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/encode.go b/internal/impl/encode.go
index c258d46..bf7308a 100644
--- a/internal/impl/encode.go
+++ b/internal/impl/encode.go
@@ -13,19 +13,19 @@
 	piface "google.golang.org/protobuf/runtime/protoiface"
 )
 
+// marshalOptions is a more efficient representation of MarshalOptions.
+//
+// We don't preserve the AllowPartial flag, because fast-path (un)marshal
+// operations always allow partial messages.
 type marshalOptions uint
 
 const (
-	marshalAllowPartial marshalOptions = 1 << iota
-	marshalDeterministic
+	marshalDeterministic marshalOptions = 1 << iota
 	marshalUseCachedSize
 )
 
 func newMarshalOptions(opts piface.MarshalOptions) marshalOptions {
 	var o marshalOptions
-	if opts.AllowPartial {
-		o |= marshalAllowPartial
-	}
 	if opts.Deterministic {
 		o |= marshalDeterministic
 	}
@@ -37,13 +37,12 @@
 
 func (o marshalOptions) Options() proto.MarshalOptions {
 	return proto.MarshalOptions{
-		AllowPartial:  o.AllowPartial(),
+		AllowPartial:  true,
 		Deterministic: o.Deterministic(),
 		UseCachedSize: o.UseCachedSize(),
 	}
 }
 
-func (o marshalOptions) AllowPartial() bool  { return o&marshalAllowPartial != 0 }
 func (o marshalOptions) Deterministic() bool { return o&marshalDeterministic != 0 }
 func (o marshalOptions) UseCachedSize() bool { return o&marshalUseCachedSize != 0 }