Fix collection-optimized paths to be at iteration-time

[atifaziz]

This PR fixes #943.

It fixes the optimised path in the following operators for when the source is a collection:

• AssertCount
• CountDown
• FallbackIfEmpty
• PadStart
• SkipLast
• TakeLast

All of the above operators have deferred execution semantics, but they were binding to the source collection count when invoked and not when the resulting sequence was iterated. Consequently, if the source collection was updated before iteration, the (cached) count and the resulting sequence would be incorrect.

Tests have been added to avoid regressions in the future.

Some tests were removed because the optimisations they were exercising couldn't be maintained anymore. For example, if AssertCount could determine that the source was a collection and had the right count, then it would return the source reference as-is. Now, the count is asserted when iteration start.

[viceroypenguin]

The only concern I'd have is that in some cases, there is a performance advantage to have in .net6+ by using IEnumerable.TryGetNonEnumeratedCount().

The differences are:

• TryGetNonEnumeratedCount tests ICollection<>, an internal IIListProvider<>, and ICollection
• TryGetNonEnumeratedCount does not test IReadOnlyCollection<>

The cases where this difference applies are:

• IReadOnlyCollection<>
• Certain IEnumerables returned by System.Linq.Enumerable, which implement IIListProvider<>, but not ICollection<>.

[atifaziz]

The only concern I'd have is that in some cases, there is a performance advantage to have in .net6+ by using IEnumerable.TryGetNonEnumeratedCount().

That's an improvement rather than a concern 😉 and happy to take a PR on it, but the focus in this PR was to fix the bug as a priority.

Thanks for the review though and reporting the issue in the first place!

[codecov]

Codecov Report

Merging #946 (f6fde96) into master (5b49471) will increase coverage by 0.04%.
The diff coverage is 97.05%.

❗ Current head f6fde96 differs from pull request most recent head 4910703. Consider uploading reports for the commit 4910703 to get more accurate results

@@            Coverage Diff             @@
##           master     #946      +/-   ##
==========================================
+ Coverage   92.39%   92.44%   +0.04%     
==========================================
  Files         112      113       +1     
  Lines        3434     3453      +19     
  Branches     1021     1025       +4     
==========================================
+ Hits         3173     3192      +19     
+ Misses        199      196       -3     
- Partials       62       65       +3     

Impacted Files	Coverage Δ
MoreLinq/SkipLast.cs	100.00% <ø> (ø)
MoreLinq/CollectionLike.cs	80.00% <80.00%> (ø)
MoreLinq/AssertCount.cs	96.29% <100.00%> (-0.14%)	⬇️
MoreLinq/CountDown.cs	90.32% <100.00%> (+0.32%)	⬆️
MoreLinq/CountMethods.cs	100.00% <100.00%> (ø)
MoreLinq/EndsWith.cs	100.00% <100.00%> (ø)
MoreLinq/FallbackIfEmpty.cs	96.22% <100.00%> (+7.54%)	⬆️
MoreLinq/MoreEnumerable.cs	94.11% <100.00%> (ø)
MoreLinq/PadStart.cs	97.91% <100.00%> (+0.13%)	⬆️
MoreLinq/StartsWith.cs	100.00% <100.00%> (ø)
... and 4 more

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