diff --git a/serde/build.rs b/serde/build.rs
index 391c5a2e..fcff8e6d 100644
--- a/serde/build.rs
+++ b/serde/build.rs
@@ -39,6 +39,13 @@ fn main() {
         println!("cargo:rustc-cfg=de_boxed_c_str");
     }
 
+    // From<Box<T>> for Rc<T> / Arc<T> stabilized in Rust 1.21:
+    // https://doc.rust-lang.org/std/rc/struct.Rc.html#impl-From<Box<T>>
+    // https://doc.rust-lang.org/std/sync/struct.Arc.html#impl-From<Box<T>>
+    if minor >= 21 {
+        println!("cargo:rustc-cfg=de_rc_dst");
+    }
+
     // 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 6d325883..342ca3be 100644
--- a/serde/src/de/impls.rs
+++ b/serde/src/de/impls.rs
@@ -1528,7 +1528,7 @@ forwarded_impl!((T), Box<[T]>, Vec::into_boxed_slice);
 #[cfg(any(feature = "std", feature = "alloc"))]
 forwarded_impl!((), Box<str>, String::into_boxed_str);
 
-#[cfg(all(not(feature = "unstable"), feature = "rc", any(feature = "std", feature = "alloc")))]
+#[cfg(all(not(de_rc_dst), feature = "rc", any(feature = "std", feature = "alloc")))]
 forwarded_impl! {
     /// This impl requires the [`"rc"`] Cargo feature of Serde.
     ///
@@ -1540,7 +1540,7 @@ forwarded_impl! {
     (T), Arc<T>, Arc::new
 }
 
-#[cfg(all(not(feature = "unstable"), feature = "rc", any(feature = "std", feature = "alloc")))]
+#[cfg(all(not(de_rc_dst), feature = "rc", any(feature = "std", feature = "alloc")))]
 forwarded_impl! {
     /// This impl requires the [`"rc"`] Cargo feature of Serde.
     ///
@@ -1607,7 +1607,7 @@ where
 
 ////////////////////////////////////////////////////////////////////////////////
 
-#[cfg(all(feature = "unstable", feature = "rc", any(feature = "std", feature = "alloc")))]
+#[cfg(all(de_rc_dst, feature = "rc", any(feature = "std", feature = "alloc")))]
 macro_rules! box_forwarded_impl {
     (
         $(#[doc = $doc:tt])*
@@ -1628,7 +1628,7 @@ macro_rules! box_forwarded_impl {
     };
 }
 
-#[cfg(all(feature = "unstable", feature = "rc", any(feature = "std", feature = "alloc")))]
+#[cfg(all(de_rc_dst, feature = "rc", any(feature = "std", feature = "alloc")))]
 box_forwarded_impl! {
     /// This impl requires the [`"rc"`] Cargo feature of Serde.
     ///
@@ -1640,7 +1640,7 @@ box_forwarded_impl! {
     Rc
 }
 
-#[cfg(all(feature = "unstable", feature = "rc", any(feature = "std", feature = "alloc")))]
+#[cfg(all(de_rc_dst, feature = "rc", any(feature = "std", feature = "alloc")))]
 box_forwarded_impl! {
     /// This impl requires the [`"rc"`] Cargo feature of Serde.
     ///