diff --git a/polonius-engine/src/facts.rs b/polonius-engine/src/facts.rs
index eb90b3f2846..edd23b25f5d 100644
--- a/polonius-engine/src/facts.rs
+++ b/polonius-engine/src/facts.rs
@@ -64,6 +64,10 @@ pub struct AllFacts<Origin: Atom, Loan: Atom, Point: Atom, Variable: Atom, MoveP
     pub path_accessed_at: Vec<(MovePath, Point)>,
 }
 
+// We impl Default manually because the more convenient
+// `#[derive(Default)]` syntax would result in, for example,
+// `rustc::ty::RegionVid`, an `Atom`, also having to impl `Default`,
+// something that does not make sense for that type.
 impl<Origin: Atom, Loan: Atom, Point: Atom, Variable: Atom, MovePath: Atom> Default
     for AllFacts<Origin, Loan, Point, Variable, MovePath>
 {