Backsert tests don't cover enumerator disposal

[atifaziz]

If the using is removed from the following line in Backsert:

MoreLINQ/MoreLinq/Backsert.cs

Line 70 in ce57548

using var e = first.CountDown(index, ValueTuple.Create).GetEnumerator();

then its tests, like the following that uses TestingSequence:

MoreLINQ/MoreLinq.Test/BacksertTest.cs

Lines 56 to 63 in ce57548

	public IEnumerable<int> Backsert(int[] seq1, int index, int[] seq2)
	{
	using (var test1 = seq1.AsTestingSequence())
	using (var test2 = seq2.AsTestingSequence())
	{
	return test1.Backsert(test2, index).ToArray();
	}
	}

don't break!

This is because Backsert internally uses other operators like Concat and CountDown and so provide a false positive because while Backsert is modified to not dispose anymore, Concat and CountDown do! This highlights a flaw that relying on TestingSequence is weak and won't cover what it's designed to test if an implementation is refactored internally to rely on other operations. TestingSequence is only as good as the implementation fully stand on its own.

This came up while reviewing issue #649. Thanks @Orace!

[Orace]

A solution is to build hand crafted implementation for every methods and prohibit nested enumerable ;)

[atifaziz]

A solution is to build hand crafted implementation for every methods and prohibit nested enumerable ;)

Or hijack MoreEnumerable extension methods within the test project as is done for Enumerable and inject TestingSequence. 💭