[CustomDescriptors] RemoveUnusedModuleElements: Fix StructNew optimization of descriptor (#7650)

We have very special optimizations for StructNew in that pass: we see if they
have reads (struct.get) and remove them if not. That explicit handling of
StructNew was not aware of the new descriptor field, and so it could error,
removing things that were still needed.

Author: kripken

scripts/test/fuzzing.py

@@ -112,6 +112,7 @@
     'unsubtyping-stack-switching.wast',
     'vacuum-stack-switching.wast',
     # TODO: fuzzer support for custom descriptors
+    'remove-unused-module-elements-refs-descriptors.wast',
     'custom-descriptors.wast',
     'br_on_cast_desc.wast',
     'ref.get_desc.wast',

src/passes/RemoveUnusedModuleElements.cpp

@@ -482,6 +482,13 @@ struct Analyzer {
     }
 
     auto* new_ = curr->cast<StructNew>();
+
+    // Use the descriptor right now, normally. (We only have special
+    // optimization for struct.new operands, below.)
+    if (new_->descriptor) {
+      use(new_->descriptor);
+    }
+
     auto type = new_->type.getHeapType();
 
     for (Index i = 0; i < new_->operands.size(); i++) {

test/lit/passes/remove-unused-module-elements-refs-descriptors.wast

@@ -0,0 +1,37 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
+;; RUN: foreach %s %t wasm-opt --remove-unused-module-elements --closed-world -all -S -o - | filecheck %s
+
+;; The global.get of $global is the descriptor, not a field of the struct.
+;; We should not try to optimize it away and error.
+(module
+ (rec
+  ;; CHECK:      (rec
+  ;; CHECK-NEXT:  (type $B (sub (descriptor $A (struct))))
+  (type $B (sub (descriptor $A (struct))))
+  ;; CHECK:       (type $A (sub (describes $B (struct))))
+  (type $A (sub (describes $B (struct))))
+ )
+
+ ;; CHECK:      (type $2 (func))
+
+ ;; CHECK:      (global $global (ref (exact $A)) (struct.new_default $A))
+ (global $global (ref (exact $A)) (struct.new $A))
+
+ ;; CHECK:      (export "export" (func $export))
+
+ ;; CHECK:      (func $export (type $2)
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (struct.new_default $B
+ ;; CHECK-NEXT:    (global.get $global)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $export (export "export")
+  (drop
+   (struct.new $B
+    (global.get $global)
+   )
+  )
+ )
+)
+