Function keep_host_facts 

pub fn keep_host_facts(
    shell_allow: &[String],
    staged: &[StagedRuntimeGrant],
    capsule_writable: &[&Path],
    cache: &HostFactsCache,
) -> HostFactsKept

Fills cache with the entries derive_interpreter_runtimes would keep for the same arguments, ahead of the launch that would otherwise work them out, and counts them.

The derivation is the launch’s own, with the same PATH, interpreter table and query timeout, so an entry written here is the one that launch would write and the launch reports it kept. A staging passing other capsule_writable trees still keeps every entry whose recorded paths lie outside both sets. Nothing is written to standard error: a query that fails stores no entry, and the launch that works it out again reports it.

Off Linux this keeps nothing and runs nothing.