Introduce ConstAllocation.

Currently some `Allocation`s are interned, some are not, and it's very
hard to tell at a use point which is which.

This commit introduces `ConstAllocation` for the known-interned ones,
which makes the division much clearer. `ConstAllocation::inner()` is
used to get the underlying `Allocation`.

In some places it's natural to use an `Allocation`, in some it's natural
to use a `ConstAllocation`, and in some places there's no clear choice.
I've tried to make things look as nice as possible, while generally
favouring `ConstAllocation`, which is the type that embodies more
information. This does require quite a few calls to `inner()`.

The commit also tweaks how `PartialOrd` works for `Interned`. The
previous code was too clever by half, building on `T: Ord` to make the
code shorter. That caused problems with deriving `PartialOrd` and `Ord`
for `ConstAllocation`, so I changed it to build on `T: PartialOrd`,
which is slightly more verbose but much more standard and avoided the
problems.

compiler/rustc_data_structures/src/intern.rs

@@ -62,13 +62,17 @@ impl<'a, T> PartialEq for Interned<'a, T> {
 
 impl<'a, T> Eq for Interned<'a, T> {}
 
-// In practice you can't intern any `T` that doesn't implement `Eq`, because
-// that's needed for hashing. Therefore, we won't be interning any `T` that
-// implements `PartialOrd` without also implementing `Ord`. So we can have the
-// bound `T: Ord` here and avoid duplication with the `Ord` impl below.
-impl<'a, T: Ord> PartialOrd for Interned<'a, T> {
+impl<'a, T: PartialOrd> PartialOrd for Interned<'a, T> {
     fn partial_cmp(&self, other: &Interned<'a, T>) -> Option<Ordering> {
-        Some(self.cmp(other))
+        // Pointer equality implies equality, due to the uniqueness constraint,
+        // but the contents must be compared otherwise.
+        if ptr::eq(self.0, other.0) {
+            Some(Ordering::Equal)
+        } else {
+            let res = self.0.partial_cmp(&other.0);
+            debug_assert_ne!(res, Some(Ordering::Equal));
+            res
+        }
     }
 }