feat(build): Better support for custom codecs (#999)

BREAKING CHANGE: `CODEC_PATH` moved from const to fn

examples/src/json-codec/common.rs

@@ -0,0 +1,80 @@
+//! This module defines common request/response types as well as the JsonCodec that is used by the
+//! json.helloworld.Greeter service which is defined manually (instead of via proto files) by the
+//! `build_json_codec_service` function in the `examples/build.rs` file.
+
+use bytes::{Buf, BufMut};
+use serde::{Deserialize, Serialize};
+use std::marker::PhantomData;
+use tonic::{
+    codec::{Codec, DecodeBuf, Decoder, EncodeBuf, Encoder},
+    Status,
+};
+
+#[derive(Debug, Deserialize, Serialize)]
+pub struct HelloRequest {
+    pub name: String,
+}
+
+#[derive(Debug, Deserialize, Serialize)]
+pub struct HelloResponse {
+    pub message: String,
+}
+
+#[derive(Debug)]
+pub struct JsonEncoder<T>(PhantomData<T>);
+
+impl<T: serde::Serialize> Encoder for JsonEncoder<T> {
+    type Item = T;
+    type Error = Status;
+
+    fn encode(&mut self, item: Self::Item, buf: &mut EncodeBuf<'_>) -> Result<(), Self::Error> {
+        serde_json::to_writer(buf.writer(), &item).map_err(|e| Status::internal(e.to_string()))
+    }
+}
+
+#[derive(Debug)]
+pub struct JsonDecoder<U>(PhantomData<U>);
+
+impl<U: serde::de::DeserializeOwned> Decoder for JsonDecoder<U> {
+    type Item = U;
+    type Error = Status;
+
+    fn decode(&mut self, buf: &mut DecodeBuf<'_>) -> Result<Option<Self::Item>, Self::Error> {
+        if !buf.has_remaining() {
+            return Ok(None);
+        }
+
+        let item: Self::Item =
+            serde_json::from_reader(buf.reader()).map_err(|e| Status::internal(e.to_string()))?;
+        Ok(Some(item))
+    }
+}
+
+/// A [`Codec`] that implements `application/grpc+json` via the serde library.
+#[derive(Debug, Clone)]
+pub struct JsonCodec<T, U>(PhantomData<(T, U)>);
+
+impl<T, U> Default for JsonCodec<T, U> {
+    fn default() -> Self {
+        Self(PhantomData)
+    }
+}
+
+impl<T, U> Codec for JsonCodec<T, U>
+where
+    T: serde::Serialize + Send + 'static,
+    U: serde::de::DeserializeOwned + Send + 'static,
+{
+    type Encode = T;
+    type Decode = U;
+    type Encoder = JsonEncoder<T>;
+    type Decoder = JsonDecoder<U>;
+
+    fn encoder(&mut self) -> Self::Encoder {
+        JsonEncoder(PhantomData)
+    }
+
+    fn decoder(&mut self) -> Self::Decoder {
+        JsonDecoder(PhantomData)
+    }
+}