1 files changed, 58 insertions, 0 deletions

diff --git a/crates/libsyntax2/src/algo/visit.rs b/crates/libsyntax2/src/algo/visit.rs
index a36c8f45e..9f1c127c7 100644
--- a/crates/libsyntax2/src/algo/visit.rs
+++ b/crates/libsyntax2/src/algo/visit.rs
@@ -6,6 +6,10 @@ pub fn visitor<'a, T>() -> impl Visitor<'a, Output=T> {
     EmptyVisitor { ph: PhantomData }
 }
 
+pub fn visitor_ctx<'a, T, C>(ctx: C) -> impl VisitorCtx<'a, Output=T, Ctx=C> {
+    EmptyVisitorCtx { ph: PhantomData, ctx }
+}
+
 pub trait Visitor<'a>: Sized {
     type Output;
     fn accept(self, node: SyntaxNodeRef<'a>) -> Option<Self::Output>;
@@ -17,6 +21,18 @@ pub trait Visitor<'a>: Sized {
     }
 }
 
+pub trait VisitorCtx<'a>: Sized {
+    type Output;
+    type Ctx;
+    fn accept(self, node: SyntaxNodeRef<'a>) -> Result<Self::Output, Self::Ctx>;
+    fn visit<N, F>(self, f: F) -> VisCtx<Self, N, F>
+        where N: AstNode<'a>,
+              F: FnOnce(N, Self::Ctx) -> Self::Output,
+    {
+        VisCtx { inner: self, f, ph: PhantomData }
+    }
+}
+
 #[derive(Debug)]
 struct EmptyVisitor<T> {
     ph: PhantomData<fn() -> T>
@@ -31,6 +47,21 @@ impl<'a, T> Visitor<'a> for EmptyVisitor<T> {
 }
 
 #[derive(Debug)]
+struct EmptyVisitorCtx<T, C> {
+    ctx: C,
+    ph: PhantomData<fn() -> T>,
+}
+
+impl<'a, T, C> VisitorCtx<'a> for EmptyVisitorCtx<T, C> {
+    type Output = T;
+    type Ctx = C;
+
+    fn accept(self, _node: SyntaxNodeRef<'a>) -> Result<T, C> {
+        Err(self.ctx)
+    }
+}
+
+#[derive(Debug)]
 pub struct Vis<V, N, F> {
     inner: V,
     f: F,
@@ -50,3 +81,30 @@ impl<'a, V, N, F> Visitor<'a> for Vis<V, N, F>
         inner.accept(node).or_else(|| N::cast(node).map(f))
     }
 }
+
+#[derive(Debug)]
+pub struct VisCtx<V, N, F> {
+    inner: V,
+    f: F,
+    ph: PhantomData<fn(N)>,
+}
+
+impl<'a, V, N, F> VisitorCtx<'a> for VisCtx<V, N, F>
+    where
+        V: VisitorCtx<'a>,
+        N: AstNode<'a>,
+        F: FnOnce(N, <V as VisitorCtx<'a>>::Ctx) -> <V as VisitorCtx<'a>>::Output,
+{
+    type Output = <V as VisitorCtx<'a>>::Output;
+    type Ctx = <V as VisitorCtx<'a>>::Ctx;
+
+    fn accept(self, node: SyntaxNodeRef<'a>) -> Result<Self::Output, Self::Ctx> {
+        let VisCtx { inner, f, .. } = self;
+        inner.accept(node).or_else(|ctx|
+            match N::cast(node) {
+                None => Err(ctx),
+                Some(node) => Ok(f(node, ctx))
+            }
+        )
+    }
+}