diff --git a/language-rust.cabal b/language-rust.cabal
index 5606bae..6871c88 100644
--- a/language-rust.cabal
+++ b/language-rust.cabal
@@ -42,6 +42,9 @@ library
                        -Wmissing-signatures
 
   build-tools:         alex >=3.1, happy >=1.19.8 && < 2.1
+  -- We restrict to < 2.1, because of
+  -- https://github.com/haskell/happy/issues/320
+
   default-language:    Haskell2010
 
   exposed-modules:     Language.Rust.Syntax