Hey there, I encountered this when attempting to use require.Len as shown below.

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")
	}

	require.Len(t, bigSlice, 10)
}

Output of test:

=== RUN   TestVeryLongSlice
    tester_test.go:15:
                Error Trace:    C:/Users/Fots/source/tester/tester/tester_test.go:15
                Error:
                Test:           TestVeryLongSlice
--- FAIL: TestVeryLongSlice (0.00s)
FAIL
FAIL    github.com/fgimian/tester/tester        0.150s
FAIL

I haven't quite figured out why this happens, although I can imagine the output would be impractically long anyway. Worst of all, the main assertion message is not displayed which is quite confusing (especially to someone new using the library, like me 😄).

Personally, I think that it would be best not to display the slice at all for the Len function, and instead just display:

Should have X item(s), but has Y

However, I suspect my view on that may not be what some people want. The reason I think it is unwise to print the slice in any form, is that typically when many people are testing for the length of a slice, they may be comparing to a relatively large slice of items (e.g. the output of a mocked API call or similar).

An additional point that I believe has been discussed before is the inconsistency with argument ordering for the Len function which takes the actual first and expected second.

Cheers
Fotis