[pointer] Clarify semantics of aliasing invariants

Previously, we supported the `AtLeast` bound, which was used to describe
a subset relationship in which `I: AtLeast<J>` implied that `I` as at
least as restrictive as `J`. However, as described in #1866, this
incorrectly models invariants as monotonic. In reality, invariants both
provide guarantees but also *require* guarantees.

This commit takes a step in the direction of resolving #1866 by removing
`AtLeast`. Uses of `AtLeast<Shared>` are replaced by a new `Reference`
trait, which is implemented for `Shared` and `Exclusive`. This serves
two purposes: First, it makes it explicit what this bound means.
Previously, `AtLeast<Shared>` had an ambiguous meaning, while
`Reference` means precisely that an invariant is either `Shared` or
`Exclusive` and nothing else. Second, it paves the way for #1183, in
which we may add new aliasing invariants which convey ownership. In that
case, it will be important for existing methods to add `Reference`
bounds when those methods would not be sound in the face of ownership
semantics.

We also inline the items in the `invariant` module, which were
previously generated by macro. The addition of the `Reference` trait did
not play nicely with that macro, and we will likely need to go further
from the macro in order to fix #1839 – this fix will likely require
making aliasing invariants meaningfully different than other invariants,
for example by adding an associated type.

Makes progress on #1866

src/impls.rs

@@ -657,9 +657,7 @@ unsafe impl<T: TryFromBytes + ?Sized> TryFromBytes for UnsafeCell<T> {
     }
 
     #[inline]
-    fn is_bit_valid<A: invariant::Aliasing + invariant::AtLeast<invariant::Shared>>(
-        candidate: Maybe<'_, Self, A>,
-    ) -> bool {
+    fn is_bit_valid<A: invariant::Reference>(candidate: Maybe<'_, Self, A>) -> bool {
         // The only way to implement this function is using an exclusive-aliased
         // pointer. `UnsafeCell`s cannot be read via shared-aliased pointers
         // (other than by using `unsafe` code, which we can't use since we can't
@@ -1134,21 +1132,15 @@ mod tests {
 
             pub(super) trait TestIsBitValidShared<T: ?Sized> {
                 #[allow(clippy::needless_lifetimes)]
-                fn test_is_bit_valid_shared<
-                    'ptr,
-                    A: invariant::Aliasing + invariant::AtLeast<invariant::Shared>,
-                >(
+                fn test_is_bit_valid_shared<'ptr, A: invariant::Reference>(
                     &self,
                     candidate: Maybe<'ptr, T, A>,
                 ) -> Option<bool>;
             }
 
             impl<T: TryFromBytes + Immutable + ?Sized> TestIsBitValidShared<T> for AutorefWrapper<T> {
                 #[allow(clippy::needless_lifetimes)]
-                fn test_is_bit_valid_shared<
-                    'ptr,
-                    A: invariant::Aliasing + invariant::AtLeast<invariant::Shared>,
-                >(
+                fn test_is_bit_valid_shared<'ptr, A: invariant::Reference>(
                     &self,
                     candidate: Maybe<'ptr, T, A>,
                 ) -> Option<bool> {
@@ -1238,7 +1230,7 @@ mod tests {
                 #[allow(unused, non_local_definitions)]
                 impl AutorefWrapper<$ty> {
                     #[allow(clippy::needless_lifetimes)]
-                    fn test_is_bit_valid_shared<'ptr, A: invariant::Aliasing + invariant::AtLeast<invariant::Shared>>(
+                    fn test_is_bit_valid_shared<'ptr, A: invariant::Reference>(
                         &mut self,
                         candidate: Maybe<'ptr, $ty, A>,
                     ) -> Option<bool> {

src/lib.rs

@@ -1321,9 +1321,7 @@ pub unsafe trait TryFromBytes {
     /// [`UnsafeCell`]: core::cell::UnsafeCell
     /// [`Shared`]: invariant::Shared
     #[doc(hidden)]
-    fn is_bit_valid<A: invariant::Aliasing + invariant::AtLeast<invariant::Shared>>(
-        candidate: Maybe<'_, Self, A>,
-    ) -> bool;
+    fn is_bit_valid<A: invariant::Reference>(candidate: Maybe<'_, Self, A>) -> bool;
 
     /// Attempts to interpret the given `source` as a `&Self`.
     ///

src/pointer/mod.rs

@@ -35,7 +35,7 @@ pub type MaybeAligned<'a, T, Aliasing = invariant::Shared, Alignment = invariant
 impl<'a, T, Aliasing, Alignment> MaybeAligned<'a, T, Aliasing, Alignment>
 where
     T: 'a + ?Sized,
-    Aliasing: invariant::Aliasing + invariant::AtLeast<invariant::Shared>,
+    Aliasing: invariant::Reference,
     Alignment: invariant::Alignment,
 {
     /// Reads the value from `MaybeAligned`.
@@ -47,11 +47,20 @@ where
     {
         let raw = self.as_non_null().as_ptr();
         // SAFETY: By invariant on `MaybeAligned`, `raw` contains
-        // validly-initialized data for `T`. The value is safe to read and
-        // return, because `T` is copy.
+        // validly-initialized data for `T`. By `Aliasing: Reference`,
+        // `Aliasing` is either `Shared` or `Exclusive`, both of which ensure
+        // that it is sound to perform this read. By `T: Copy`, the value is
+        // safe to return.
         unsafe { core::ptr::read_unaligned(raw) }
     }
+}
 
+impl<'a, T, Aliasing, Alignment> MaybeAligned<'a, T, Aliasing, Alignment>
+where
+    T: 'a + ?Sized,
+    Aliasing: invariant::Reference,
+    Alignment: invariant::Alignment,
+{
     /// Views the value as an aligned reference.
     ///
     /// This is only available if `T` is [`Unaligned`].
@@ -70,7 +79,7 @@ pub(crate) fn is_zeroed<T, I>(ptr: Ptr<'_, T, I>) -> bool
 where
     T: crate::Immutable + crate::KnownLayout,
     I: invariant::Invariants<Validity = invariant::Initialized>,
-    I::Aliasing: invariant::AtLeast<invariant::Shared>,
+    I::Aliasing: invariant::Reference,
 {
     ptr.as_bytes::<BecauseImmutable>().as_ref().iter().all(|&byte| byte == 0)
 }