Function external_commands_unavailable_error 

pub fn external_commands_unavailable_error(
    name: &str,
    reason: ExternalCommandsUnavailable,
) -> ExecResult

Refuse to run name as an external command, exit 127 — the same code POSIX gives “command not found”, since from here it’s indistinguishable to a caller checking only the exit code. The message is what carries the distinction; see ExternalCommandsUnavailable::condition.