encoding/json: avoid misleading errors under goexperiment.jsonv2

The jsontext package represents the location of JSON errors
using a JSON Pointer (RFC 6901). This uses the JSON type system.

Unfortunately the v1 json.UnmarshalTypeError assumes a Go struct-based
mechanism for reporting the location of errors
(and has historically never been implemented correctly since
it was a weird mix of both JSON and Go namespaces; see #43126).
Trying to map a JSON Pointer into UnmarshalTypeError.{Struct,Field}
is difficult to get right without teaching jsontext
about the Go type system.

To reduce the probability of misleading errors,
check whether the last token looks like a JSON array index
and if so, elide the phrase "into Go struct field".

Fixes #74801

Change-Id: Id2088ffb9c339a9238ed38c90223d86a89422842
Reviewed-on: https://go-review.googlesource.com/c/go/+/710676
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

src/encoding/json/v2_decode.go

@@ -14,6 +14,7 @@ import (
 	"fmt"
 	"reflect"
 	"strconv"
+	"strings"
 
 	"encoding/json/internal/jsonwire"
 	"encoding/json/jsontext"
@@ -119,7 +120,20 @@ type UnmarshalTypeError struct {
 func (e *UnmarshalTypeError) Error() string {
 	var s string
 	if e.Struct != "" || e.Field != "" {
-		s = "json: cannot unmarshal " + e.Value + " into Go struct field " + e.Struct + "." + e.Field + " of type " + e.Type.String()
+		// The design of UnmarshalTypeError overly assumes a struct-based
+		// Go representation for the JSON value.
+		// The logic in jsontext represents paths using a JSON Pointer,
+		// which is agnostic to the Go type system.
+		// Trying to convert a JSON Pointer into a UnmarshalTypeError.Field
+		// is difficult. As a heuristic, if the last path token looks like
+		// an index into a JSON array (e.g., ".foo.bar.0"),
+		// avoid the phrase "Go struct field ".
+		intoWhat := "Go struct field "
+		i := strings.LastIndexByte(e.Field, '.') + len(".")
+		if len(e.Field[i:]) > 0 && strings.TrimRight(e.Field[i:], "0123456789") == "" {
+			intoWhat = "" // likely a Go slice or array
+		}
+		s = "json: cannot unmarshal " + e.Value + " into " + intoWhat + e.Struct + "." + e.Field + " of type " + e.Type.String()
 	} else {
 		s = "json: cannot unmarshal " + e.Value + " into Go value of type " + e.Type.String()
 	}

src/encoding/json/v2_decode_test.go

@@ -2363,6 +2363,34 @@ func TestUnmarshalTypeError(t *testing.T) {
 	}
 }
 
+func TestUnmarshalTypeErrorMessage(t *testing.T) {
+	err := &UnmarshalTypeError{
+		Value:  "number 5",
+		Type:   reflect.TypeFor[int](),
+		Offset: 1234,
+		Struct: "Root",
+	}
+
+	for _, tt := range []struct {
+		field string
+		want  string
+	}{
+		{"", "json: cannot unmarshal number 5 into Go struct field Root. of type int"},
+		{"1", "json: cannot unmarshal number 5 into Root.1 of type int"},
+		{"foo", "json: cannot unmarshal number 5 into Go struct field Root.foo of type int"},
+		{"foo.1", "json: cannot unmarshal number 5 into Root.foo.1 of type int"},
+		{"foo.bar", "json: cannot unmarshal number 5 into Go struct field Root.foo.bar of type int"},
+		{"foo.bar.1", "json: cannot unmarshal number 5 into Root.foo.bar.1 of type int"},
+		{"foo.bar.baz", "json: cannot unmarshal number 5 into Go struct field Root.foo.bar.baz of type int"},
+	} {
+		err.Field = tt.field
+		got := err.Error()
+		if got != tt.want {
+			t.Errorf("Error:\n\tgot:  %v\n\twant: %v", got, tt.want)
+		}
+	}
+}
+
 func TestUnmarshalSyntax(t *testing.T) {
 	var x any
 	tests := []struct {