Store original Impl in explicit impls set
diff --git a/syntax/impls.rs b/syntax/impls.rs
index ebebb3e..6a177d5 100644
--- a/syntax/impls.rs
+++ b/syntax/impls.rs
@@ -1,4 +1,5 @@
-use crate::syntax::{ExternFn, Receiver, Ref, Signature, Slice, Ty1, Type};
+use crate::syntax::{ExternFn, Impl, Receiver, Ref, Signature, Slice, Ty1, Type};
+use std::borrow::Borrow;
 use std::hash::{Hash, Hasher};
 use std::mem;
 use std::ops::{Deref, DerefMut};
@@ -238,3 +239,38 @@
         ty.hash(state);
     }
 }
+
+impl Hash for Impl {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        let Impl {
+            impl_token: _,
+            ty,
+            brace_token: _,
+        } = self;
+        ty.hash(state);
+    }
+}
+
+impl Eq for Impl {}
+
+impl PartialEq for Impl {
+    fn eq(&self, other: &Impl) -> bool {
+        let Impl {
+            impl_token: _,
+            ty,
+            brace_token: _,
+        } = self;
+        let Impl {
+            impl_token: _,
+            ty: ty2,
+            brace_token: _,
+        } = other;
+        ty == ty2
+    }
+}
+
+impl Borrow<Type> for &Impl {
+    fn borrow(&self) -> &Type {
+        &self.ty
+    }
+}