The easiest workaround for now seems to be:

package tester_test

import (
	"testing"

	"github.com/stretchr/testify/require"
)

func TestVeryLongSlice(t *testing.T) {
	var bigSlice []string
	for i := 0; i < 20000; i++ {
		bigSlice = append(bigSlice, "hello")
	}

	expected := 10
	require.Len(
		t,
		bigSlice,
		expected,
		"Should have %d item(s), but has %d",
		expected,
		len(bigSlice),
	)
}

We use a scanner to get the text which should go there line by line (it can be multiline) to indent each line until after the field name:

This mishandles the scanner.Scan() call. If scanner.Scan() returns false then you need to check scanner.Err() in case it isn't io.EOF. If we did that we'd have found it is bufio.Scanner: token too long.

We need to think of a thing to do about very long lines in this code. We should also probably not produce such a long line from assert.Len, maybe we should truncate after a sane number of elements?

We use a scanner to get the text which should go there line by line (it can be multiline) to indent each line until after the field name:

testify/assert/assertions.go

Lines 304 to 313 in 7caada5

	for i, scanner := 0, bufio.NewScanner(strings.NewReader(message)); scanner.Scan(); i++ {
	// no need to align first line because it starts at the correct location (after the label)
	if i != 0 {
	// append alignLen+1 spaces to align with "{{longestLabel}}:" before adding tab
	outBuf.WriteString("\n\t" + strings.Repeat(" ", longestLabelLen+1) + "\t")
	}
	outBuf.WriteString(scanner.Text())
	}
	return outBuf.String()

This mishandles the scanner.Scan() call. If scanner.Scan() returns false then you need to check scanner.Err() in case it isn't io.EOF. If we did that we'd have found it is bufio.Scanner: token too long.

We need to think of a thing to do about very long lines in this code. We should also probably not produce such a long line from assert.Len, maybe we should truncate after a sane number of elements?

That sounds good to me. Personally I'd even be happy for the contents of the slice not to be printed at all, but I suppose this output can be useful for shorter slices when troubleshooting why a test is failing.

Cheers
Fotis

Hello
For the argument ordering, you can enable https://github.com/Antonboom/testifylint via golangci

Hello For the argument ordering, you can enable https://github.com/Antonboom/testifylint via golangci

Thanks, yep I'm using this already, but I think my point was more around the inconsistency of the API and how that particular function feels odd in the context of the rest.

Of course I appreciate that this would be a breaking change but just thought I'd bring it up for consideration in case a major new version of testify was being planned.

Cheers
Fotis

The argument order is covered by #146. Let's keep this issue about the missing message.

This mishandles the scanner.Scan() call. If scanner.Scan() returns false then you need to check scanner.Err() in case it isn't io.EOF. If we did that we'd have found it is bufio.Scanner: token too long.

I've raised a PR that will make sure that the TokenTooLong never happens.

We should also probably not produce such a long line from assert.Len, maybe we should truncate after a sane number of elements?

Agreed. But regardless of the call we take about displaying long messages, I think this fix will be useful.