diff --git a/syntax/check.rs b/syntax/check.rs
index 2e94f4d..1ec6178 100644
--- a/syntax/check.rs
+++ b/syntax/check.rs
@@ -178,7 +178,10 @@
 }
 
 fn check_type_slice(cx: &mut Check, ty: &Slice) {
-    cx.error(ty, "only &[u8] is supported so far, not other slice types");
+    cx.error(
+        ty,
+        "only &[u8] and &mut [u8] are supported so far, not other slice types",
+    );
 }
 
 fn check_type_fn(cx: &mut Check, ty: &Signature) {
@@ -489,7 +492,10 @@
         Type::Str(_) => "&str".to_owned(),
         Type::CxxVector(_) => "C++ vector".to_owned(),
         Type::Slice(_) => "slice".to_owned(),
-        Type::SliceRefU8(_) => "&[u8]".to_owned(),
+        Type::SliceRefU8(ty) => match ty.mutable {
+            false => "&[u8]".to_owned(),
+            true => "&mut [u8]".to_owned(),
+        },
         Type::Fn(_) => "function pointer".to_owned(),
         Type::Void(_) => "()".to_owned(),
     }