diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 3e3c16f5100..ef3c3da89c5 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -913,7 +913,10 @@ pub struct Resolver<'a> {
     label_res_map: NodeMap<NodeId>,
     /// Resolutions for lifetimes.
     lifetimes_res_map: NodeMap<LifetimeRes>,
-    /// Mapping from generics def-id to RPIT copied generic def-id
+    /// Mapping from generics `def_id`s to TAIT generics `def_id`s.
+    /// For each captured lifetime (e.g., 'a), we create a new lifetime parameter that is a generic
+    /// defined on the TAIT, so we have type Foo<'a1> = ... and we establish a mapping in this
+    /// field from the original parameter 'a to the new parameter 'a1.
     generics_def_id_map: Vec<FxHashMap<LocalDefId, LocalDefId>>,
     /// Lifetime parameters that lowering will have to introduce.
     extra_lifetime_params_map: NodeMap<Vec<(Ident, NodeId, LifetimeRes)>>,