clang's -Wimplicit-fallthrough warnings (not yet enabled in mozilla-central) warn about switch cases that fall through without a break or return statement. MOZ_FALLTHROUGH (bug 1215411) is an annotation to suppress -Wimplicit-fallthrough warnings where the fallthrough is intentional. MOZ_FALLTHROUGH_ASSERT will suppress -Wimplicit-fallthrough warnings about switch cases that MOZ_ASSERT(false) (or its alias MOZ_ASSERT_UNREACHABLE) in debug builds, but intentionally fall through in release builds. Without MOZ_FALLTHROUGH_ASSERT, release builds will fail with -Wimplicit-fallthrough without a MOZ_FALLTHROUGH, but debug builds will fail with -Wunreachable-code warnings about the MOZ_ASSERT(false) prevents the MOZ_FALLTHROUGH from being reached. The MOZ_FALLTHROUGH_ASSERT macro breaks this warning stalemate. dom/media/MediaManager.cpp:1905:7 [-Wimplicit-fallthrough] unannotated fall-through between switch labels dom/media/webm/WebMBufferedParser.cpp:133:7 [-Wimplicit-fallthrough] unannotated fall-through between switch labels