diff --git a/serde/src/private/size_hint.rs b/serde/src/private/size_hint.rs
index f1e34183..ca71e616 100644
--- a/serde/src/private/size_hint.rs
+++ b/serde/src/private/size_hint.rs
@@ -7,6 +7,7 @@ where
     helper(iter.size_hint())
 }
 
+#[cfg(any(feature = "std", feature = "alloc"))]
 #[inline]
 pub fn cautious(hint: Option<usize>) -> usize {
     cmp::min(hint.unwrap_or(0), 4096)