Add unmarshal_unsafe feature

Makefile

@@ -61,6 +61,7 @@ gen-testproto: gen-wkt-testproto install
 		testproto/pool/pool_with_oneof.proto \
 		testproto/proto3opt/opt.proto \
 		testproto/proto2/scalars.proto \
+		testproto/unsafe/unsafe.proto \
 		|| exit 1;
 
 gen-wkt-testproto: install

README.md

@@ -37,6 +37,8 @@ The following features can be generated:
 
 - `unmarshal`: generates a `func (p *YourProto) UnmarshalVT(data []byte)` that behaves similarly to calling `proto.Unmarshal(data, p)` on the message, except the unmarshalling is performed by unrolled codegen without using reflection and allocating as little memory as possible. If the receiver `p` is **not** fully zeroed-out, the unmarshal call will actually behave like `proto.Merge(data, p)`. This is because the `proto.Unmarshal` in the ProtoBuf API is implemented by resetting the destination message and then calling `proto.Merge` on it. To ensure proper `Unmarshal` semantics, ensure you've called `proto.Reset` on your message before calling `UnmarshalVT`, or that your message has been newly allocated.
 
+- `unmarshal_unsafe` generates a `func (p *YourProto) UnmarshalVTUnsafe(data []byte)` that behaves like `UnmarshalVT`, except it unsafely casts slices of data to `bytes` and `string` fields instead of copying them to newly allocated arrays, so that it performs less allocations. **Data received from the wire has to be left untouched for the lifetime of the message.** Otherwise, the message's `bytes` and `string` fields can be corrupted.
+
 - `pool`: generates the following helper methods
 
     - `func (p *YourProto) ResetVT()`: this function behaves similarly to `proto.Reset(p)`, except it keeps as much memory as possible available on the message, so that further calls to `UnmarshalVT` on the same message will need to allocate less memory. This an API meant to be used with memory pools and does not need to be used directly.
@@ -221,4 +223,4 @@ plugins:
     opt: paths=source_relative
 ```
 
-Running `buf generate` will now also include the `vtprotobuf` optimized helpers.
+Running `buf generate` will now also include the `vtprotobuf` optimized helpers.