Improved error message when checks with variables fail

[alecthomas]

I realise this would be difficult (or potentially impossible) to solve, but when an expression fails it would be very useful to have the value available.

eg.

The following test code:

a := 1
assert.Check(t, a == 2)

Produces the error:

	parser_test.go:20: assertion failed: expression is false: a == 2

This is not that helpful, as I now need to edit the code to print the actual value.

Ideally it would print something like:

	parser_test.go:20: assertion failed: expression is false: a (1) == 2

[dnephin]

That would be great, but I'm not sure it's possible. I think it will be necessary to keep assert.Equal() for those comparisons. assert.Equal() will print the ast of the arguments if it's a simple expression.

[dnephin]

Another option is something like this:

assert.Check(t, a == 2, "a=%s", a)
// parser_test.go:20: assertion failed: expression is false: a == 2: a=1

If printing the args as part of the "extra messages" is a common pattern it could be done with a convenience function:

assert.Check(t, x > y, assert.Args(x, y))
// parser_test.go:20: assertion failed: expression is false: x > y: x=2, y=100

I don't know if we can do any better than that.

[thaJeztah]

Would assert.Equal() satisfy your use case?

func TestAreYouTheSame(t *testing.T) {
	a := 1
	assert.Equal(t, a, 2)
}

Produces:

=== RUN   TestAreYouTheSame
--- FAIL: TestAreYouTheSame (0.00s)
    bla_test.go:16: assertion failed: 1 (a int) != 2 (int)
FAIL

Process finished with exit code 1

[thaJeztah]

Or, if you need assert.Check;

func TestAreYouTheSame(t *testing.T) {
	a := 1
	assert.Check(t, cmp.Equal(a, 2))
	assert.Equal(t, a, 2)
}

=== RUN   TestAreYouTheSame
--- FAIL: TestAreYouTheSame (0.00s)
    bla_test.go:16: assertion failed: 1 (a int) != 2 (int)
    bla_test.go:17: assertion failed: 1 (a int) != 2 (int)
FAIL

Process finished with exit code 1