Clearer error message in assert_changes

[lxxxvi]

If the to: argument is passed to assert_changes and the comparison with the after value fails, the error message only revealed the "actual" value, not the "expected" value.

Example:

test 'after value correct' do
  value = 'x'
  assert_changes 'value', from: 'x', to: 'y' do
    value = 'z'
  end
end

Before the error message said:

"value" didn't change to y

Now it says:

"value" didn't change to as expected
Expected: "y"
  Actual: "z"

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[sikachu]

I think this change looks fine. However, I wonder, is this the first time we introduce multi-line assertion failure message (apart from when you set the assertion message yourself)?

[lxxxvi]

@sikachu Thanks for asking this question.

The multi-line assertion failure message would normally come from minitest's assert_equal (or other assertions).

In assert_changes the assertion method assert is used which does not return a multi-line failure message.

So I tested, if assert_equal could be used instead

# Option without manually creating the multi-line message,
# `assert_equal` creates it instead
unless to == UNTRACKED
  error = "#{expression.inspect} didn't change to as expected"
  error = "#{message}.\n#{error}" if message

  assert_equal to, after, error
end

This works. It also returns a multi-line failure:

@object.num should 1.
"@object.num" didn't change to as expected.
Expected: 1
  Actual: -1

But it breaks if to: is a Regexp, because assert_equal does not cover regular expressions.

In order to workaround this, I could come up with this

# Option that also considers if `to:` is a regular expression
unless to == UNTRACKED
  error = "#{expression.inspect} didn't change to as expected"
  error = "#{message}.\n#{error}" if message

  assert_match to, after, error if to.class == Regexp
  assert_equal to, after, error
end

Could be that there's even a better approach...?