1 files changed, 20 insertions, 10 deletions
diff --git a/src/eval/capture.rs b/src/eval/capture.rs
index 42b617a7..9ef55fb2 100644
--- a/src/eval/capture.rs
+++ b/src/eval/capture.rs
@@ -3,17 +3,27 @@ use std::rc::Rc;
 use super::*;
 use crate::syntax::visit::*;
 
-/// A visitor that replaces all captured variables with their values.
+/// A visitor that captures variable slots.
 #[derive(Debug)]
 pub struct CapturesVisitor<'a> {
     external: &'a Scopes<'a>,
     internal: Scopes<'a>,
+    captures: Scope,
 }
 
 impl<'a> CapturesVisitor<'a> {
     /// Create a new visitor for the given external scopes.
     pub fn new(external: &'a Scopes) -> Self {
-        Self { external, internal: Scopes::default() }
+        Self {
+            external,
+            internal: Scopes::default(),
+            captures: Scope::new(),
+        }
+    }
+
+    /// Return the scope of capture variables.
+    pub fn finish(self) -> Scope {
+        self.captures
     }
 
     /// Define an internal variable.
@@ -23,14 +33,14 @@ impl<'a> CapturesVisitor<'a> {
 }
 
 impl<'ast> Visit<'ast> for CapturesVisitor<'_> {
-    fn visit_expr(&mut self, item: &'ast mut Expr) {
+    fn visit_expr(&mut self, item: &'ast Expr) {
         match item {
             Expr::Ident(ident) => {
                 // Find out whether the identifier is not locally defined, but
                 // captured, and if so, replace it with its value.
                 if self.internal.get(ident).is_none() {
-                    if let Some(value) = self.external.get(ident) {
-                        *item = Expr::Captured(Rc::clone(&value));
+                    if let Some(slot) = self.external.get(ident) {
+                        self.captures.def_slot(ident.as_str(), Rc::clone(slot));
                     }
                 }
             }
@@ -38,7 +48,7 @@ impl<'ast> Visit<'ast> for CapturesVisitor<'_> {
         }
     }
 
-    fn visit_block(&mut self, item: &'ast mut ExprBlock) {
+    fn visit_block(&mut self, item: &'ast ExprBlock) {
         // Blocks create a scope except if directly in a template.
         if item.scopes {
             self.internal.push();
@@ -49,20 +59,20 @@ impl<'ast> Visit<'ast> for CapturesVisitor<'_> {
         }
     }
 
-    fn visit_template(&mut self, item: &'ast mut ExprTemplate) {
+    fn visit_template(&mut self, item: &'ast ExprTemplate) {
         // Templates always create a scope.
         self.internal.push();
         visit_template(self, item);
         self.internal.pop();
     }
 
-    fn visit_let(&mut self, item: &'ast mut ExprLet) {
+    fn visit_let(&mut self, item: &'ast ExprLet) {
         self.define(&item.pat.v);
         visit_let(self, item);
     }
 
-    fn visit_for(&mut self, item: &'ast mut ExprFor) {
-        match &mut item.pat.v {
+    fn visit_for(&mut self, item: &'ast ExprFor) {
+        match &item.pat.v {
             ForPattern::Value(value) => self.define(value),
             ForPattern::KeyValue(key, value) => {
                 self.define(key);