Rename PtrKey as Interned and improve it.

In particular, there's now more protection against incorrect usage,
because you can only create one via `Interned::new_unchecked`, which
makes it more obvious that you must be careful.

There are also some tests.

compiler/rustc_data_structures/src/intern.rs

@@ -0,0 +1,102 @@
+use std::cmp::Ordering;
+use std::hash::{Hash, Hasher};
+use std::ops::Deref;
+use std::ptr;
+
+mod private {
+    #[derive(Clone, Copy, Debug)]
+    pub struct PrivateZst;
+}
+
+/// A reference to a value that is interned, and is known to be unique.
+///
+/// Note that it is possible to have a `T` and a `Interned<T>` that are (or
+/// refer to) equal but different values. But if you have two different
+/// `Interned<T>`s, they both refer to the same value, at a single location in
+/// memory. This means that equality and hashing can be done on the value's
+/// address rather than the value's contents, which can improve performance.
+///
+/// The `PrivateZst` field means you can pattern match with `Interned(v, _)`
+/// but you can only construct a `Interned` with `new_unchecked`, and not
+/// directly.
+#[derive(Debug)]
+#[cfg_attr(not(bootstrap), rustc_pass_by_value)]
+pub struct Interned<'a, T>(pub &'a T, pub private::PrivateZst);
+
+impl<'a, T> Interned<'a, T> {
+    /// Create a new `Interned` value. The value referred to *must* be interned
+    /// and thus be unique, and it *must* remain unique in the future. This
+    /// function has `_unchecked` in the name but is not `unsafe`, because if
+    /// the uniqueness condition is violated condition it will cause incorrect
+    /// behaviour but will not affect memory safety.
+    #[inline]
+    pub const fn new_unchecked(t: &'a T) -> Self {
+        Interned(t, private::PrivateZst)
+    }
+}
+
+impl<'a, T> Clone for Interned<'a, T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<'a, T> Copy for Interned<'a, T> {}
+
+impl<'a, T> Deref for Interned<'a, T> {
+    type Target = T;
+
+    #[inline]
+    fn deref(&self) -> &T {
+        self.0
+    }
+}
+
+impl<'a, T> PartialEq for Interned<'a, T> {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        // Pointer equality implies equality, due to the uniqueness constraint.
+        ptr::eq(self.0, other.0)
+    }
+}
+
+impl<'a, T> Eq for Interned<'a, T> {}
+
+impl<'a, T: PartialOrd> PartialOrd for Interned<'a, T> {
+    fn partial_cmp(&self, other: &Interned<'a, T>) -> Option<Ordering> {
+        // 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!(res != Some(Ordering::Equal));
+            res
+        }
+    }
+}
+
+impl<'a, T: Ord> Ord for Interned<'a, T> {
+    fn cmp(&self, other: &Interned<'a, T>) -> Ordering {
+        // Pointer equality implies equality, due to the uniqueness constraint,
+        // but the contents must be compared otherwise.
+        if ptr::eq(self.0, other.0) {
+            Ordering::Equal
+        } else {
+            let res = self.0.cmp(&other.0);
+            debug_assert!(res != Ordering::Equal);
+            res
+        }
+    }
+}
+
+impl<'a, T> Hash for Interned<'a, T> {
+    #[inline]
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        // Pointer hashing is sufficient, due to the uniqueness constraint.
+        ptr::hash(self.0, s)
+    }
+}
+
+#[cfg(test)]
+mod tests;