diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index 671a0fc2d5d7aa96fed93c20b0ed1681c787807a..6d49da5d002ba227dcebfbd139284326a4277bf6 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -107,6 +107,7 @@ pub enum TyKind<'tcx> {
     /// definition and not a concrete use of it.
     Adt(&'tcx AdtDef, &'tcx Substs<'tcx>),
 
+    /// An unsized FFI type that is opaque to Rust. Written as `extern type T`.
     Foreign(DefId),
 
     /// The pointee of a string slice. Written as `str`.