Add early return in TryGetLast for empty results.

[prozolic]

Summary

This PR adds a fast path in IEnumerableSkipTakeIterator.TryGetLast. When source is an Iterator and GetCount(onlyIfCheap: true) returns a valid count (not -1), check if count <= _minIndexInclusive. In this case, immediately return with found=false.

Benchmark

| Method                    | Toolchain         | Mean     | Error    | StdDev   | Median   | Code Size | Gen0   | Allocated |
|-------------------------- |------------------ |---------:|---------:|---------:|---------:|----------:|-------:|----------:|
| LastOrDefault_PrependSkip | \main\corerun.exe | 54.05 ns | 4.295 ns | 4.774 ns | 53.12 ns |   2,616 B | 0.0180 |     152 B |
| LastOrDefault_PrependSkip | \pr\corerun.exe   | 25.75 ns | 0.948 ns | 1.014 ns | 25.67 ns |   1,777 B | 0.0181 |     152 B |
|                           |                   |          |          |          |          |           |        |           |
| LastOrDefault_AppendSkip  | \main\corerun.exe | 54.89 ns | 3.638 ns | 4.190 ns | 53.72 ns |   2,603 B | 0.0181 |     152 B |
| LastOrDefault_AppendSkip  | \pr\corerun.exe   | 25.78 ns | 1.001 ns | 1.112 ns | 25.53 ns |   1,770 B | 0.0181 |     152 B |
|                           |                   |          |          |          |          |           |        |           |
| LastOrDefault_ConcatSkip  | \main\corerun.exe | 85.73 ns | 4.710 ns | 5.424 ns | 87.06 ns |   4,306 B | 0.0255 |     216 B |
| LastOrDefault_ConcatSkip  | \pr\corerun.exe   | 41.26 ns | 2.351 ns | 2.613 ns | 40.88 ns |   2,089 B | 0.0220 |     184 B |

    [MemoryDiagnoser]
    [BenchmarkCategory(Categories.Libraries, Categories.LINQ)]
    public class SkipTakeTryGetLastBenchmarks
    {
        [Benchmark]
        public int LastOrDefault_PrependSkip()
        {
            return Enumerable.Range(0, 4).Prepend(4).Skip(5).LastOrDefault();
        }

        [Benchmark]
        public int LastOrDefault_AppendSkip()
        {
            return Enumerable.Range(0, 4).Append(4).Skip(5).LastOrDefault();
        }

        [Benchmark]
        public int LastOrDefault_ConcatSkip()
        {
            return Enumerable.Range(0, 3).Concat(new []{ 3, 4 }).Skip(5).LastOrDefault();
        }
    }

[Copilot]

Pull request overview

This PR adds a performance optimization to IEnumerableSkipTakeIterator.TryGetLast by introducing an early return path when the source iterator can cheaply determine that there are no elements after skipping.

Changes:

• Modified the condition from count > _minIndexInclusive to count != -1 to enter the fast path whenever a cheap count is available
• Added an early return when count <= _minIndexInclusive, avoiding enumeration when there are no elements to return
• Performance improvement demonstrated via benchmarks: ~2x faster for scenarios like Prepend().Skip(), Append().Skip(), and Concat().Skip() when skipping beyond the available elements

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[stephentoub]

Looks reasonable. Thanks!