Reverse impls available for Rust >=1.19

serde/build.rs

@@ -23,6 +23,12 @@ fn main() {
         println!("cargo:rustc-cfg=collections_bound");
     }
 
+    // core::cmp::Reverse stabilized in Rust 1.19:
+    // https://doc.rust-lang.org/stable/core/cmp/struct.Reverse.html
+    if minor >= 19 {
+        println!("cargo:rustc-cfg=core_reverse");
+    }
+
     // 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 {

serde/src/de/impls.rs

@@ -578,6 +578,7 @@ macro_rules! forwarded_impl {
 #[cfg(all(feature = "std", de_boxed_c_str))]
 forwarded_impl!((), Box<CStr>, CString::into_boxed_c_str);
 
+#[cfg(core_reverse)]
 forwarded_impl!((T), Reverse<T>, Reverse);
 
 ////////////////////////////////////////////////////////////////////////////////

serde/src/lib.rs

@@ -139,6 +139,7 @@ mod lib {
 
     pub use self::core::cell::{Cell, RefCell};
     pub use self::core::clone::{self, Clone};
+    #[cfg(core_reverse)]
     pub use self::core::cmp::Reverse;
     pub use self::core::convert::{self, From, Into};
     pub use self::core::default::{self, Default};

serde/src/ser/impls.rs

@@ -825,6 +825,7 @@ where
     }
 }
 
+#[cfg(core_reverse)]
 impl<T> Serialize for Reverse<T>
     where
         T: Serialize,