Sandboxing in the act of running a compilation into a separate directory, for the purpose of enforcing that dependencies and targets be properly specified (instead of trusting the user to get them right).
val hardlink_ignore_targets : tSandbox an action by hard-linking dependencies. Ignore misspecified targets.
val default : tEquivalent to none unless the --sandbox-actions command-line flag is active, in
which case it is equivalent to hardlink.