encoding/json: improve decoder alloc count

Author: oiweiwei

src/encoding/json/scanner.go

@@ -45,11 +45,18 @@ func checkValid(data []byte, scan *scanner) error {
 // A SyntaxError is a description of a JSON syntax error.
 // [Unmarshal] will return a SyntaxError if the JSON can't be parsed.
 type SyntaxError struct {
-	msg    string // description of error
-	Offset int64  // error occurred after reading Offset bytes
+	msg                string // description of error
+	Offset             int64  // error occurred after reading Offset bytes
+	invalidCharContext string // invalid character error context
+	invalidChar        byte   // the invalid character
 }
 
-func (e *SyntaxError) Error() string { return e.msg }
+func (e *SyntaxError) Error() string {
+	if e.invalidCharContext != "" {
+		return "invalid character " + quoteChar(e.invalidChar) + " " + e.invalidCharContext
+	}
+	return e.msg
+}
 
 // A scanner is a JSON scanning state machine.
 // Callers call scan.reset and then pass bytes in one at a time
@@ -168,7 +175,7 @@ func (s *scanner) eof() int {
 		return scanEnd
 	}
 	if s.err == nil {
-		s.err = &SyntaxError{"unexpected end of JSON input", s.bytes}
+		s.err = &SyntaxError{"unexpected end of JSON input", s.bytes, "", 0}
 	}
 	return scanError
 }
@@ -590,7 +597,7 @@ func stateError(s *scanner, c byte) int {
 // error records an error and switches to the error state.
 func (s *scanner) error(c byte, context string) int {
 	s.step = stateError
-	s.err = &SyntaxError{"invalid character " + quoteChar(c) + " " + context, s.bytes}
+	s.err = &SyntaxError{invalidCharContext: context, invalidChar: c, Offset: s.bytes}
 	return scanError
 }
 

src/encoding/json/stream.go

@@ -312,7 +312,7 @@ func (dec *Decoder) tokenPrepareForDecode() error {
 			return err
 		}
 		if c != ',' {
-			return &SyntaxError{"expected comma after array element", dec.InputOffset()}
+			return &SyntaxError{"expected comma after array element", dec.InputOffset(), "", 0}
 		}
 		dec.scanp++
 		dec.tokenState = tokenArrayValue
@@ -322,7 +322,7 @@ func (dec *Decoder) tokenPrepareForDecode() error {
 			return err
 		}
 		if c != ':' {
-			return &SyntaxError{"expected colon after object key", dec.InputOffset()}
+			return &SyntaxError{"expected colon after object key", dec.InputOffset(), "", 0}
 		}
 		dec.scanp++
 		dec.tokenState = tokenObjectValue
@@ -475,7 +475,7 @@ func (dec *Decoder) tokenError(c byte) (Token, error) {
 	case tokenObjectComma:
 		context = " after object key:value pair"
 	}
-	return nil, &SyntaxError{"invalid character " + quoteChar(c) + context, dec.InputOffset()}
+	return nil, &SyntaxError{invalidChar: c, invalidCharContext: context, Offset: dec.InputOffset()}
 }
 
 // More reports whether there is another element in the