Inline the dep-graph reverse-index lookup fast path

node_to_index_opt is the reverse-index lookup used while marking dep nodes
green, so it runs many times per incremental rebuild. The per-kind map build
sat inside the get_or_init closure, which kept the lookup from inlining, so
every lookup paid an out-of-line call even after the map was built.

Move the build into build_fingerprint_map and call get_or_init directly in
node_to_index_opt, so the lookup fast path is a plain OnceLock load. No data
structures change.

Author: xmakro

compiler/rustc_middle/src/dep_graph/serialized.rs

@@ -168,39 +168,35 @@ struct LazyKindIndex {
 
 impl LazyKindIndex {
     /// Returns this kind's `key_fingerprint -> node index` map.
-    fn fingerprint_map(
+    fn build_fingerprint_map(
         &self,
         kind: DepKind,
         nodes: &IndexSlice<SerializedDepNodeIndex, DepNode>,
         nodes_by_kind: &[Option<SerializedDepNodeIndex>],
         profiler: &Option<SelfProfilerRef>,
-    ) -> &UnhashMap<PackedFingerprint, SerializedDepNodeIndex> {
-        self.map.get_or_init(|| {
-            let _prof_timer = profiler
-                .as_ref()
-                .map(|p| p.generic_activity("incr_comp_load_dep_graph_reverse_index"));
-            let range = (self.start as usize)..(self.start as usize + self.len as usize);
-            let mut map =
-                UnhashMap::with_capacity_and_hasher(self.len as usize, Default::default());
-            for &idx in &nodes_by_kind[range] {
-                let idx = idx.expect("counting sort fills every slot of a kind's range");
-                let node = nodes[idx];
-                debug_assert_eq!(node.kind, kind);
-                if map.insert(node.key_fingerprint, idx).is_some()
-                    // Side effect nodes can legitimately share a fingerprint.
-                    && node.kind != DepKind::SideEffect
-                {
-                    panic!(
-                        "Error: A dep graph node ({kind:?}) does not have an unique index. \
-                         Running a clean build on a nightly compiler with \
-                         `-Z incremental-verify-ich` can help narrow down the issue for reporting. \
-                         A clean build may also work around the issue.\n
-                         DepNode: {node:?}"
-                    )
-                }
+    ) -> UnhashMap<PackedFingerprint, SerializedDepNodeIndex> {
+        let _prof_timer =
+            profiler.as_ref().map(|p| p.generic_activity("incr_comp_load_dep_graph_reverse_index"));
+        let range = (self.start as usize)..(self.start as usize + self.len as usize);
+        let mut map = UnhashMap::with_capacity_and_hasher(self.len as usize, Default::default());
+        for &idx in &nodes_by_kind[range] {
+            let idx = idx.expect("counting sort fills every slot of a kind's range");
+            let node = nodes[idx];
+            debug_assert_eq!(node.kind, kind);
+            if map.insert(node.key_fingerprint, idx).is_some()
+                // Side effect nodes can legitimately share a fingerprint.
+                && node.kind != DepKind::SideEffect
+            {
+                panic!(
+                    "Error: A dep graph node ({kind:?}) does not have an unique index. \
+                     Running a clean build on a nightly compiler with \
+                     `-Z incremental-verify-ich` can help narrow down the issue for reporting. \
+                     A clean build may also work around the issue.\n
+                     DepNode: {node:?}"
+                )
             }
-            map
-        })
+        }
+        map
     }
 }
 
@@ -235,12 +231,14 @@ impl SerializedDepGraph {
     #[inline]
     pub fn node_to_index_opt(&self, dep_node: &DepNode) -> Option<SerializedDepNodeIndex> {
         let kind = self.reverse_index.kinds.get(dep_node.kind.as_usize())?;
-        let map = kind.fingerprint_map(
-            dep_node.kind,
-            &self.nodes,
-            &self.reverse_index.nodes_by_kind,
-            &self.profiler,
-        );
+        let map = kind.map.get_or_init(|| {
+            kind.build_fingerprint_map(
+                dep_node.kind,
+                &self.nodes,
+                &self.reverse_index.nodes_by_kind,
+                &self.profiler,
+            )
+        });
         map.get(&dep_node.key_fingerprint).copied()
     }