encoding/json/v2: remove `unknown` tag option and DiscardUnknownMembers

WARNING: This commit contains breaking changes
for those already using GOEXPERIMENT=jsonv2.

This removes support for the `unknown` tag option and
the DiscardUnknownMembers marshal option.

The `unknown` tag option semantics are a bit too subtle
even for experienced Go programmers to understand.
Remove support for it. The exact same feature (or something similar)
can be added back into a future release of json/v2.

We already support the `inline` tag option,
which can handle most cases of what someone might want to do
with unknown fields (such as preserve them).

Fixes #77271
Updates #76444

Change-Id: I875952f0755e58aac4c571869b2cdb56e75cfda9
Reviewed-on: https://go-review.googlesource.com/c/go/+/741320
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

1 files changed, 0 insertions, 62 deletions

diff --git a/src/encoding/json/v2/example_test.go b/src/encoding/json/v2/example_test.go
index dc1f06674c..684ca9c6a2 100644
--- a/src/encoding/json/v2/example_test.go
+++ b/src/encoding/json/v2/example_test.go
@@ -8,7 +8,6 @@ package json_test
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"log"
 	"math"
@@ -338,67 +337,6 @@ func Example_inlinedFields() {
 	// }
 }
 
-// Due to version skew, the set of JSON object members known at compile-time
-// may differ from the set of members encountered at execution-time.
-// As such, it may be useful to have finer grain handling of unknown members.
-// This package supports preserving, rejecting, or discarding such members.
-func Example_unknownMembers() {
-	const input = `{
-		"Name": "Teal",
-		"Value": "#008080",
-		"WebSafe": false
-	}`
-	type Color struct {
-		Name  string
-		Value string
-
-		// Unknown is a Go struct field that holds unknown JSON object members.
-		// It is marked as having this behavior with the "unknown" tag option.
-		//
-		// The type may be a jsontext.Value or map[string]T.
-		Unknown jsontext.Value `json:",unknown"`
-	}
-
-	// By default, unknown members are stored in a Go field marked as "unknown"
-	// or ignored if no such field exists.
-	var color Color
-	err := json.Unmarshal([]byte(input), &color)
-	if err != nil {
-		log.Fatal(err)
-	}
-	fmt.Println("Unknown members:", string(color.Unknown))
-
-	// Specifying RejectUnknownMembers causes Unmarshal
-	// to reject the presence of any unknown members.
-	err = json.Unmarshal([]byte(input), new(Color), json.RejectUnknownMembers(true))
-	serr, ok := errors.AsType[*json.SemanticError](err)
-	if ok && serr.Err == json.ErrUnknownName {
-		fmt.Println("Unmarshal error:", serr.Err, strconv.Quote(serr.JSONPointer.LastToken()))
-	}
-
-	// By default, Marshal preserves unknown members stored in
-	// a Go struct field marked as "unknown".
-	b, err := json.Marshal(color)
-	if err != nil {
-		log.Fatal(err)
-	}
-	fmt.Println("Output with unknown members:   ", string(b))
-
-	// Specifying DiscardUnknownMembers causes Marshal
-	// to discard any unknown members.
-	b, err = json.Marshal(color, json.DiscardUnknownMembers(true))
-	if err != nil {
-		log.Fatal(err)
-	}
-	fmt.Println("Output without unknown members:", string(b))
-
-	// Output:
-	// Unknown members: {"WebSafe":false}
-	// Unmarshal error: unknown object member name "WebSafe"
-	// Output with unknown members:    {"Name":"Teal","Value":"#008080","WebSafe":false}
-	// Output without unknown members: {"Name":"Teal","Value":"#008080"}
-}
-
 // The "format" tag option can be used to alter the formatting of certain types.
 func Example_formatFlags() {
 	value := struct {