switch to deterministic hasher

src/util.rs

@@ -2,15 +2,21 @@
 
 use std::fmt::Display;
 
+use symbol_table::DeterministicHashBuilder;
+
 use crate::core::SpecializedPrimitive;
 #[allow(unused_imports)]
 use crate::*;
 
 pub(crate) type BuildHasher = std::hash::BuildHasherDefault<rustc_hash::FxHasher>;
 
-pub(crate) type HashMap<K, V> = hashbrown::HashMap<K, V, BuildHasher>;
-pub(crate) type HashSet<K> = hashbrown::HashSet<K, BuildHasher>;
+/// Use deterministic hasher to make egglog deterministic
+/// when rule application order matters.
+pub(crate) type HashMap<K, V> = hashbrown::HashMap<K, V, DeterministicHashBuilder>;
+pub(crate) type HashSet<K> = hashbrown::HashSet<K, DeterministicHashBuilder>;
 
+/// Index maps don't need deterministic hashing,
+/// since iteration order is guaranteed to be insertion order.
 pub type IndexMap<K, V> = indexmap::IndexMap<K, V, BuildHasher>;
 pub type IndexSet<K> = indexmap::IndexSet<K, BuildHasher>;