flag to control zeroing of target structs before decode/unmarshal

cfg/config.go

@@ -11,6 +11,8 @@ type MsgpConfig struct {
 	Marshal    bool
 	Tests      bool
 	Unexported bool
+
+	SkipStructZeroingOnDecode bool
 }
 
 // call DefineFlags before myflags.Parse()
@@ -21,6 +23,7 @@ func (c *MsgpConfig) DefineFlags(fs *flag.FlagSet) {
 	fs.BoolVar(&c.Marshal, "marshal", true, "create Marshal and Unmarshal methods")
 	fs.BoolVar(&c.Tests, "tests", true, "create tests and benchmarks")
 	fs.BoolVar(&c.Unexported, "unexported", false, "also process unexported types")
+	fs.BoolVar(&c.SkipStructZeroingOnDecode, "skip-decode-struct-zeroing", false, "possibly dangerous option: don't zero out the target struct before decoding/unmarshalling into it.")
 }
 
 // call c.ValidateConfig() after myflags.Parse()

gen/decode.go

@@ -3,12 +3,15 @@ package gen
 import (
 	"io"
 	"strconv"
+
+	"github.com/tinylib/msgp/cfg"
 )
 
-func decode(w io.Writer) *decodeGen {
+func decode(w io.Writer, cfg *cfg.MsgpConfig) *decodeGen {
 	return &decodeGen{
 		p:        printer{w: w},
 		hasfield: false,
+		cfg:      cfg,
 	}
 }
 
@@ -17,6 +20,7 @@ type decodeGen struct {
 	p        printer
 	hasfield bool
 	depth    int
+	cfg      *cfg.MsgpConfig
 }
 
 func (d *decodeGen) Method() Method { return Decode }
@@ -93,9 +97,11 @@ func (d *decodeGen) structAsTuple(s *Struct) {
 }
 
 func (d *decodeGen) structAsMap(s *Struct) {
-	if d.depth == 1 {
-		d.p.printf("\n\n// zero the target:\n")
-		d.p.printf("*%s = %s{}\n", s.vname, s.TypeName())
+	if !d.cfg.SkipStructZeroingOnDecode {
+		if d.depth == 1 {
+			d.p.printf("\n\n// zero the target:\n")
+			d.p.printf("*%s = %s{}\n", s.vname, s.TypeName())
+		}
 	}
 	d.needsField()
 	sz := randIdent()

gen/spec.go

@@ -2,6 +2,7 @@ package gen
 
 import (
 	"fmt"
+	"github.com/tinylib/msgp/cfg"
 	"io"
 )
 
@@ -101,17 +102,19 @@ const (
 
 type Printer struct {
 	gens []generator
+	cfg  *cfg.MsgpConfig
 }
 
-func NewPrinter(m Method, out io.Writer, tests io.Writer) *Printer {
+func NewPrinter(m Method, out io.Writer, tests io.Writer, cfg *cfg.MsgpConfig) *Printer {
 	if m.isset(Test) && tests == nil {
 		panic("cannot print tests with 'nil' tests argument!")
 	}
 	gens := make([]generator, 0, 8)
 	if m.isset(Decode) {
-		gens = append(gens, decode(out))
+		gens = append(gens, decode(out, cfg))
 	}
-	// must run FieldsEmpty before Encode/Marshal, to set Struct.hasOmitEmptyTags
+	// must run FieldsEmpty before Encode/Marshal, so as
+	// to set Struct.hasOmitEmptyTags
 	if m.isset(FieldsEmpty) {
 		gens = append(gens, fieldsempty(out))
 	}
@@ -122,7 +125,7 @@ func NewPrinter(m Method, out io.Writer, tests io.Writer) *Printer {
 		gens = append(gens, marshal(out))
 	}
 	if m.isset(Unmarshal) {
-		gens = append(gens, unmarshal(out))
+		gens = append(gens, unmarshal(out, cfg))
 	}
 	if m.isset(Size) {
 		gens = append(gens, sizes(out))
@@ -136,7 +139,7 @@ func NewPrinter(m Method, out io.Writer, tests io.Writer) *Printer {
 	if len(gens) == 0 {
 		panic("NewPrinter called with invalid method flags")
 	}
-	return &Printer{gens: gens}
+	return &Printer{gens: gens, cfg: cfg}
 }
 
 // TransformPass is a pass that transforms individual

gen/unmarshal.go

@@ -3,11 +3,14 @@ package gen
 import (
 	"io"
 	"strconv"
+
+	"github.com/tinylib/msgp/cfg"
 )
 
-func unmarshal(w io.Writer) *unmarshalGen {
+func unmarshal(w io.Writer, cfg *cfg.MsgpConfig) *unmarshalGen {
 	return &unmarshalGen{
-		p: printer{w: w},
+		p:   printer{w: w},
+		cfg: cfg,
 	}
 }
 
@@ -16,6 +19,7 @@ type unmarshalGen struct {
 	p        printer
 	hasfield bool
 	depth    int
+	cfg      *cfg.MsgpConfig
 }
 
 func (u *unmarshalGen) Method() Method { return Unmarshal }
@@ -90,9 +94,11 @@ func (u *unmarshalGen) tuple(s *Struct) {
 }
 
 func (u *unmarshalGen) mapstruct(s *Struct) {
-	if u.depth == 1 {
-		u.p.printf("\n\n// zero the target:\n")
-		u.p.printf("*%s = %s{}\n", s.vname, s.TypeName())
+	if !u.cfg.SkipStructZeroingOnDecode {
+		if u.depth == 1 {
+			u.p.printf("\n\n// zero the target:\n")
+			u.p.printf("*%s = %s{}\n", s.vname, s.TypeName())
+		}
 	}
 	u.needsField()
 	sz := randIdent()

printer/print.go

@@ -104,7 +104,7 @@ func generate(f *parse.FileSet, mode gen.Method) (*bytes.Buffer, *bytes.Buffer,
 		}
 		testwr = testbuf
 	}
-	return outbuf, testbuf, f.PrintTo(gen.NewPrinter(mode, outbuf, testwr))
+	return outbuf, testbuf, f.PrintTo(gen.NewPrinter(mode, outbuf, testwr, f.Cfg))
 }
 
 func writePkgHeader(b *bytes.Buffer, name string) {