2 files changed, 53 insertions, 0 deletions

diff --git a/crates/libsyntax2/src/algo/mod.rs b/crates/libsyntax2/src/algo/mod.rs
index d2de70fd4..263b58d97 100644
--- a/crates/libsyntax2/src/algo/mod.rs
+++ b/crates/libsyntax2/src/algo/mod.rs
@@ -1,4 +1,5 @@
 pub mod walk;
+pub mod visit;
 
 use {SyntaxNodeRef, TextUnit, TextRange};
 
diff --git a/crates/libsyntax2/src/algo/visit.rs b/crates/libsyntax2/src/algo/visit.rs
new file mode 100644
index 000000000..dc5afa5a9
--- /dev/null
+++ b/crates/libsyntax2/src/algo/visit.rs
@@ -0,0 +1,52 @@
+use std::marker::PhantomData;
+use {SyntaxNodeRef, AstNode, SyntaxRoot};
+
+
+pub fn visitor<'a, T>() -> impl Visitor<'a, Output=T> {
+    EmptyVisitor { ph: PhantomData }
+}
+
+pub trait Visitor<'a>: Sized {
+    type Output;
+    fn accept(self, node: SyntaxNodeRef<'a>) -> Option<Self::Output>;
+    fn visit<N, F>(self, f: F) -> Vis<Self, N, F>
+        where N: AstNode<&'a SyntaxRoot>,
+              F: FnOnce(N) -> Self::Output,
+    {
+        Vis { inner: self, f, ph: PhantomData }
+    }
+}
+
+#[derive(Debug)]
+struct EmptyVisitor<T> {
+    ph: PhantomData<fn() -> T>
+}
+
+impl<'a, T> Visitor<'a> for EmptyVisitor<T> {
+    type Output = T;
+
+    fn accept(self, _node: SyntaxNodeRef<'a>) -> Option<T> {
+        None
+    }
+}
+
+#[derive(Debug)]
+pub struct Vis<V, N, F> {
+    inner: V,
+    f: F,
+    ph: PhantomData<fn(N)>,
+}
+
+impl<'a, V, N, F> Visitor<'a> for Vis<V, N, F>
+    where
+        V: Visitor<'a>,
+        N: AstNode<&'a SyntaxRoot>,
+        F: FnOnce(N) -> <V as Visitor<'a>>::Output,
+{
+    type Output = <V as Visitor<'a>>::Output;
+
+    fn accept(self, node: SyntaxNodeRef<'a>) -> Option<Self::Output> {
+        let Vis { inner, f, .. } = self;
+        inner.accept(node).or_else(|| N::cast(node).map(f))
+    }
+}