I think this patch is still incomplete. This will just turn the timeout into an early error. Obviously a fast error is better than a long timeout, but it would be better to exclude the test from the non-fuzz-only builds in the CI and mention in the pull request description that require_build_for_fuzzing targets must be excluded manually in non-fuzz-only builds.

An alternative would be to not register it, as explained by Cory.

style nit: Seems fine, but I preferred the previous approach where all fuzz targets are registered with the same macro. This should be possible by guarding the FuzzFrameworkRegisterTarget behind a if constexpr (opts.require_build_for_fuzzing and not build_for_fuzzing).

This should also make #30882 easier to implement, because it doesn't have to deal with two macros.

This may also work around the CI false positive?

Agreed, that's a better way to do it. Let me know what you think about d470af3. Another possibility I was thinking of was something like:

#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
#define CONDITIONAL_REGISTER(name, target, opts) FuzzFrameworkRegisterTarget(name, target, opts)
#else
#define CONDITIONAL_REGISTER(name, target, opts)                     
    do {                                                                                                   
        if (!(opts).requires_fuzz_build) {                                                  
            FuzzFrameworkRegisterTarget(name, target, opts);               
        }               
    } while(0)
#endif

Is there an approach I'm missing?

Dealing with the logic in DETAIL_FUZZ seems better. And not as many macros.

nit: Could rename, so that it can be re-used in the future for other stuff, if needed?