[nyberg]

One would just run tests and rebuild for the supported targets? That will hit all comptime branches that you care about without needing to do anything else. I don't see how this is much worse? What you seem to be looking for is an "unused function" (https://github.com/ziglang/zig/issues/335) error instead as it catches that case unless you use the function in a branch that's not part of your target set (truly dead unused code).

There's support for invoking with qemu, wine, etc in the build system which allows you to run tests for other platforms.

A case where I've taken advantage if this is to have data structures and code adjust based on the expected page size and cache-line. There are also cases where things may be both comptime and runtime known depending on which platform that you target which is easy to handle by having the maybe comptime value be first in conditional branches.