Improve error positioning of SemanticError

Changes made:
* Always populate SemanticError.JSONPointer.
* Export ErrUnknownName to programmatically identify an unknown name.
* Reduce verbosity of SemanticError.Error method.
* Apply Hyrum-proofing on a per-process basis,
  rather than on a per-error.Error method call.
* Cleanup error wrapping for user-provided method and function calls.
  Some heuristics were used to provided reasonable position injection
  into user-provided errors.

arshal.go

@@ -428,6 +428,8 @@ func UnmarshalDecode(in *jsontext.Decoder, out any, opts ...Options) (err error)
 	return unmarshalDecode(in, out, uo)
 }
 
+var errNonNilReference = errors.New("value must be passed as a non-nil pointer reference")
+
 func unmarshalDecode(in *jsontext.Decoder, out any, uo *jsonopts.Struct) (err error) {
 	v := reflect.ValueOf(out)
 	if !v.IsValid() || v.Kind() != reflect.Pointer || v.IsNil() {
@@ -438,8 +440,7 @@ func unmarshalDecode(in *jsontext.Decoder, out any, uo *jsonopts.Struct) (err er
 				t = t.Elem()
 			}
 		}
-		err := errors.New("value must be passed as a non-nil pointer reference")
-		return &SemanticError{action: "unmarshal", GoType: t, Err: err}
+		return &SemanticError{action: "unmarshal", GoType: t, Err: errNonNilReference}
 	}
 	va := addressableValue{v.Elem()} // dereferenced pointer is always addressable
 	t := va.Type()

arshal_any.go

@@ -73,7 +73,7 @@ func unmarshalValueAny(dec *jsontext.Decoder, uo *jsonopts.Struct) (any, error)
 		case '0':
 			fv, ok := jsonwire.ParseFloat(val, 64)
 			if !ok && uo.Flags.Get(jsonflags.RejectFloatOverflow) {
-				return nil, &SemanticError{action: "unmarshal", JSONKind: k, GoType: float64Type, Err: strconv.ErrRange}
+				return nil, newUnmarshalErrorAfter(dec, float64Type, strconv.ErrRange)
 			}
 			return fv, nil
 		default:
@@ -88,7 +88,7 @@ func marshalObjectAny(enc *jsontext.Encoder, obj map[string]any, mo *jsonopts.St
 	if xe.Tokens.Depth() > startDetectingCyclesAfter {
 		v := reflect.ValueOf(obj)
 		if err := visitPointer(&xe.SeenPointers, v); err != nil {
-			return err
+			return newMarshalErrorBefore(enc, anyType, err)
 		}
 		defer leavePointer(&xe.SeenPointers, v)
 	}
@@ -176,7 +176,7 @@ func unmarshalObjectAny(dec *jsontext.Decoder, uo *jsonopts.Struct) (map[string]
 
 			// Manually check for duplicate names.
 			if _, ok := obj[name]; ok {
-				name := xd.PreviousBuffer()
+				name := xd.PreviousTokenOrValue()
 				err := newDuplicateNameError(dec.StackPointer(), nil, dec.InputOffset()-len64(name))
 				return obj, err
 			}
@@ -192,7 +192,7 @@ func unmarshalObjectAny(dec *jsontext.Decoder, uo *jsonopts.Struct) (map[string]
 		}
 		return obj, nil
 	}
-	return nil, &SemanticError{action: "unmarshal", JSONKind: k, GoType: mapStringAnyType}
+	return nil, newUnmarshalErrorAfter(dec, mapStringAnyType, nil)
 }
 
 func marshalArrayAny(enc *jsontext.Encoder, arr []any, mo *jsonopts.Struct) error {
@@ -201,7 +201,7 @@ func marshalArrayAny(enc *jsontext.Encoder, arr []any, mo *jsonopts.Struct) erro
 	if xe.Tokens.Depth() > startDetectingCyclesAfter {
 		v := reflect.ValueOf(arr)
 		if err := visitPointer(&xe.SeenPointers, v); err != nil {
-			return err
+			return newMarshalErrorBefore(enc, sliceAnyType, err)
 		}
 		defer leavePointer(&xe.SeenPointers, v)
 	}
@@ -259,5 +259,5 @@ func unmarshalArrayAny(dec *jsontext.Decoder, uo *jsonopts.Struct) ([]any, error
 		}
 		return arr, nil
 	}
-	return nil, &SemanticError{action: "unmarshal", JSONKind: k, GoType: sliceAnyType}
+	return nil, newUnmarshalErrorAfter(dec, sliceAnyType, nil)
 }