1 files changed, 66 insertions, 0 deletions

diff --git a/crates/ra_hir/src/arena.rs b/crates/ra_hir/src/arena.rs
new file mode 100644
index 000000000..d4f9d9cb9
--- /dev/null
+++ b/crates/ra_hir/src/arena.rs
@@ -0,0 +1,66 @@
+//! A simple id-based arena, similar to https://github.com/fitzgen/id-arena.
+//! We use our own version for more compact id's and to allow inherent impls
+//! on Ids.
+
+use std::{
+    fmt,
+    hash::{Hash, Hasher},
+    marker::PhantomData,
+};
+
+pub struct Id<T> {
+    idx: u32,
+    _ty: PhantomData<fn() -> T>,
+}
+
+impl<T> fmt::Debug for Id<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_tuple("Id").field(&self.idx).finish()
+    }
+}
+impl<T> Copy for Id<T> {}
+impl<T> Clone for Id<T> {
+    fn clone(&self) -> Id<T> {
+        *self
+    }
+}
+
+impl<T> PartialEq for Id<T> {
+    fn eq(&self, other: &Id<T>) -> bool {
+        self.idx == other.idx
+    }
+}
+
+impl<T> Eq for Id<T> {}
+
+impl<T> Hash for Id<T> {
+    fn hash<H: Hasher>(&self, h: &mut H) {
+        self.idx.hash(h);
+    }
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub(crate) struct ArenaBehavior<T> {
+    _ty: PhantomData<T>,
+}
+
+impl<T> id_arena::ArenaBehavior for ArenaBehavior<T> {
+    type Id = Id<T>;
+    fn new_arena_id() -> u32 {
+        0
+    }
+    fn new_id(_arena_id: u32, index: usize) -> Id<T> {
+        Id {
+            idx: index as u32,
+            _ty: PhantomData,
+        }
+    }
+    fn index(id: Id<T>) -> usize {
+        id.idx as usize
+    }
+    fn arena_id(_id: Id<T>) -> u32 {
+        0
+    }
+}
+
+pub(crate) type Arena<T> = id_arena::Arena<T, ArenaBehavior<T>>;