keep the range of @foo in ExprSource

src/eval.rs

@@ -128,7 +128,7 @@ pub enum ExprSource {
         /// whether the patter is incomplete (contains `...`)
         ellipsis: bool,
         /// the identifier bound by `@`
-        at: Option<String>,
+        at: Option<ExprResultBox>,
     },
     Lambda {
         /// A `Pattern` or an `Identifier`

src/parse.rs

@@ -406,7 +406,7 @@ impl Expr {
                     Some(at) => {
                         let string = at.as_str().to_string();
                         names.insert(string.clone());
-                        Some(string)
+                        Some(recurse_box(at.node().clone()))
                     }
                 };
                 for entry in pattern.entries() {