Polyfill SearchValues and optimize Activity, W3CPropagator

[MihaZupan]

Polyfilling such helpers seems like a neat way of avoiding duplication in a bunch of places where we might want to use newer APIs.

W3cPropagator being an extreme example where we were already duplicating a bunch of logic, but still not using SearchValues everywhere because it would add even more.

Benchmark source

public class ActivityBench
{
    private static readonly DistributedContextPropagator s_propagator = DistributedContextPropagator.CreateW3CPropagator();
    private readonly Activity _activity = new Activity("Test");

    public ActivityBench()
    {
        _activity.AddBaggage("a", "aaaaabbbbbcccccddddd");
        _activity.Start();
    }

    [Benchmark]
    public void Extract() => s_propagator.ExtractTraceIdAndState(null, static (carrier, name, out value, out values) =>
    {
        value = name == "traceparent" ? "00-0af7651916cd43dd8448eb211c80319c-b9c7c989f97918e1-01" : null;
        values = null;
    }, out _, out _);

    [Benchmark]
    public void InjectBaggage() => s_propagator.Inject(_activity, null, static (carrier, name, value) => { });
}

Method	Toolchain	Mean	Error	Ratio
Extract	\main\corerun.exe	27.15 ns	0.407 ns	1.00
Extract	\pr\corerun.exe	9.75 ns	0.133 ns	0.36
InjectBaggage	\main\corerun.exe	55.39 ns	1.121 ns	1.00
InjectBaggage	\pr\corerun.exe	44.18 ns	0.730 ns	0.80

[Copilot]

Pull Request Overview

This PR adds polyfills for modern .NET APIs like SearchValues to enable optimization of string searching operations across different target frameworks. The key change is replacing many manual character-by-character validation loops and character array lookups with more efficient SearchValues operations.

Key changes:

• Adds polyfill infrastructure for SearchValues<char>, Ascii.IsValid, and span extension methods for downlevel targets
• Optimizes W3CPropagator by replacing bitmask-based validation with SearchValues and simplifying trace parent validation logic
• Converts various libraries to use SearchValues instead of IndexOfAny with character arrays or manual loops

Reviewed Changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/Directory.Build.targets	Adds MSBuild logic to include polyfill files when IncludeSpanPolyfills is enabled
src/libraries/Common/src/System/Text/AsciiPolyfills.cs	Polyfill implementation of Ascii.IsValid methods for downlevel targets
src/libraries/Common/src/System/MemoryExtensionsPolyfills.cs	Polyfill for ContainsAnyExcept span extension method
src/libraries/Common/src/System/Buffers/SearchValuesPolyfills.cs	Complete polyfill implementation of SearchValues APIs using bitmap for ASCII chars
Multiple library projects	Enable polyfills and convert to use SearchValues instead of manual validation loops

[dotnet-policy-service]

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

[stephentoub]

LGTM