Preserve exception message in iterator finalization.

src/ast/ast.h

Index: src/ast/ast.h
diff --git a/src/ast/ast.h b/src/ast/ast.h
index 539a848370a22f39b11dd13467f34c8308183c65..397d0b4663ceef629bfe841b0d0b95129afd9bdf 100644
--- a/src/ast/ast.h
+++ b/src/ast/ast.h
@@ -1182,19 +1182,34 @@ class TryCatchStatement final : public TryStatement {
   Variable* variable() { return variable_; }
   Block* catch_block() const { return catch_block_; }
   void set_catch_block(Block* b) { catch_block_ = b; }
+  bool clear_pending_message() { return clear_pending_message_; }

[Michael Starzinger, 2016/04/01 13:07:41]

nit: I love the awesome comment below. Can we move the accessor to right below
the comment? That way it shows up in various GUI editors as being associated
with the accessor (improves readability and helps keeping it in sync).

+
+  // The clear_pending_message flag indicates whether or not to clear the
+  // isolate's pending exception message before executing the catch_block.  In
+  // the normal use case, this flag is always on because the message object
+  // is not needed anymore when entering the catch block and should not be kept
+  // alive.
+  // The use case where the flag is off is when the catch block is guaranteed to
+  // rethrow the caught exception (using %ReThrow), which reuses the pending
+  // message instead of generating a new one.
+  // (When the catch block doesn't rethrow but is guaranteed to perform an
+  // ordinary throw, not clearing the old message is safe but not very useful.)
 
  protected:
   TryCatchStatement(Zone* zone, Block* try_block, Scope* scope,
-                    Variable* variable, Block* catch_block, int pos)
+                    Variable* variable, Block* catch_block,
+                    bool clear_pending_message, int pos)
       : TryStatement(zone, try_block, pos),
         scope_(scope),
         variable_(variable),
-        catch_block_(catch_block) {}
+        catch_block_(catch_block),
+        clear_pending_message_(clear_pending_message) {}
 
  private:
   Scope* scope_;
   Variable* variable_;
   Block* catch_block_;
+  bool clear_pending_message_;
 };
 
 
@@ -3134,8 +3149,17 @@ class AstNodeFactory final BASE_EMBEDDED {
   TryCatchStatement* NewTryCatchStatement(Block* try_block, Scope* scope,
                                           Variable* variable,
                                           Block* catch_block, int pos) {
-    return new (local_zone_) TryCatchStatement(local_zone_, try_block, scope,
-                                               variable, catch_block, pos);
+    return new (local_zone_) TryCatchStatement(
+        local_zone_, try_block, scope, variable, catch_block, true, pos);
+  }
+
+  TryCatchStatement* NewTryCatchStatementForReThrow(Block* try_block,
+                                                    Scope* scope,
+                                                    Variable* variable,
+                                                    Block* catch_block,
+                                                    int pos) {
+    return new (local_zone_) TryCatchStatement(
+        local_zone_, try_block, scope, variable, catch_block, false, pos);
   }
 
   TryFinallyStatement* NewTryFinallyStatement(Block* try_block,