Optimize Stack<T>'s enumerator

[stephentoub]

There's a non-trivial amount of unnecessary overhead/complexity in Stack<T>'s enumerator. This streamlines it.

(It does also change the undefined behavior of using Current erroneously. We can put back most of its current semantics if we really care. This change puts it on a plan closer to List<T>.)

Method	Toolchain	Mean	Error	StdDev	Ratio	Allocated	Alloc Ratio
IntsEnumerable	\main\corerun.exe	232.50 ns	1.022 ns	0.956 ns	1.00	40 B	1.00
IntsEnumerable	\pr\corerun.exe	57.13 ns	0.401 ns	0.356 ns	0.25	-	0.00
IntsStack	\main\corerun.exe	220.76 ns	1.008 ns	0.943 ns	1.00	-	NA
IntsStack	\pr\corerun.exe	54.96 ns	0.377 ns	0.335 ns	0.25	-	NA
StringsEnumerable	\main\corerun.exe	345.99 ns	1.714 ns	1.604 ns	1.00	40 B	1.00
StringsEnumerable	\pr\corerun.exe	67.93 ns	0.639 ns	0.598 ns	0.20	-	0.00
StringsStack	\main\corerun.exe	243.20 ns	1.808 ns	1.603 ns	1.00	-	NA
StringsStack	\pr\corerun.exe	55.86 ns	0.603 ns	0.534 ns	0.23	-	NA

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

[MemoryDiagnoser(false)]
public class Bench
{
    const int Length = 100;

    private IEnumerable<int> _intsEnumerable = new Stack<int>(Enumerable.Range(0, Length));
    private Stack<int> _intsStack = new Stack<int>(Enumerable.Range(0, Length));
    private IEnumerable<string> _stringsEnumerable = new Stack<string>(Enumerable.Range(0, Length).Select(i => i.ToString()));
    private Stack<string> _stringsStack = new Stack<string>(Enumerable.Range(0, Length).Select(i => i.ToString()));

    [Benchmark]
    public int IntsEnumerable()
    {
        int sum = 0;
        foreach (var i in _intsEnumerable) sum += i;
        return sum;
    }

    [Benchmark]
    public int IntsStack()
    {
        int sum = 0;
        foreach (var i in _intsStack) sum += i;
        return sum;
    }

    [Benchmark]
    public int StringsEnumerable()
    {
        int sum = 0;
        foreach (var s in _stringsEnumerable) sum += s.Length;
        return sum;
    }

    [Benchmark]
    public int StringsStack()
    {
        int sum = 0;
        foreach (var s in _stringsStack) sum += s.Length;
        return sum;
    }
}

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-collections
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull Request Overview

Streamlines Stack<T>'s enumerator by removing extra state checks and aligning its behavior more closely with List<T>, resulting in a significant performance improvement.

• Renames test properties related to undefined Current behavior.
• Refactors Enumerator to use a single index and simpler MoveNext/Current logic.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/libraries/System.Collections/tests/Generic/Stack/Stack.Tests.cs	Renamed test property for undefined Current behavior
src/libraries/System.Collections/tests/Generic/Stack/Stack.Generic.Tests.cs	Renamed test property for undefined Current behavior
src/libraries/System.Collections/src/System/Collections/Generic/Stack.cs	Simplified Enumerator implementation, updated index handling and removed old guards

Comments suppressed due to low confidence (1)

src/libraries/System.Collections/tests/Generic/Stack/Stack.Tests.cs:27

• The property name looks like it has a typo. It should be Enumerator_Empty_Current_UndefinedOperation_Throws (with a trailing 's') to match the naming pattern and the corresponding base property.

        protected override bool Enumerator_Empty_Current_UndefinedOperation_Throw => true;