Omit derive helpers in versions older than serde_derive msrv

serde/build.rs

@@ -71,6 +71,11 @@ fn main() {
         println!("cargo:rustc-cfg=num_nonzero");
     }
 
+    // Current minimum supported version of serde_derive crate is Rust 1.31.
+    if minor >= 31 {
+        println!("cargo:rustc-cfg=serde_derive");
+    }
+
     // TryFrom, Atomic types, and non-zero signed integers stabilized in Rust 1.34:
     // https://blog.rust-lang.org/2019/04/11/Rust-1.34.0.html#tryfrom-and-tryinto
     // https://blog.rust-lang.org/2019/04/11/Rust-1.34.0.html#library-stabilizations

serde/src/private/mod.rs

@@ -1,5 +1,8 @@
+#[cfg(serde_derive)]
 pub mod de;
+#[cfg(serde_derive)]
 pub mod ser;
+
 pub mod size_hint;
 
 // FIXME: #[cfg(doctest)] once https://github.com/rust-lang/rust/issues/67295 is fixed.