use scratch buffers to avoid even more allocations

gen.go

@@ -16,7 +16,7 @@ func doTemplate(w io.Writer, info interface{}, templ string) error {
 	t := template.Must(template.New("").
 		Funcs(template.FuncMap{
 			"MajorType": func(wname string, tname string, val string) string {
-				return fmt.Sprintf(`if err := cbg.WriteMajorTypeHeader(%s, %s, uint64(%s)); err != nil {
+				return fmt.Sprintf(`if err := cbg.WriteMajorTypeHeaderBuf(scratch, %s, %s, uint64(%s)); err != nil {
 	return err
 }`, wname, tname, val)
 			},
@@ -94,6 +94,34 @@ type GenTypeInfo struct {
 	Fields []Field
 }
 
+func (gti *GenTypeInfo) NeedsScratch() bool {
+	for _, f := range gti.Fields {
+		switch f.Type.Kind() {
+		case reflect.String,
+			reflect.Uint64,
+			reflect.Int64,
+			reflect.Uint8,
+			reflect.Array,
+			reflect.Slice,
+			reflect.Map:
+			return true
+
+		case reflect.Struct:
+			fname := f.Type.PkgPath() + "." + f.Type.Name()
+			switch fname {
+			case "math/big.Int":
+				return true
+			case "github.com/ipfs/go-cid.Cid":
+				return true
+			}
+			// nope
+		case reflect.Bool:
+			// nope
+		}
+	}
+	return false
+}
+
 func nameIsExported(name string) bool {
 	return strings.ToUpper(name[0:1]) == name[0:1]
 }
@@ -168,7 +196,7 @@ func emitCborMarshalStringField(w io.Writer, f Field) error {
 	}
 
 	{{ MajorType "w" "cbg.MajTextString" (print "len(" .Name ")") }}
-	if _, err := w.Write([]byte({{ .Name }})); err != nil {
+	if _, err := io.WriteString(w, {{ .Name }}); err != nil {
 		return err
 	}
 `)
@@ -204,12 +232,12 @@ func emitCborMarshalStructField(w io.Writer, f Field) error {
 			return err
 		}
 	} else {
-		if err := cbg.WriteCid(w, *{{ .Name }}); err != nil {
+		if err := cbg.WriteCidBuf(scratch, w, *{{ .Name }}); err != nil {
 			return xerrors.Errorf("failed to write cid field {{ .Name }}: %w", err)
 		}
 	}
 {{ else }}
-	if err := cbg.WriteCid(w, {{ .Name }}); err != nil {
+	if err := cbg.WriteCidBuf(scratch, w, {{ .Name }}); err != nil {
 		return xerrors.Errorf("failed to write cid field {{ .Name }}: %w", err)
 	}
 {{ end }}
@@ -372,7 +400,7 @@ func emitCborMarshalSliceField(w io.Writer, f Field) error {
 		switch fname {
 		case "github.com/ipfs/go-cid.Cid":
 			err := doTemplate(w, f, `
-		if err := cbg.WriteCid(w, v); err != nil {
+		if err := cbg.WriteCidBuf(scratch, w, v); err != nil {
 			return xerrors.Errorf("failed writing cid field {{ .Name }}: %w", err)
 		}
 `)
@@ -436,6 +464,9 @@ func (t *{{ .Name }}) MarshalCBOR(w io.Writer) error {
 	if _, err := w.Write(lengthBuf{{ .Name }}); err != nil {
 		return err
 	}
+{{ if .NeedsScratch }}
+	scratch := make([]byte, 9)
+{{ end }}
 `)
 	if err != nil {
 		return err
@@ -1039,6 +1070,9 @@ func emitCborMarshalStructMap(w io.Writer, gti *GenTypeInfo) error {
 	if _, err := w.Write({{ .MapHeaderAsByteString }}); err != nil {
 		return err
 	}
+{{ if .NeedsScratch }}
+	scratch := make([]byte, 9)
+{{ end }}
 `)
 	if err != nil {
 		return err