Use rustversion for conditional compilation

Because `std::num::Saturating` is only available
since Rust 1.74.

serde/Cargo.toml

@@ -16,6 +16,7 @@ rust-version = "1.31"
 
 [dependencies]
 serde_derive = { version = "1", optional = true, path = "../serde_derive" }
+rustversion = "1.0"
 
 [dev-dependencies]
 serde_derive = { version = "1", path = "../serde_derive" }

serde/src/de/impls.rs

@@ -1,4 +1,5 @@
 use crate::lib::*;
+use rustversion;
 
 use crate::de::{
     Deserialize, Deserializer, EnumAccess, Error, MapAccess, SeqAccess, Unexpected, VariantAccess,
@@ -3004,6 +3005,7 @@ where
     }
 }
 
+#[rustversion::since(1.74)]
 impl<'de, T> Deserialize<'de> for Saturating<T>
 where
     T: Deserialize<'de>,

serde/src/lib.rs

@@ -185,7 +185,6 @@ mod lib {
     pub use self::core::default;
     pub use self::core::fmt::{self, Debug, Display, Write as FmtWrite};
     pub use self::core::marker::{self, PhantomData};
-    pub use self::core::num::Saturating;
     pub use self::core::num::Wrapping;
     pub use self::core::ops::{Bound, Range, RangeFrom, RangeInclusive, RangeTo};
     pub use self::core::option;
@@ -275,6 +274,9 @@ mod lib {
     pub use std::sync::atomic::{AtomicI64, AtomicU64};
     #[cfg(all(feature = "std", not(no_target_has_atomic), target_has_atomic = "ptr"))]
     pub use std::sync::atomic::{AtomicIsize, AtomicUsize};
+
+    #[rustversion::since(1.74)]
+    pub use self::core::num::Saturating;
 }
 
 // None of this crate's error handling needs the `From::from` error conversion

serde/src/ser/impls.rs

@@ -1026,6 +1026,7 @@ where
     }
 }
 
+#[rustversion::since(1.74)]
 impl<T> Serialize for Saturating<T>
 where
     T: Serialize,