Introduce a collections crate w/ deterministic hashmap, hashset in tests

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

crates/collections/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "collections"
+version = "0.1.0"
+edition = "2021"
+
+[features]
+test-support = ["seahash"]
+
+[dependencies]
+seahash = { version = "4.1", optional = true }

crates/collections/src/lib.rs

@@ -0,0 +1,26 @@
+#[cfg(feature = "test-support")]
+#[derive(Clone, Default)]
+pub struct DeterministicState;
+
+#[cfg(feature = "test-support")]
+impl std::hash::BuildHasher for DeterministicState {
+    type Hasher = seahash::SeaHasher;
+
+    fn build_hasher(&self) -> Self::Hasher {
+        seahash::SeaHasher::new()
+    }
+}
+
+#[cfg(feature = "test-support")]
+pub type HashMap<K, V> = std::collections::HashMap<K, V, DeterministicState>;
+
+#[cfg(feature = "test-support")]
+pub type HashSet<T> = std::collections::HashSet<T, DeterministicState>;
+
+#[cfg(not(feature = "test-support"))]
+pub type HashMap<K, V> = std::collections::HashMap<K, V>;
+
+#[cfg(not(feature = "test-support"))]
+pub type HashSet<T> = std::collections::HashSet<T>;
+
+pub use std::collections::*;