Constant BUILT_WITH_METAL 

pub const BUILT_WITH_METAL: bool = false;

Whether this build of the server has the Metal kernels compiled in.

Exists for the front ends that link this library: ferrox-cli’s metal feature has to forward into ferrox-server (ferrox-server?/metal) or ferrox serve --device metal refuses on a Metal host while ferrox run on the same binary uses it. That mismatch is one Cargo manifest edit away and compiles cleanly, so ferrox-cli asserts on this constant at compile time.