impl StrongHash for Hashed

Summary:
Implement `StrongHash` for `Hashed<T>` if T also uses `StrongHash`.

This is working up to implementing `StrongHash` for configuration data structures so we can fix hash collision in configuration hashes.

Reviewed By: samkevich

Differential Revision: D68197230

fbshipit-source-id: aa236b070fb20004930e631abab352bd017d8be9

Author: Scott Cao

Cargo.toml

@@ -4,6 +4,8 @@ members = [
     "allocative/allocative_derive",
     "gazebo/display_container",
     "gazebo/dupe",
+    "gazebo/strong_hash",
+    "gazebo/strong_hash_derive",
     "starlark",
     "starlark_bin",
     "starlark_derive",
@@ -24,6 +26,8 @@ allocative_derive = { version = "0.3.3", path = "allocative/allocative_derive" }
 cmp_any = { version = "0.8.1", path = "gazebo/cmp_any" }
 display_container = { version = "0.9.0", path = "gazebo/display_container" }
 dupe = { version = "0.9.1", path = "gazebo/dupe" }
+strong_hash = { path = "gazebo/strong_hash" }
+strong_hash_derive = { path = "gazebo/strong_hash_derive" }
 
 starlark_lsp = { version = "0.13", path = "starlark_lsp" }
 starlark_map = { version = "0.13", path = "starlark_map" }

starlark_map/Cargo.toml

@@ -18,6 +18,7 @@ equivalent = { workspace = true }
 fxhash = "0.2.1"
 hashbrown = { version = "0.14.3", features = ["raw"] }
 serde = { version = "1.0", features = ["derive"] }
+strong_hash = { workspace = true }
 
 [dev-dependencies]
 serde_json = "1.0.48"

starlark_map/src/hashed.rs

@@ -24,6 +24,8 @@ use std::ops::Deref;
 use allocative::Allocative;
 use dupe::Dupe;
 use equivalent::Equivalent;
+use strong_hash::StrongHash;
+use strong_hash::StrongHasher;
 
 use crate::hash_value::StarlarkHashValue;
 
@@ -43,6 +45,13 @@ impl<K> Hash for Hashed<K> {
     }
 }
 
+impl<K: StrongHash> StrongHash for Hashed<K> {
+    fn strong_hash<S: StrongHasher>(&self, state: &mut S) {
+        // Only hash the key, not the (weak) hash.
+        self.key.strong_hash(state);
+    }
+}
+
 impl<K> Deref for Hashed<K> {
     type Target = K;