feat: TypeUrl trait + Any encoding support

As discussed in tokio-rs#299, adds a `TypeUrl` trait which associates a type URL
constant with a `Message` type.

This enables adding methods to `Any` for decoding/encoding messages:

- `Any::from_message`: encodes a given `Message`, returning `Any`.
- `Any::to_message`: decodes `Any::value` as the given `Message`, first
  validating the message type has the expected type URL.

prost-types/src/lib.rs

@@ -24,6 +24,8 @@ use core::i64;
 use core::str::FromStr;
 use core::time;
 
+use prost::{DecodeError, EncodeError, Message, TypeUrl};
+
 pub use protobuf::*;
 
 // The Protobuf `Duration` and `Timestamp` types can't delegate to the standard library equivalents
@@ -33,6 +35,39 @@ pub use protobuf::*;
 const NANOS_PER_SECOND: i32 = 1_000_000_000;
 const NANOS_MAX: i32 = NANOS_PER_SECOND - 1;
 
+impl Any {
+    /// Serialize this message proto as [`Any`].
+    pub fn from_message<M>(msg: &M) -> Result<Self, EncodeError>
+    where
+        M: TypeUrl,
+    {
+        let type_url = M::TYPE_URL.to_owned();
+        let mut value = Vec::new();
+        Message::encode(msg, &mut value)?;
+
+        Ok(Any { type_url, value })
+    }
+
+    /// Decode the given message type `M` from [`Any`], validating that it has
+    /// the expected [`TypeUrl`].
+    pub fn to_message<M>(&self) -> Result<M, DecodeError>
+    where
+        M: Default + Sized + TypeUrl,
+    {
+        if self.type_url == M::TYPE_URL {
+            Ok(M::decode(&*self.value)?)
+        } else {
+            let mut err = DecodeError::new(format!(
+                "expected type URL: \"{}\" (got: \"{}\")",
+                M::TYPE_URL,
+                &self.type_url
+            ));
+            err.push("unexpected type URL", "type_url");
+            Err(err)
+        }
+    }
+}
+
 impl Duration {
     /// Normalizes the duration to a canonical format.
     ///

src/lib.rs

@@ -11,13 +11,15 @@ pub use bytes;
 
 mod error;
 mod message;
+mod type_url;
 mod types;
 
 #[doc(hidden)]
 pub mod encoding;
 
 pub use crate::error::{DecodeError, EncodeError};
 pub use crate::message::Message;
+pub use crate::type_url::TypeUrl;
 
 use bytes::{Buf, BufMut};
 

src/type_url.rs

@@ -0,0 +1,19 @@
+//! Support for associating a type URL with a [`Message`].
+
+use crate::Message;
+
+/// Associate a type URL with the given [`Message]`.
+pub trait TypeUrl: Message {
+    /// Type URL for this [`Message`]. They take the form:
+    ///
+    /// ```text
+    /// /<package>.<TypeName>
+    /// ```
+    ///
+    /// For example:
+    ///
+    /// ```text
+    /// /foo.bar.baz.MyTypeName
+    /// ```
+    const TYPE_URL: &'static str;
+}