Optimize deserialization of recursive buffered types

serde/src/de/mod.rs

@@ -1213,6 +1213,20 @@ pub trait Deserializer<'de>: Sized {
     fn is_human_readable(&self) -> bool {
         true
     }
+
+    // Not public API.
+    #[cfg(all(serde_derive, any(feature = "std", feature = "alloc")))]
+    #[doc(hidden)]
+    fn __deserialize_content<V>(
+        self,
+        _: ::actually_private::T,
+        visitor: V,
+    ) -> Result<::private::de::Content<'de>, Self::Error>
+    where
+        V: Visitor<'de, Value = ::private::de::Content<'de>>,
+    {
+        self.deserialize_any(visitor)
+    }
 }
 
 ////////////////////////////////////////////////////////////////////////////////

serde/src/lib.rs

@@ -295,3 +295,8 @@ extern crate serde_derive;
 #[cfg(feature = "serde_derive")]
 #[doc(hidden)]
 pub use serde_derive::*;
+
+#[cfg(all(serde_derive, any(feature = "std", feature = "alloc")))]
+mod actually_private {
+    pub struct T;
+}

serde/src/private/de.rs

@@ -206,6 +206,7 @@ mod content {
     use lib::*;
 
     use __private::size_hint;
+    use actually_private;
     use de::{
         self, Deserialize, DeserializeSeed, Deserializer, EnumAccess, Expected, IgnoredAny,
         MapAccess, SeqAccess, Unexpected, Visitor,
@@ -215,7 +216,7 @@ mod content {
     /// deserializing untagged enums and internally tagged enums.
     ///
     /// Not public API. Use serde-value instead.
-    #[derive(Debug)]
+    #[derive(Debug, Clone)]
     pub enum Content<'de> {
         Bool(bool),
 
@@ -294,7 +295,7 @@ mod content {
             // Untagged and internally tagged enums are only supported in
             // self-describing formats.
             let visitor = ContentVisitor { value: PhantomData };
-            deserializer.deserialize_any(visitor)
+            deserializer.__deserialize_content(actually_private::T, visitor)
         }
     }
 
@@ -1427,6 +1428,18 @@ mod content {
             drop(self);
             visitor.visit_unit()
         }
+
+        fn __deserialize_content<V>(
+            self,
+            _: actually_private::T,
+            visitor: V,
+        ) -> Result<Content<'de>, Self::Error>
+        where
+            V: Visitor<'de, Value = Content<'de>>,
+        {
+            let _ = visitor;
+            Ok(self.content)
+        }
     }
 
     impl<'de, E> ContentDeserializer<'de, E> {
@@ -2138,6 +2151,18 @@ mod content {
         {
             visitor.visit_unit()
         }
+
+        fn __deserialize_content<V>(
+            self,
+            _: actually_private::T,
+            visitor: V,
+        ) -> Result<Content<'de>, Self::Error>
+        where
+            V: Visitor<'de, Value = Content<'de>>,
+        {
+            let _ = visitor;
+            Ok(self.content.clone())
+        }
     }
 
     impl<'a, 'de, E> ContentRefDeserializer<'a, 'de, E> {