2 files changed, 74 insertions, 0 deletions
diff --git a/src/util/mod.rs b/src/util/mod.rs
index 32ec8dc4..84bd1aa1 100644
--- a/src/util/mod.rs
+++ b/src/util/mod.rs
@@ -3,9 +3,11 @@
 #[macro_use]
 mod eco_string;
 mod mac_roman;
+mod prehashed;
 
 pub use eco_string::EcoString;
 pub use mac_roman::decode_mac_roman;
+pub use prehashed::Prehashed;
 
 use std::cell::RefMut;
 use std::cmp::Ordering;
diff --git a/src/util/prehashed.rs b/src/util/prehashed.rs
new file mode 100644
index 00000000..bab1c8f8
--- /dev/null
+++ b/src/util/prehashed.rs
@@ -0,0 +1,72 @@
+use std::any::Any;
+use std::fmt::{self, Debug, Formatter};
+use std::hash::{Hash, Hasher};
+use std::ops::Deref;
+
+/// A wrapper around a type that precomputes its hash.
+#[derive(Copy, Clone)]
+pub struct Prehashed<T: ?Sized> {
+    /// The precomputed hash.
+    #[cfg(feature = "layout-cache")]
+    hash: u64,
+    /// The wrapped item.
+    item: T,
+}
+
+impl<T: Hash + 'static> Prehashed<T> {
+    /// Compute an item's hash and wrap it.
+    pub fn new(item: T) -> Self {
+        Self {
+            #[cfg(feature = "layout-cache")]
+            hash: {
+                // Also hash the TypeId because the type might be converted
+                // through an unsized coercion.
+                let mut state = fxhash::FxHasher64::default();
+                item.type_id().hash(&mut state);
+                item.hash(&mut state);
+                state.finish()
+            },
+            item,
+        }
+    }
+
+    /// Return the wrapped value.
+    pub fn into_iter(self) -> T {
+        self.item
+    }
+}
+
+impl<T: ?Sized> Deref for Prehashed<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.item
+    }
+}
+
+impl<T: Debug + ?Sized> Debug for Prehashed<T> {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        self.item.fmt(f)
+    }
+}
+
+impl<T: Hash + ?Sized> Hash for Prehashed<T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        // Hash the node.
+        #[cfg(feature = "layout-cache")]
+        state.write_u64(self.hash);
+        #[cfg(not(feature = "layout-cache"))]
+        self.item.hash(state);
+    }
+}
+
+impl<T: Eq + ?Sized> Eq for Prehashed<T> {}
+
+impl<T: PartialEq + ?Sized> PartialEq for Prehashed<T> {
+    fn eq(&self, other: &Self) -> bool {
+        #[cfg(feature = "layout-cache")]
+        return self.hash == other.hash;
+        #[cfg(not(feature = "layout-cache"))]
+        self.item.eq(&other.item)
+    }
+}