Support Duration in no-std mode on new compilers

serde/build.rs

@@ -24,6 +24,12 @@ fn main() {
         println!("cargo:rustc-cfg=de_rc_dst");
     }
 
+    // Duration available in core since Rust 1.25:
+    // https://blog.rust-lang.org/2018/03/29/Rust-1.25.html#library-stabilizations
+    if minor >= 25 {
+        println!("cargo:rustc-cfg=core_duration");
+    }
+
     // 128-bit integers stabilized in Rust 1.26:
     // https://blog.rust-lang.org/2018/05/10/Rust-1.26.html
     if minor >= 26 {

serde/src/de/impls.rs

@@ -12,7 +12,7 @@ use de::{
     Deserialize, Deserializer, EnumAccess, Error, SeqAccess, Unexpected, VariantAccess, Visitor,
 };
 
-#[cfg(any(feature = "std", feature = "alloc"))]
+#[cfg(any(core_duration, feature = "std", feature = "alloc"))]
 use de::MapAccess;
 
 use de::from_primitive::FromPrimitive;
@@ -1694,7 +1694,7 @@ forwarded_impl!((T), RwLock<T>, RwLock::new);
 //         secs: u64,
 //         nanos: u32,
 //     }
-#[cfg(feature = "std")]
+#[cfg(any(core_duration, feature = "std"))]
 impl<'de> Deserialize<'de> for Duration {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
@@ -1742,7 +1742,7 @@ impl<'de> Deserialize<'de> for Duration {
                             b"secs" => Ok(Field::Secs),
                             b"nanos" => Ok(Field::Nanos),
                             _ => {
-                                let value = String::from_utf8_lossy(value);
+                                let value = ::export::from_utf8_lossy(value);
                                 Err(Error::unknown_field(&value, FIELDS))
                             }
                         }

serde/src/lib.rs

@@ -211,7 +211,10 @@ mod lib {
     #[cfg(feature = "std")]
     pub use std::sync::{Mutex, RwLock};
     #[cfg(feature = "std")]
-    pub use std::time::{Duration, SystemTime, UNIX_EPOCH};
+    pub use std::time::{SystemTime, UNIX_EPOCH};
+
+    #[cfg(any(core_duration, feature = "std"))]
+    pub use self::core::time::Duration;
 }
 
 ////////////////////////////////////////////////////////////////////////////////

serde/src/ser/impls.rs

@@ -527,7 +527,7 @@ where
 
 ////////////////////////////////////////////////////////////////////////////////
 
-#[cfg(feature = "std")]
+#[cfg(any(core_duration, feature = "std"))]
 impl Serialize for Duration {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where