diff --git a/serde/build.rs b/serde/build.rs
index 28321278d..391c5a2ef 100644
--- a/serde/build.rs
+++ b/serde/build.rs
@@ -33,6 +33,12 @@ fn main() {
         Err(_) => return,
     };
 
+    // CString::into_boxed_c_str stabilized in Rust 1.20:
+    // https://doc.rust-lang.org/std/ffi/struct.CString.html#method.into_boxed_c_str
+    if minor >= 20 {
+        println!("cargo:rustc-cfg=de_boxed_c_str");
+    }
+
     // 128-bit integers stabilized in Rust 1.26:
     // https://blog.rust-lang.org/2018/05/10/Rust-1.26.html
     if minor >= 26 {
diff --git a/serde/src/de/impls.rs b/serde/src/de/impls.rs
index fdade5b39..6d3258832 100644
--- a/serde/src/de/impls.rs
+++ b/serde/src/de/impls.rs
@@ -573,7 +573,7 @@ macro_rules! forwarded_impl {
     }
 }
 
-#[cfg(all(feature = "std", feature = "unstable"))]
+#[cfg(all(feature = "std", de_boxed_c_str))]
 forwarded_impl!((), Box<CStr>, CString::into_boxed_c_str);
 
 ////////////////////////////////////////////////////////////////////////////////