internal/impl: add fast-path unmarshal

Benchmarks run with:
  go test ./benchmarks/ -bench=Wire  -benchtime=500ms -benchmem -count=8

Fast-path vs. parent commit:

  name                                      old time/op    new time/op    delta
  Wire/Unmarshal/google_message1_proto2-12    1.35µs ± 2%    0.45µs ± 4%  -67.01%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message1_proto3-12    1.07µs ± 1%    0.31µs ± 1%  -71.04%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message2-12            691µs ± 2%     188µs ± 2%  -72.78%  (p=0.000 n=7+8)

  name                                      old allocs/op  new allocs/op  delta
  Wire/Unmarshal/google_message1_proto2-12      60.0 ± 0%      25.0 ± 0%  -58.33%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message1_proto3-12      42.0 ± 0%       7.0 ± 0%  -83.33%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message2-12            28.6k ± 0%      8.5k ± 0%  -70.34%  (p=0.000 n=8+8)

Fast-path vs. -v1:

  name                                      old time/op    new time/op    delta
  Wire/Unmarshal/google_message1_proto2-12     702ns ± 1%     445ns ± 4%   -36.58%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message1_proto3-12     604ns ± 1%     311ns ± 1%   -48.54%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message2-12            179µs ± 3%     188µs ± 2%    +5.30%  (p=0.000 n=7+8)

  name                                      old allocs/op  new allocs/op  delta
  Wire/Unmarshal/google_message1_proto2-12      26.0 ± 0%      25.0 ± 0%    -3.85%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message1_proto3-12      8.00 ± 0%      7.00 ± 0%   -12.50%  (p=0.000 n=8+8)
  Wire/Unmarshal/google_message2-12            8.49k ± 0%     8.49k ± 0%    -0.01%  (p=0.000 n=8+8)

Change-Id: I6247ac3fd66a63d9acb902cbd192094ee3d151c3
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185147
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/message.go b/internal/impl/message.go
index e9a38c2..fe7cd37 100644
--- a/internal/impl/message.go
+++ b/internal/impl/message.go
@@ -516,6 +516,7 @@
 	return &piface.Methods{
 		Flags:         piface.MethodFlagDeterministicMarshal,
 		MarshalAppend: m.marshalAppend,
+		Unmarshal:     m.unmarshal,
 		Size:          m.size,
 		IsInitialized: m.isInitialized,
 	}
@@ -526,9 +527,13 @@
 func (m *messageIfaceWrapper) marshalAppend(b []byte, _ pref.ProtoMessage, opts piface.MarshalOptions) ([]byte, error) {
 	return m.mi.marshalAppendPointer(b, m.p, newMarshalOptions(opts))
 }
+func (m *messageIfaceWrapper) unmarshal(b []byte, _ pref.ProtoMessage, opts piface.UnmarshalOptions) error {
+	_, err := m.mi.unmarshalPointer(b, m.p, 0, newUnmarshalOptions(opts))
+	return err
+}
 func (m *messageIfaceWrapper) size(msg pref.ProtoMessage) (size int) {
 	return m.mi.sizePointer(m.p, 0)
 }
-func (m *messageIfaceWrapper) isInitialized(msg pref.ProtoMessage) error {
+func (m *messageIfaceWrapper) isInitialized(_ pref.ProtoMessage) error {
 	return m.mi.isInitializedPointer(m.p)
 }