Add ignore reason to placate `tidy`
diff --git a/src/librustc_mir/dataflow/generic.rs b/src/librustc_mir/dataflow/generic.rs
index e890b2c..47d6bd9 100644
--- a/src/librustc_mir/dataflow/generic.rs
+++ b/src/librustc_mir/dataflow/generic.rs
@@ -21,7 +21,7 @@
/// You should use an `Engine` to actually run an analysis, and a `ResultsCursor` to inspect the
/// results of that analysis like so:
///
-/// ```ignore
+/// ```ignore(cross-crate-imports)
/// fn do_my_analysis(body: &mir::Body<'tcx>, dead_unwinds: &BitSet<BasicBlock>) {
/// let analysis = MyAnalysis::new();
/// let results = Engine::new(body, dead_unwinds, analysis).iterate_to_fixpoint();
