Prevent various panics when deserializing malformed SystemTime

serde/build.rs

@@ -76,12 +76,14 @@ fn main() {
         println!("cargo:rustc-cfg=serde_derive");
     }
 
-    // TryFrom, Atomic types, and non-zero signed integers stabilized in Rust 1.34:
+    // TryFrom, Atomic types, non-zero signed integers, and `SystemTime::checked_add`
+    // 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
     if minor >= 34 {
         println!("cargo:rustc-cfg=core_try_from");
         println!("cargo:rustc-cfg=num_nonzero_signed");
+        println!("cargo:rustc-cfg=systemtime_checked_add");
 
         // Whitelist of archs that support std::sync::atomic module. Ideally we
         // would use #[cfg(target_has_atomic = "...")] but it is not stable yet.