[pointer][invariant] Remove AliasingMapping, Inaccessible (#2301) (#2386

)

We previously used `AliasingMapping`s and `Inaccessible` to model
`UnsafeCell` agreement. This abuses the notion of a mapping since one
doesn't ever actually want to change the aliasing of a pointer (and
certainly not to `Inaccessible`) - really this was meant to model
pointer casts which should never be performed. In addition to being an
awkward fit, the presence of `Inaccessible` meant that code could not
assume that any `Aliasing` invariant permitted reading, and so we had to
add extra machinery to work around this.

Future commits will use a different, simpler model for denoting
`UnsafeCell` agreement or disagreement.

While we're here, make `Read` slightly more permissive, implemented for
`A: Aliasing, T: Immutable` rather than just `A: Reference, T:
Immutable`.

Makes progress on #1122, #1866

gherrit-pr-id: I1ac2ae177a235083e33b09fc848423220d3da042

src/pointer/invariant.rs

@@ -27,6 +27,11 @@ impl<A: Aliasing, AA: Alignment, V: Validity> Invariants for (A, AA, V) {
 }
 
 /// The aliasing invariant of a [`Ptr`][super::Ptr].
+///
+/// All aliasing invariants must permit reading from the bytes of a pointer's
+/// referent which are not covered by [`UnsafeCell`]s.
+///
+/// [`UnsafeCell`]: core::cell::UnsafeCell
 pub trait Aliasing: Sealed {
     /// Is `Self` [`Exclusive`]?
     #[doc(hidden)]
@@ -144,7 +149,7 @@ impl Validity for Valid {}
 /// permitted to perform unsynchronized reads from its referent.
 pub trait Read<A: Aliasing, R: ReadReason> {}
 
-impl<A: Reference, T: ?Sized + crate::Immutable> Read<A, BecauseImmutable> for T {}
+impl<A: Aliasing, T: ?Sized + crate::Immutable> Read<A, BecauseImmutable> for T {}
 impl<T: ?Sized> Read<Exclusive, BecauseExclusive> for T {}
 
 /// Used to disambiguate [`Read`] impls.