diff --git a/.cargo/config b/.cargo/config
index 463a715..93640ad 100644
--- a/.cargo/config
+++ b/.cargo/config
@@ -7,7 +7,9 @@ rustflags = [
   "-C", "link-arg=-Tdefmt.x",
 
   # Code-size optimizations.
-  "-Z", "trap-unreachable=no",
+  #   trap unreachable can save a lot of space, but requires nightly compiler.
+  #   uncomment the next line if you wish to enable it
+  # "-Z", "trap-unreachable=no",
   "-C", "inline-threshold=5",
   "-C", "no-vectorize-loops",
 ]