Reviving tagged values in Serde

revive-tagged-values.diff

commit 56080751ae37a4202b0681b36ab066bc838d9537
Author: Volker Mische <[email protected]>
Date:   Fri Sep 27 00:28:51 2019 +0200

    WIP

diff --git a/serde/src/de/mod.rs b/serde/src/de/mod.rs
index f1b8b312..646714c0 100644
--- a/serde/src/de/mod.rs
+++ b/serde/src/de/mod.rs
@@ -1615,6 +1615,7 @@ pub trait Visitor<'de>: Sized {
     {
         let _ = deserializer;
         Err(Error::invalid_type(Unexpected::NewtypeStruct, &self))
+        //Deserialize::deserialize(deserializer)
     }
 
     /// The input contains a sequence of elements.
@@ -1650,6 +1651,17 @@ pub trait Visitor<'de>: Sized {
         Err(Error::invalid_type(Unexpected::Enum, &self))
     }
 
+    /// The input contains a tagged value
+    fn visit_tagged_value<T, D>(self, _tag: T, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        T: ::Tagger,
+        D: Deserializer<'de>,
+    {
+        //Deserialize::deserialize(deserializer)
+        Deserializer::deserialize_any(deserializer)
+        //Err(Error::invalid_type(Unexpected::NewtypeStruct, &self))
+    }
+
     // Used when deserializing a flattened Option field. Not public API.
     #[doc(hidden)]
     fn __private_visit_untagged_option<D>(self, _: D) -> Result<Self::Value, ()>
diff --git a/serde/src/lib.rs b/serde/src/lib.rs
index b52c9501..5779d7fc 100644
--- a/serde/src/lib.rs
+++ b/serde/src/lib.rs
@@ -237,11 +237,14 @@ mod integer128;
 
 pub mod de;
 pub mod ser;
+mod tagger;
 
 #[doc(inline)]
 pub use de::{Deserialize, Deserializer};
 #[doc(inline)]
 pub use ser::{Serialize, Serializer};
+#[doc(inline)]
+pub use tagger::Tagger;
 
 // Generated code uses these to support no_std. Not public API.
 #[doc(hidden)]
diff --git a/serde/src/ser/mod.rs b/serde/src/ser/mod.rs
index 8fb2434f..98bef6de 100644
--- a/serde/src/ser/mod.rs
+++ b/serde/src/ser/mod.rs
@@ -1247,6 +1247,17 @@ pub trait Serializer: Sized {
         len: usize,
     ) -> Result<Self::SerializeStructVariant, Self::Error>;
 
+    /// Serialize a tagged value.
+    ///
+    /// By default, the tag is discarded and the value is serialized as-is.
+    fn serialize_tagged_value<T, V>(self, _tag: T, value: V) -> Result<Self::Ok, Self::Error>
+    where
+        T: ::Tagger,
+        V: Serialize,
+    {
+        value.serialize(self)
+    }
+
     /// Collect an iterator as a sequence.
     ///
     /// The default implementation serializes each item yielded by the iterator
diff --git a/serde/src/tagger.rs b/serde/src/tagger.rs
new file mode 100644
index 00000000..f1938f3c
--- /dev/null
+++ b/serde/src/tagger.rs
@@ -0,0 +1,32 @@
+/// Provides the correct tag for a given format.
+pub trait Tagger {
+    /// Returns a `u8` tag if available.
+    fn u8_tag(&self, _format: &'static str) -> Option<u8> { None }
+
+    /// Returns a `u16` tag if available.
+    fn u16_tag(&self, _format: &'static str) -> Option<u16> { None }
+
+    /// Returns a `u32` tag if available.
+    fn u32_tag(&self, _format: &'static str) -> Option<u32> { None }
+
+    /// Returns a `u64` tag if available.
+    fn u64_tag(&self, _format: &'static str) -> Option<u64> { None }
+
+    /// Returns an `i8` tag if available.
+    fn i8_tag(&self, _format: &'static str) -> Option<i8> { None }
+
+    /// Returns an `i16` tag if available.
+    fn i16_tag(&self, _format: &'static str) -> Option<i16> { None }
+
+    /// Returns an `i32` tag if available.
+    fn i32_tag(&self, _format: &'static str) -> Option<i32> { None }
+
+    /// Returns an `i64` tag if available.
+    fn i64_tag(&self, _format: &'static str) -> Option<i64> { None }
+
+    /// Provides a string tag if available.
+    fn string_tag(&self, _format: &'static str) -> Option<&str> { None }
+
+    /// Provides a binary tag if available.
+    fn bytes_tag(&self, _format: &'static str) -> Option<&[u8]> { None }
+}