diff --git a/compiler/rustc_trait_selection/src/traits/wf.rs b/compiler/rustc_trait_selection/src/traits/wf.rs
index 47bccb9588e8..cad7e42fd821 100644
--- a/compiler/rustc_trait_selection/src/traits/wf.rs
+++ b/compiler/rustc_trait_selection/src/traits/wf.rs
@@ -1,3 +1,8 @@
+//! Core logic responsible for determining what it means for various type system
+//! primitives to be "well formed". Actually checking whether these primitives are
+//! well formed is performed elsewhere (e.g. during type checking or item well formedness
+//! checking).
+
 use std::iter;
 
 use rustc_hir as hir;