I'm sure this was mentioned somewhere but could not find an issue: The gist is as follows:

if (@areWeInComptime()) {
   // use a slow fallback which we can compute in zig
} else {
   // use a fast runtime version that uses assembly (or features not supported at comptime)
}

This is present in D for example via the __ctfe variable. The current reason I need this is that I want to call sqrt at compile-time but on x86_64 this uses an assembly routine which can not be evaluated at compile-time.