Problem

When using pools, the ResetVT method is able to reuse allocated memory of bytes. It looks like this with a minimal example:

syntax = "proto2";
package TEST;
option go_package = "./testpkg";

message Foo {
	required bytes raw = 1;
}

func (m *Foo) ResetVT() {
	if m != nil {
		f0 := m.Raw[:0]
		m.Reset()
		m.Raw = f0
	}
}

When the bytes are inside a oneof field, the memory is no longer reused:

syntax = "proto2";
package TEST;
option go_package = "./testpkg";

message Foo {
	oneof data {
		bytes raw = 1;
		bytes zlib_data = 2;
	}
}

func (m *Foo) ResetVT() {
	if m != nil {
		m.Reset()
	}
}

Use Case

This is a real problem for me when parsing open streetmap's PBF files, because they use this construct for large blobs of data that occur hundreds to thousands of times in a PBF file: https://github.com/openstreetmap/OSM-binary/blob/65e7e976f5c8e47f057a0d921639ea8e6309ef06/osmpbf/fileformat.proto#L38

Maybe it would have been better if they used a single bytes field and a separate type field, but I'm afraid the design is set in stone as the format is already widely used.

Suggested Goal

Unfortunately I don't understand the code well enough to provide a pull request, but this is how I would imagine the generated code to look like in order to fix the problem:

func (m *Foo) ResetVT() {
	if m != nil {
		var f0 isFoo_Data
		switch t := m.Data.(type) {
		case *Foo_Raw:
			t.Raw = t.Raw[:0]
			f0 = t
		case *Foo_ZlibData:
			t.ZlibData = t.ZlibData[:0]
			f0 = t
		}
		m.Reset()
		m.Data = f0
	}
}

Inside the UnmarshalVT method, the code would look something like this; As a first step I'm only reusing the memory, if the Data field previously had the same type:

...
			var v []byte
			if m.Data == nil {
				v = make([]byte, postIndex-iNdEx)
			} else {
				switch t := m.Data.(type) {
				case *Foo_Raw:
					if cap(t.Raw) < postIndex-iNdEx {
						v = make([]byte, postIndex-iNdEx)
					} else {
						v = t.Raw[:postIndex-iNdEx]
					}
				default:
					v = make([]byte, postIndex-iNdEx)
				}
			}
			copy(v, dAtA[iNdEx:postIndex])
			m.Data = &Foo_Raw{Raw: v}
...