Use the interned stable hash as plain hash.

Author: cjgillot

compiler/rustc_data_structures/src/intern.rs

@@ -156,7 +156,11 @@ impl<T> Deref for WithStableHash<T> {
 impl<T: Hash> Hash for WithStableHash<T> {
     #[inline]
     fn hash<H: Hasher>(&self, s: &mut H) {
-        self.internee.hash(s)
+        if self.stable_hash != Fingerprint::ZERO {
+            self.stable_hash.hash(s)
+        } else {
+            self.internee.hash(s)
+        }
     }
 }
 

compiler/rustc_middle/src/ty/context.rs

@@ -192,9 +192,7 @@ impl<'tcx> CtxtInterners<'tcx> {
 
                     // It's impossible to hash inference variables (and will ICE), so we don't need to try to cache them.
                     // Without incremental, we rarely stable-hash types, so let's not do it proactively.
-                    let stable_hash = if flags.flags.intersects(TypeFlags::NEEDS_INFER)
-                        || sess.opts.incremental.is_none()
-                    {
+                    let stable_hash = if flags.flags.intersects(TypeFlags::NEEDS_INFER) {
                         Fingerprint::ZERO
                     } else {
                         let mut hasher = StableHasher::new();