1. dom/ipc/ContentChild.cpp:2175:9 [-Wunreachable-code] code will never be executed 2. objdir-osx/dom/bindings/TestJSImplGenBinding.cpp:47640:9: warning: code will never be executed [-Wunreachable-code] ThrowErrorMessage(cx, MSG_PERMISSION_DENIED_TO_PASS_ARG, "element of return value of TestJSImplInterface.receiveAnySequence"); ^~~~~~~~~~~~~~~~~ objdir-osx/dom/bindings/TestJSImplGenBinding.cpp:47639:20: note: silence by adding parentheses to mark code as explicitly dead if (false && !CallerSubsumes(temp)) { ^ /* DISABLES CODE */ () 3. Also suppress clang's -Wimplicit-fallthrough warnings (not yet enabled by default in mozilla-central) in generated code about switch cases that fall through without a break or return statement. The warnings can be suppressed with the MOZ_FALLTHROUGH, a MFBT macro for a clang compiler annotation. MOZ_FALLTHROUGH is only needed on cases that have code: switch (foo) { case 1: // These cases have no code. No fallthrough annotations are needed. case 2: case 3: foo = 4; // This case has code, so a fallthrough annotation is needed: MOZ_FALLTHROUGH; default: return foo; }