MOZ_FALLTHROUGH (bug 1215411) is an annotation to suppress -Wimplicit-fallthrough warnings about switch cases that fall through without a break or return statement. 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. The problem with this common case is that, in release builds, the MOZ_ASSERT(false) will expand to `do { } while (0)`, requiring a MOZ_FALLTHROUGH annotation to suppress a -Wimplicit-fallthrough warning. In debug builds, the MOZ_ASSERT(false) will expand to something like `if (true) { MOZ_CRASH(); }` and the MOZ_FALLTHROUGH annotation will cause a -Wunreachable-code warning. The MOZ_FALLTHROUGH_ASSERT macro breaks this warning stalemate. BAD EXAMPLE: switch (foo) { default: // This case wants to assert in debug builds, fall through in release. MOZ_ASSERT(false); // -Wimplicit-fallthrough warning in release builds! MOZ_FALLTHROUGH; // but -Wunreachable-code warning in debug builds! case 5: return 5; } GOOD EXAMPLE: switch (foo) { default: // This case asserts in debug builds, falls through in release. MOZ_FALLTHROUGH_ASSERT("Unexpected foo value?!"); case 5: return 5; } Unfortunately, the MOZ_FALLTHROUGH definition and comments are in Attributes.h, but MOZ_FALLTHROUGH_ASSERT definition and comments are in Assertions.h. These macro definitions depend on other macro definitions in their respective header files.

I filed a clang feature request to make this MOZ_FALLTHROUGH_ASSERT() macro unnecessary: https://llvm.org/bugs/show_bug.cgi?id=25966 clang could allow a [[clang::fallthrough]] annotation after a call to a noreturn function like abort(), but still warn about unreachable [[clang::fallthrough]] annotations after an early break or return.