Fix allownil propagated to byte slices as slice elements (#374)

`[][]byte` would assume that the byte slices had allownil set. This would cause an extra `if x == nil {x = make([]byte,0)}` to be inserted on elements.

Regression from #363

_generated/omitempty.go

@@ -29,9 +29,11 @@ type OmitEmpty0 struct {
 
 	APtrNamedStr *NamedString `msg:"aptrnamedstr,omitempty"`
 
-	AString      string `msg:"astring,omitempty"`
-	ANamedString string `msg:"anamedstring,omitempty"`
-	AByteSlice   []byte `msg:"abyteslice,omitempty"`
+	AString         string            `msg:"astring,omitempty"`
+	ANamedString    string            `msg:"anamedstring,omitempty"`
+	AByteSlice      []byte            `msg:"abyteslice,omitempty"`
+	ASliceByteSlice [][]byte          `msg:"aslicebyteslice,omitempty"`
+	AMapByteSlice   map[string][]byte `msg:"amapbyteslice,omitempty"`
 
 	ASliceString      []string      `msg:"aslicestring,omitempty"`
 	ASliceNamedString []NamedString `msg:"aslicenamedstring,omitempty"`

gen/spec.go

@@ -385,6 +385,14 @@ func (p *printer) closeblock() { p.print("\n}") }
 //	}
 func (p *printer) rangeBlock(ctx *Context, idx string, iter string, t traversal, inner Elem) {
 	ctx.PushVar(idx)
+	// Tags on slices do not extend to the elements, so we always disable allownil on elements.
+	// If we want this to happen in the future, it should be a unique tag.
+	type an interface {
+		SetIsAllowNil(b bool)
+	}
+	if set, ok := inner.(an); ok {
+		set.SetIsAllowNil(false)
+	}
 	p.printf("\n for %s := range %s {", idx, iter)
 	next(t, inner)
 	p.closeblock()

msgp/write_bytes_test.go

@@ -199,7 +199,7 @@ func BenchmarkAppendFloat(b *testing.B) {
 	b.ReportAllocs()
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
-		AppendFloat64(buf, src[i&(n-1)])
+		AppendFloat(buf, src[i&(n-1)])
 	}
 }