doc(hidden)]
         #[inline]
-        pub unsafe fn cast_unsized<U, F, R, S>(self, cast: F) -> Ptr<'a, U, (I::Aliasing, Any, Any)>
+        pub unsafe fn cast_unsized<U, F, R, S>(
+            self,
+            cast: F,
+        ) -> Ptr<'a, U, (I::Aliasing, Unknown, Unknown)>
         where
             T: Read<I::Aliasing, R>,
             U: 'a + ?Sized + Read<I::Aliasing, S>,
@@ -1070,7 +1073,7 @@ mod _project {
         pub unsafe fn project<U: 'a + ?Sized>(
             self,
             projector: impl FnOnce(*mut T) -> *mut U,
-        ) -> Ptr<'a, U, (I::Aliasing, Any, Initialized)> {
+        ) -> Ptr<'a, U, (I::Aliasing, Unknown, Initialized)> {
             // TODO(#1122): If `cast_unsized` were able to reason that, when
             // casting from an `Initialized` pointer, the result is another
             // `Initialized` pointer, we could remove this method entirely.

src/util/macro_util.rs

@@ -520,7 +520,7 @@ pub unsafe fn transmute_mut<'dst, 'src: 'dst, Src: 'src, Dst: 'dst>(
 fn try_cast_or_pme<Src, Dst, I, R>(
     src: Ptr<'_, Src, I>,
 ) -> Result<
-    Ptr<'_, Dst, (I::Aliasing, invariant::Any, invariant::Valid)>,
+    Ptr<'_, Dst, (I::Aliasing, invariant::Unknown, invariant::Valid)>,
     ValidityError<Ptr<'_, Src, I>, Dst>,
 >
 where

src/util/mod.rs

@@ -99,11 +99,11 @@ impl<I: invariant::Validity> ValidityVariance<I> for Covariant {
 pub enum Invariant {}
 
 impl<I: invariant::Alignment> AlignmentVariance<I> for Invariant {
-    type Applied = invariant::Any;
+    type Applied = invariant::Unknown;
 }
 
 impl<I: invariant::Validity> ValidityVariance<I> for Invariant {
-    type Applied = invariant::Any;
+    type Applied = invariant::Unknown;
 }
 
 // SAFETY: