diff --git a/compiler/rustc_middle/src/ty/subst.rs b/compiler/rustc_middle/src/ty/subst.rs
index 0660e9b79a700..c1d9f496c5b22 100644
--- a/compiler/rustc_middle/src/ty/subst.rs
+++ b/compiler/rustc_middle/src/ty/subst.rs
@@ -506,6 +506,9 @@ impl<'tcx, T: TypeVisitable<'tcx>> TypeVisitable<'tcx> for &'tcx ty::List<T> {
     }
 }
 
+/// Similar to [`Binder`] except that it tracks early bound generics, i.e. `struct Foo<T>(T)`
+/// needs `T` substituted immediately. This type primarily exists to avoid forgetting to call
+/// `subst`.
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
 #[derive(Encodable, Decodable, HashStable)]
 pub struct EarlyBinder<T>(pub T);