Cannot collect facets starting from the root in Tantivy 0.20

[adamreichold]

Applying the following diff

diff --git a/src/collector/facet_collector.rs b/src/collector/facet_collector.rs
index 6861a9faa..597e4ba12 100644
--- a/src/collector/facet_collector.rs
+++ b/src/collector/facet_collector.rs
@@ -119,17 +119,16 @@ fn facet_depth(facet_bytes: &[u8]) -> usize {
 ///
 ///     {
 ///         let mut facet_collector = FacetCollector::for_field("facet");
-///         facet_collector.add_facet("/lang");
-///         facet_collector.add_facet("/category");
+///         facet_collector.add_facet("/");
 ///         let facet_counts = searcher.search(&AllQuery, &facet_collector)?;
 ///
 ///         // This lists all of the facet counts
 ///         let facets: Vec<(&Facet, u64)> = facet_counts
-///             .get("/category")
+///             .get("/")
 ///             .collect();
 ///         assert_eq!(facets, vec![
-///             (&Facet::from("/category/biography"), 1),
-///             (&Facet::from("/category/fiction"), 3)
+///             (&Facet::from("/lang"), 4),
+///             (&Facet::from("/category"), 4)
 ///         ]);
 ///     }
 ///

to the example for FacetCollector makes it fail with

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `[(Facet(/lang), 4)]`,
 right: `[(Facet(/lang), 4), (Facet(/category), 4)]`', src/collector/facet_collector.rs:59:9

using the just released Tantitvy version 0.20, i.e. it is not possible to collect facets starting with the root.

I am not sure what the problem is, but I suspect that compute_collapse_mapping_one does not handle facet_bytes.len() == 0 which is the case when collecting facets from the root.

[adamreichold]

I see that 0.20.1 was just released so I understand this will not happen immediately, but since this is the last known blocker for us to upgrade to 0.20.x, I would be glad if this could find its way into an upcoming bug fix release as well.

[PSeitz]

It's released with 0.20.2