[Wasm GC] Fix StructSet::finalize on an unreachable value (#3874)

Also fix printing of unreachable StructSets, which must handle the case
of an unreachable reference, which means we do not know the RTT,
and so we must print a replacement for the StructSet somehow. Emit a
block with drops, fixing the old behavior which was missing the drops.

2 files changed, 29 insertions, 2 deletions

diff --git a/src/passes/Print.cpp b/src/passes/Print.cpp
index 1392ccb6f..ef78bf582 100644
--- a/src/passes/Print.cpp
+++ b/src/passes/Print.cpp
@@ -1926,7 +1926,7 @@ struct PrintExpressionContents
     // where if the ref is unreachable, we don't know what heap type to print),
     // then print the children in a block, which is good enough as this
     // instruction is never reached anyhow.
-    printMedium(o, "block ");
+    printMedium(o, "block");
   }
   void printFieldName(HeapType type, Index index) {
     processFieldName(wasm, type, index, [&](Name name) {
@@ -2346,6 +2346,33 @@ struct PrintSExpression : public UnifiedExpressionVisitor<PrintSExpression> {
       o << " ;; end try";
     }
   }
+  void printUnreachableReplacement(Expression* curr) {
+    // See the parallel function in PrintExpressionContents for background.
+    //
+    // Emit a block with drops of the children.
+    o << "(block";
+    incIndent();
+    for (auto* child : ChildIterator(curr)) {
+      Drop drop;
+      drop.value = child;
+      printFullLine(&drop);
+    }
+    decIndent();
+  }
+  void visitStructSet(StructSet* curr) {
+    if (curr->ref->type == Type::unreachable) {
+      printUnreachableReplacement(curr);
+      return;
+    }
+    visitExpression(curr);
+  }
+  void visitStructGet(StructGet* curr) {
+    if (curr->ref->type == Type::unreachable) {
+      printUnreachableReplacement(curr);
+      return;
+    }
+    visitExpression(curr);
+  }
   // Module-level visitors
   void handleSignature(Signature curr, Name name = Name()) {
     o << "(func";
diff --git a/src/wasm/wasm.cpp b/src/wasm/wasm.cpp
index 410ba0573..69464e73f 100644
--- a/src/wasm/wasm.cpp
+++ b/src/wasm/wasm.cpp
@@ -977,7 +977,7 @@ void StructGet::finalize() {
 }
 
 void StructSet::finalize() {
-  if (ref->type == Type::unreachable) {
+  if (ref->type == Type::unreachable || value->type == Type::unreachable) {
     type = Type::unreachable;
   } else {
     type = Type::none;