Auto merge of #95524 - oli-obk:cached_stable_hash_cleanups, r=nnethercote

Cached stable hash cleanups

r? `@nnethercote`

Add a sanity assertion in debug mode to check that the cached hashes are actually the ones we get if we compute the hash each time.

Add a new data structure that bundles all the hash-caching work to make it easier to re-use it for different interned data structures

compiler/rustc_data_structures/src/intern.rs

@@ -4,6 +4,8 @@ use std::hash::{Hash, Hasher};
 use std::ops::Deref;
 use std::ptr;
 
+use crate::fingerprint::Fingerprint;
+
 mod private {
     #[derive(Clone, Copy, Debug)]
     pub struct PrivateZst;
@@ -108,5 +110,87 @@ where
     }
 }
 
+/// A helper trait so that `Interned` things can cache stable hashes reproducibly.
+pub trait InternedHashingContext {
+    fn with_def_path_and_no_spans(&mut self, f: impl FnOnce(&mut Self));
+}
+
+/// A helper type that you can wrap round your own type in order to automatically
+/// cache the stable hash on creation and not recompute it whenever the stable hash
+/// of the type is computed.
+/// This is only done in incremental mode. You can also opt out of caching by using
+/// StableHash::ZERO for the hash, in which case the hash gets computed each time.
+/// This is useful if you have values that you intern but never (can?) use for stable
+/// hashing.
+#[derive(Copy, Clone)]
+pub struct WithStableHash<T> {
+    pub internee: T,
+    pub stable_hash: Fingerprint,
+}
+
+impl<T: PartialEq> PartialEq for WithStableHash<T> {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        self.internee.eq(&other.internee)
+    }
+}
+
+impl<T: Eq> Eq for WithStableHash<T> {}
+
+impl<T: Ord> PartialOrd for WithStableHash<T> {
+    fn partial_cmp(&self, other: &WithStableHash<T>) -> Option<Ordering> {
+        Some(self.internee.cmp(&other.internee))
+    }
+}
+
+impl<T: Ord> Ord for WithStableHash<T> {
+    fn cmp(&self, other: &WithStableHash<T>) -> Ordering {
+        self.internee.cmp(&other.internee)
+    }
+}
+
+impl<T> Deref for WithStableHash<T> {
+    type Target = T;
+
+    #[inline]
+    fn deref(&self) -> &T {
+        &self.internee
+    }
+}
+
+impl<T: Hash> Hash for WithStableHash<T> {
+    #[inline]
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        self.internee.hash(s)
+    }
+}
+
+impl<T: HashStable<CTX>, CTX: InternedHashingContext> HashStable<CTX> for WithStableHash<T> {
+    fn hash_stable(&self, hcx: &mut CTX, hasher: &mut StableHasher) {
+        if self.stable_hash == Fingerprint::ZERO || cfg!(debug_assertions) {
+            // No cached hash available. This can only mean that incremental is disabled.
+            // We don't cache stable hashes in non-incremental mode, because they are used
+            // so rarely that the performance actually suffers.
+
+            // We need to build the hash as if we cached it and then hash that hash, as
+            // otherwise the hashes will differ between cached and non-cached mode.
+            let stable_hash: Fingerprint = {
+                let mut hasher = StableHasher::new();
+                hcx.with_def_path_and_no_spans(|hcx| self.internee.hash_stable(hcx, &mut hasher));
+                hasher.finish()
+            };
+            if cfg!(debug_assertions) && self.stable_hash != Fingerprint::ZERO {
+                assert_eq!(
+                    stable_hash, self.stable_hash,
+                    "cached stable hash does not match freshly computed stable hash"
+                );
+            }
+            stable_hash.hash_stable(hcx, hasher);
+        } else {
+            self.stable_hash.hash_stable(hcx, hasher);
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests;