report line number of opening token if a pair (eg bracket) is left open

lib/rewriter.js

@@ -243,8 +243,9 @@
     // Ensure that all listed pairs of tokens are correctly balanced throughout
     // the course of the token stream.
     Rewriter.prototype.ensure_balance = function ensure_balance(pairs) {
-      var _a, _b, key, levels, unclosed, value;
+      var _a, _b, key, levels, line, open, open_line, unclosed, value;
       levels = {};
+      open_line = {};
       this.scan_tokens((function(__this) {
         var __func = function(prev, token, post, i) {
           var _a, _b, _c, _d, close, open, pair;
@@ -256,6 +257,9 @@
             close = _d[1];
             levels[open] = levels[open] || 0;
             if (token[0] === open) {
+              if (levels[open] === 0) {
+                open_line[open] = token[2];
+              }
               levels[open] += 1;
             }
             if (token[0] === close) {
@@ -282,7 +286,9 @@
         return _a;
       }).call(this);
       if (unclosed.length) {
-        throw new Error("unclosed " + (unclosed[0]));
+        open = unclosed[0];
+        line = open_line[open] + 1;
+        throw new Error("unclosed " + (open) + " on line " + (line));
       }
     };
     // We'd like to support syntax like this:

src/rewriter.coffee

@@ -174,16 +174,22 @@ exports.Rewriter: class Rewriter
   # the course of the token stream.
   ensure_balance: (pairs) ->
     levels: {}
+    open_line: {}
     @scan_tokens (prev, token, post, i) =>
       for pair in pairs
         [open, close]: pair
         levels[open] ||= 0
-        levels[open] += 1 if token[0] is open
+        if token[0] is open
+          open_line[open]: token[2] if levels[open] == 0
+          levels[open] += 1
         levels[open] -= 1 if token[0] is close
         throw new Error("too many ${token[1]} on line ${token[2] + 1}") if levels[open] < 0
       return 1
     unclosed: key for key, value of levels when value > 0
-    throw new Error("unclosed ${unclosed[0]}") if unclosed.length
+    if unclosed.length
+      open: unclosed[0]
+      line: open_line[open] + 1
+      throw new Error("unclosed ${open} on line ${line}")
 
   # We'd like to support syntax like this:
   #