diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index 3dae2aaec7c..1a9f8630632 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -931,7 +931,9 @@ pub struct GlobalCtxt<'tcx> {
 
     maybe_unused_trait_imports: FxHashSet<DefId>,
     maybe_unused_extern_crates: Vec<(DefId, Span)>,
-    pub extern_prelude: FxHashMap<ast::Name, bool /* introduced by item */>,
+    /// Extern prelude entries. The value is `true` if the entry was introduced
+    /// via `extern crate` item and not `--extern` option or compiler built-in.
+    pub extern_prelude: FxHashMap<ast::Name, bool>,
 
     // Internal cache for metadata decoding. No need to track deps on this.
     pub rcache: Lock<FxHashMap<ty::CReaderCacheKey, Ty<'tcx>>>,
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 0d4a49e95ec..6ea759dee21 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -141,7 +141,9 @@ pub struct Resolutions {
     pub maybe_unused_trait_imports: NodeSet,
     pub maybe_unused_extern_crates: Vec<(NodeId, Span)>,
     pub export_map: ExportMap,
-    pub extern_prelude: FxHashMap<Name, bool /* introduced by item */>,
+    /// Extern prelude entries. The value is `true` if the entry was introduced
+    /// via `extern crate` item and not `--extern` option or compiler built-in.
+    pub extern_prelude: FxHashMap<Name, bool>,
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]