Require std for deserializing untagged enums

serde/src/de/content.rs

@@ -1,5 +1,11 @@
-use std::fmt;
-use std::marker::PhantomData;
+use core::fmt;
+use core::marker::PhantomData;
+
+#[cfg(all(not(feature = "std"), feature = "collections"))]
+use collections::{String, Vec};
+
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+use alloc::boxed::Box;
 
 use de::{
     self,

serde/src/de/mod.rs

@@ -115,6 +115,7 @@ mod from_primitive;
 // Helpers used by generated code. Not public API.
 #[doc(hidden)]
 pub mod private;
+#[cfg(any(feature = "std", feature = "collections"))]
 mod content;
 
 ///////////////////////////////////////////////////////////////////////////////

serde/src/de/private.rs

@@ -2,6 +2,7 @@ use core::marker::PhantomData;
 
 use de::{Deserialize, Deserializer, Error, Visitor};
 
+#[cfg(any(feature = "std", feature = "collections"))]
 pub use de::content::{
     Content,
     TaggedContentVisitor,

serde/src/ser/private.rs

@@ -1,4 +1,4 @@
-use std::fmt::{self, Display};
+use core::fmt::{self, Display};
 
 use ser::{self, Serialize, Serializer, SerializeMap, SerializeStruct};