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.

2 files changed, 6 insertions, 3 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;
diff --git a/test/heap-types.wast.from-wast b/test/heap-types.wast.from-wast
index fbbc47af9..281d1983e 100644
--- a/test/heap-types.wast.from-wast
+++ b/test/heap-types.wast.from-wast
@@ -309,7 +309,7 @@
  )
  (func $unreachables-1
   (drop
-   (block
+   (block ;; (replaces something unreachable we can't emit)
     (drop
      (unreachable)
     )
@@ -323,7 +323,7 @@
   )
  )
  (func $unreachables-3
-  (block
+  (block ;; (replaces something unreachable we can't emit)
    (drop
     (unreachable)
    )
@@ -333,7 +333,7 @@
   )
  )
  (func $unreachables-4
-  (block
+  (block ;; (replaces something unreachable we can't emit)
    (drop
     (unreachable)
    )