refactor: try dashmap raw-api

Cargo.toml

@@ -64,7 +64,7 @@ name = "resolver"
 
 [dependencies]
 cfg-if = "1"
-dashmap = "6"
+dashmap = { version = "6", features = ["raw-api"] }
 indexmap = { version = "2", features = ["serde"] }
 json-strip-comments = "1"
 once_cell = "1" # Use `std::sync::OnceLock::get_or_try_init` when it is stable.

src/cache.rs

@@ -61,6 +61,19 @@ impl<Fs: FileSystem> Cache<Fs> {
             array.hash(&mut hasher);
             hasher.finish()
         };
+
+        // A DashMap is just an array of RwLock<HashSet>, sharded by hash to reduce lock contention.
+        // This uses the low level raw API to avoid cloning the value when using the `entry` method.
+        // First, find which shard the value is in, and check to see if we already have a value in the map.
+        let shard = self.paths.determine_shard(hash as usize);
+        {
+            // Scope the read lock.
+            let map = self.paths.shards()[shard].read();
+            if let Some((entry, _)) = map.get(hash, |v| v.0.path() == path) {
+                return entry.clone();
+            }
+        }
+
         if let Some(cache_entry) = self.paths.get((hash, path).borrow() as &dyn CacheKey) {
             return cache_entry.clone();
         }