Constant EXEC_BACKEND 

pub const EXEC_BACKEND: bool = false;

Which runtime::task backend this build selected — true for the reactor-free std-thread runtime::background executor, false for tokio.

The predicate is computed once, in this crate’s build.rs, from the target OS and EPICS_RS_BUILD_EXEC_BACKEND. A crate above that derives the same cfg from its own build.rs (epics-base-rs does, for server::scan) can pin the two together with a const _: () = assert!(...), so a build script that did not see the variable fails to compile instead of splitting the workspace across two backends.