diff --git a/artiq-fast/pkgs/rust/cargo.nix b/artiq-fast/pkgs/rust/cargo.nix
index 9772113..dad4047 100644
--- a/artiq-fast/pkgs/rust/cargo.nix
+++ b/artiq-fast/pkgs/rust/cargo.nix
@@ -6,6 +6,7 @@
 rustPlatform.buildRustPackage rec {
   # Note: we can't build cargo 1.28.0 because rustc tightened the borrow checker rules and broke
   # backward compatibility, which affects old cargo versions.
+  # There are also issues with asm/llvm_asm with recent rustc and cargo versions prior to 1.39.
   name = "cargo-${version}";
   version = "1.39.0";
   src = fetchurl {