Bug 1536736. Allow constexpr things in the MOZ_CAN_RUN_SCRIPT analysis. r=andi

Since these are compile-time constants, they can't exactly go away on us due to
running script, right?

Differential Revision: https://phabricator.services.mozilla.com/D24195

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

build/clang-plugin/CanRunScriptChecker.cpp

@@ -110,6 +110,10 @@ void CanRunScriptChecker::registerMatchers(MatchFinder *AstMatcher) {
           ),
           // and which is not a parameter of the parent function,
           unless(declRefExpr(to(parmVarDecl()))),
+          // and which is not a constexpr variable, since that must be
+          // computable at compile-time and therefore isn't going to be going
+          // away.
+          unless(declRefExpr(to(varDecl(isConstexpr())))),
           // and which is not a default arg with value nullptr, since those are
           // always safe.
           unless(cxxDefaultArgExpr(isNullDefaultArg())),
@@ -130,7 +134,12 @@ void CanRunScriptChecker::registerMatchers(MatchFinder *AstMatcher) {
                   // implicitCastExpr), but it's simpler to just use
                   // ignoreTrivials to strip off the cast.
                   ignoreTrivials(declRefExpr(to(parmVarDecl()))),
-                  cxxThisExpr()
+                  cxxThisExpr(),
+                  // We also allow dereferencing a constexpr variable here,
+                  // since that will just end up with a reference to the
+                  // compile-time-constant thing.  Again, use ignoreTrivials()
+                  // to stip off the LValueToRValue cast.
+                  ignoreTrivials(declRefExpr(to(varDecl(isConstexpr()))))
                 )
               )
             )

build/clang-plugin/tests/TestCanRunScript.cpp

@@ -437,3 +437,28 @@ struct DisallowRefPtrTArrayElement {
     test2(mArray[0]); // expected-error {{arguments must all be strong refs or parent parameters when calling a function marked as MOZ_CAN_RUN_SCRIPT (including the implicit object argument)}}
   }
 };
+
+struct AllowConstexprMembers {
+  static constexpr RefCountedBase* mRefCounted = nullptr;
+  MOZ_CAN_RUN_SCRIPT void foo() {
+    mRefCounted->method_test();
+  }
+  MOZ_CAN_RUN_SCRIPT void bar() {
+    test2(mRefCounted);
+  }
+  MOZ_CAN_RUN_SCRIPT void baz() {
+    test_ref(*mRefCounted);
+  }
+};
+
+MOZ_CAN_RUN_SCRIPT void test_constexpr_1() {
+  AllowConstexprMembers::mRefCounted->method_test();
+}
+
+MOZ_CAN_RUN_SCRIPT void test_constexpr_2() {
+  test2(AllowConstexprMembers::mRefCounted);
+}
+
+MOZ_CAN_RUN_SCRIPT void test_constexpr_3() {
+  test_ref(*AllowConstexprMembers::mRefCounted);
+}