Function ensure_can_skip_execution 

fn ensure_can_skip_execution<'tcx, C: QueryCache>(
    query: &'tcx QueryVTable<'tcx, C>,
    tcx: TyCtxt<'tcx>,
    key: C::Key,
    dep_node: DepNode,
    ensure_mode: EnsureMode,
) -> bool

Checks whether a tcx.ensure_ok() or tcx.ensure_done() query call can return early without actually trying to execute.

This only makes sense during incremental compilation, because it relies on having the dependency graph (and in some cases a disk-cached value) from the previous incr-comp session.