Remove some Serialize trait bounds

Containers for the most part do not have any trait requirements for
iterating over them. So these bounds are unnecessary when Serializing
only.

This relaxation is part of Rust 1.34

serde/build.rs

@@ -89,7 +89,7 @@ fn main() {
         println!("cargo:rustc-cfg=no_core_try_from");
         println!("cargo:rustc-cfg=no_num_nonzero_signed");
         println!("cargo:rustc-cfg=no_systemtime_checked_add");
-    }
+        println!("cargo:rustc-cfg=no_relaxed_trait_bounds");
 
     // Whitelist of archs that support std::sync::atomic module. Ideally we
     // would use #[cfg(target_has_atomic = "...")] but it is not stable yet.

serde/src/ser/impls.rs

@@ -182,7 +182,25 @@ where
     }
 }
 
-#[cfg(any(feature = "std", feature = "alloc"))]
+#[cfg(all(any(feature = "std", feature = "alloc"), not(no_relaxed_trait_bounds)))]
+macro_rules! seq_impl {
+    ($ty:ident < T $(: $tbound1:ident $(+ $tbound2:ident)*)* $(, $typaram:ident : $bound:ident)* >) => {
+        impl<T $(, $typaram)*> Serialize for $ty<T $(, $typaram)*>
+        where
+            T: Serialize,
+        {
+            #[inline]
+            fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+            where
+                S: Serializer,
+            {
+                serializer.collect_seq(self)
+            }
+        }
+    }
+}
+
+#[cfg(all(any(feature = "std", feature = "alloc"), no_relaxed_trait_bounds))]
 macro_rules! seq_impl {
     ($ty:ident < T $(: $tbound1:ident $(+ $tbound2:ident)*)* $(, $typaram:ident : $bound:ident)* >) => {
         impl<T $(, $typaram)*> Serialize for $ty<T $(, $typaram)*>