encoding/json: use reflect.Value.UnsafePointer over Pointer

The latter returns a uintptr, while the former returns a unsafe.Pointer.
A uintptr is unsafe if Go ever switches to a moving GC,
while a unsafe.Pointer will be properly tracked by the GC.

We do not use unsafe.Pointer for any unsafe type conversions,
and only use it for comparability purposes, which is relatively safe.

Updates #40592

Change-Id: I813e218668704b63a3043acda4331205a3835a66
Reviewed-on: https://go-review.googlesource.com/c/go/+/360855
Trust: Joseph Tsai <joetsai@digital-static.net>
Trust: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/encoding/json/encode.go

@@ -784,7 +784,7 @@ func (me mapEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
 	if e.ptrLevel++; e.ptrLevel > startDetectingCyclesAfter {
 		// We're a large number of nested ptrEncoder.encode calls deep;
 		// start checking if we've run into a pointer cycle.
-		ptr := v.Pointer()
+		ptr := v.UnsafePointer()
 		if _, ok := e.ptrSeen[ptr]; ok {
 			e.error(&UnsupportedValueError{v, fmt.Sprintf("encountered a cycle via %s", v.Type())})
 		}
@@ -877,9 +877,9 @@ func (se sliceEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
 		// Here we use a struct to memorize the pointer to the first element of the slice
 		// and its length.
 		ptr := struct {
-			ptr uintptr
+			ptr interface{} // always an unsafe.Pointer, but avoids a dependency on package unsafe
 			len int
-		}{v.Pointer(), v.Len()}
+		}{v.UnsafePointer(), v.Len()}
 		if _, ok := e.ptrSeen[ptr]; ok {
 			e.error(&UnsupportedValueError{v, fmt.Sprintf("encountered a cycle via %s", v.Type())})
 		}