Enforce T: Hash for Interned<...>

This adds panicking Hash impls for several resolver types that don't
actually satisfy this condition. It's not obvious to me that
rustc_resolve actually upholds the Interned guarantees but fixing that
seems pretty hard (the structures have at minimum some interior
mutability, so it's not really recursively hashable in place...).

Author: Mark-Simulacrum

compiler/rustc_data_structures/src/intern.rs

@@ -92,7 +92,10 @@ impl<'a, T: Ord> Ord for Interned<'a, T> {
     }
 }
 
-impl<'a, T> Hash for Interned<'a, T> {
+impl<'a, T> Hash for Interned<'a, T>
+where
+    T: Hash,
+{
     #[inline]
     fn hash<H: Hasher>(&self, s: &mut H) {
         // Pointer hashing is sufficient, due to the uniqueness constraint.

compiler/rustc_resolve/src/imports.rs

@@ -182,6 +182,19 @@ pub(crate) struct ImportData<'ra> {
 /// so we can use referential equality to compare them.
 pub(crate) type Import<'ra> = Interned<'ra, ImportData<'ra>>;
 
+// Allows us to use Interned without actually enforcing (via Hash/PartialEq/...) uniqueness of the
+// contained data.
+// FIXME: We may wish to actually have at least debug-level assertions that Interned's guarantees
+// are upheld.
+impl std::hash::Hash for ImportData<'_> {
+    fn hash<H>(&self, _: &mut H)
+    where
+        H: std::hash::Hasher,
+    {
+        unreachable!()
+    }
+}
+
 impl<'ra> ImportData<'ra> {
     pub(crate) fn is_glob(&self) -> bool {
         matches!(self.kind, ImportKind::Glob { .. })

compiler/rustc_resolve/src/lib.rs

@@ -589,6 +589,19 @@ struct ModuleData<'ra> {
 #[rustc_pass_by_value]
 struct Module<'ra>(Interned<'ra, ModuleData<'ra>>);
 
+// Allows us to use Interned without actually enforcing (via Hash/PartialEq/...) uniqueness of the
+// contained data.
+// FIXME: We may wish to actually have at least debug-level assertions that Interned's guarantees
+// are upheld.
+impl std::hash::Hash for ModuleData<'_> {
+    fn hash<H>(&self, _: &mut H)
+    where
+        H: std::hash::Hasher,
+    {
+        unreachable!()
+    }
+}
+
 impl<'ra> ModuleData<'ra> {
     fn new(
         parent: Option<Module<'ra>>,
@@ -739,6 +752,19 @@ struct NameBindingData<'ra> {
 /// so we can use referential equality to compare them.
 type NameBinding<'ra> = Interned<'ra, NameBindingData<'ra>>;
 
+// Allows us to use Interned without actually enforcing (via Hash/PartialEq/...) uniqueness of the
+// contained data.
+// FIXME: We may wish to actually have at least debug-level assertions that Interned's guarantees
+// are upheld.
+impl std::hash::Hash for NameBindingData<'_> {
+    fn hash<H>(&self, _: &mut H)
+    where
+        H: std::hash::Hasher,
+    {
+        unreachable!()
+    }
+}
+
 trait ToNameBinding<'ra> {
     fn to_name_binding(self, arenas: &'ra ResolverArenas<'ra>) -> NameBinding<'ra>;
 }