proposal: cmpopts: add convenience functions for errors about go-cmp HOT 5 CLOSED

type CheckError func(error) bool

func Test(t *testing.T) {
	// nonNilError equates any non-nil error.
	// This is usual for just checking that the result failed, but not how it failed.
	nonNilError := CheckError(func(error) bool {
		return true
	})

	// notExistError equates any error based on a function predicate.
	notExistError := CheckError(os.IsNotExist)

	// timeoutError equates any error that is a timeout error.
	timeoutError := CheckError(func(e error) bool {
		ne, ok := e.(net.Error)
		return ok && ne.Timeout()
	})

	// containsEOF equates any error with EOF in the string.
	// NOTE: string matching on error messages is heavily frowned upon.
	containsEOF := CheckError(func(e error) bool {
		return strings.Contains(e.Error(), "EOF")
	})

	tests := []struct {
		err   error
		check CheckError
		want  bool
	}{
		{io.EOF, nonNilError, true},
		{nil, nonNilError, false},

		{io.EOF, notExistError, false},
		{os.ErrNotExist, notExistError, true},
		{os.ErrPermission, notExistError, false},

		{io.EOF, timeoutError, false},
		{&net.AddrError{}, timeoutError, false},
		{syscall.ETIMEDOUT, timeoutError, true},

		{io.EOF, containsEOF, true},
		{io.ErrUnexpectedEOF, containsEOF, true},
		{&net.ParseError{}, containsEOF, false},
	}

	for _, tt := range tests {
		got := tt.check(tt.err)
		if got != tt.want {
			t.Errorf("check(%v) = %v, want %v", tt.err, got, tt.want)
		}
	}
}

type CheckError func(err error) bool

func (c CheckError) Error() string {
	return fmt.Sprintf("%#v", c)
}

func (c CheckError) CmpEqual(err error) bool {
	return c(err)
}

func EquateErrors() cmp.Option {
    return cmp.Comparerer(func (x, y error) bool {
        return errors.Is(x, y) || errors.Is(y, x) 
    })
}