Printing: Add a comment when we cannot emit something (#3878)

If we can't emit something, and instead emit a replacement for it (as is
the case for a StructSet with an unreachable RTT, so we have no known
heap type for it), add a comment that mentions it is a replacement. This
might avoid confusion while debugging.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/passes/Print.cpp b/src/passes/Print.cpp
index ef78bf582..41e800352 100644
--- a/src/passes/Print.cpp
+++ b/src/passes/Print.cpp
@@ -2351,6 +2351,9 @@ struct PrintSExpression : public UnifiedExpressionVisitor<PrintSExpression> {
     //
     // Emit a block with drops of the children.
     o << "(block";
+    if (!minify) {
+      o << " ;; (replaces something unreachable we can't emit)";
+    }
     incIndent();
     for (auto* child : ChildIterator(curr)) {
       Drop drop;