Bug 1493093 - Allow to relax MOZ_NON_TEMPORARY_CLASS for some specific constructors r=andi

Differential Revision: https://phabricator.services.mozilla.com/D6566

--HG--
extra : moz-landing-system : lando

build/clang-plugin/CustomAttributes.inc

@@ -1,3 +1,4 @@
+ATTR(moz_allow_temporary)
 ATTR(moz_can_run_script)
 ATTR(moz_can_run_script_boundary)
 ATTR(moz_global_class)

build/clang-plugin/CustomMatchers.h

@@ -29,6 +29,10 @@ AST_MATCHER(CXXRecordDecl, hasTrivialCtorDtor) {
   return hasCustomAttribute<moz_trivial_ctor_dtor>(&Node);
 }
 
+AST_MATCHER(CXXConstructExpr, allowsTemporary) {
+  return hasCustomAttribute<moz_allow_temporary>(Node.getConstructor());
+}
+
 /// This matcher will match lvalue-ref-qualified methods.
 AST_MATCHER(CXXMethodDecl, isLValueRefQualified) {
   return Node.getRefQualifier() == RQ_LValue;

build/clang-plugin/ScopeChecker.cpp

@@ -8,7 +8,10 @@
 void ScopeChecker::registerMatchers(MatchFinder *AstMatcher) {
   AstMatcher->addMatcher(varDecl().bind("node"), this);
   AstMatcher->addMatcher(cxxNewExpr().bind("node"), this);
-  AstMatcher->addMatcher(materializeTemporaryExpr().bind("node"), this);
+  AstMatcher->addMatcher(
+      materializeTemporaryExpr(
+          unless(hasDescendant(cxxConstructExpr(allowsTemporary())))
+      ).bind("node"), this);
   AstMatcher->addMatcher(
       callExpr(callee(functionDecl(heapAllocator()))).bind("node"), this);
 }

dom/bindings/BindingDeclarations.h

@@ -221,7 +221,7 @@ template<typename T>
 class Optional : public Optional_base<T, T>
 {
 public:
-  Optional() :
+  MOZ_ALLOW_TEMPORARY Optional() :
     Optional_base<T, T>()
   {}
 
@@ -235,7 +235,7 @@ class Optional<JS::Handle<T> > :
   public Optional_base<JS::Handle<T>, JS::Rooted<T> >
 {
 public:
-  Optional() :
+  MOZ_ALLOW_TEMPORARY Optional() :
     Optional_base<JS::Handle<T>, JS::Rooted<T> >()
   {}
 

mfbt/Attributes.h

@@ -598,6 +598,10 @@
  *   or constexpr constructor and a trivial destructor. Setting this attribute
  *   on a class makes it a compile-time error for that class to get a
  *   non-trivial constructor or destructor for any reason.
+ * MOZ_ALLOW_TEMPORARY: Applies to constructors. This indicates that using the
+ *   constructor is allowed in temporary expressions, if it would have otherwise
+ *   been forbidden by the type being a MOZ_NON_TEMPORARY_CLASS. Useful for
+ *   constructors like Maybe(Nothing).
  * MOZ_HEAP_ALLOCATOR: Applies to any function. This indicates that the return
  *   value is allocated on the heap, and will as a result check such allocations
  *   during MOZ_STACK_CLASS and MOZ_NONHEAP_CLASS annotation checking.
@@ -721,6 +725,7 @@
 #  define MOZ_NON_TEMPORARY_CLASS __attribute__((annotate("moz_non_temporary_class")))
 #  define MOZ_TEMPORARY_CLASS __attribute__((annotate("moz_temporary_class")))
 #  define MOZ_TRIVIAL_CTOR_DTOR __attribute__((annotate("moz_trivial_ctor_dtor")))
+#  define MOZ_ALLOW_TEMPORARY __attribute__((annotate("moz_allow_temporary")))
 #  ifdef DEBUG
      /* in debug builds, these classes do have non-trivial constructors. */
 #    define MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS __attribute__((annotate("moz_global_class")))
@@ -776,6 +781,7 @@
 #  define MOZ_NON_TEMPORARY_CLASS /* nothing */
 #  define MOZ_TEMPORARY_CLASS /* nothing */
 #  define MOZ_TRIVIAL_CTOR_DTOR /* nothing */
+#  define MOZ_ALLOW_TEMPORARY /* nothing */
 #  define MOZ_ONLY_USED_TO_AVOID_STATIC_CONSTRUCTORS /* nothing */
 #  define MOZ_IMPLICIT /* nothing */
 #  define MOZ_IS_SMARTPTR_TO_REFCOUNTED /* nothing */

mfbt/Maybe.h

@@ -182,12 +182,12 @@ class MOZ_NON_PARAM MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS Maybe
 public:
   using ValueType = T;
 
-  Maybe() : mIsSome(false)
+  MOZ_ALLOW_TEMPORARY Maybe() : mIsSome(false)
   {
   }
   ~Maybe() { reset(); }
 
-  MOZ_IMPLICIT Maybe(Nothing) : mIsSome(false)
+  MOZ_ALLOW_TEMPORARY MOZ_IMPLICIT Maybe(Nothing) : mIsSome(false)
   {
   }