Add MemoryExtensions.CountAny/ReplaceAny

[stephentoub]

Closes #109859

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[Copilot]

PR Overview

This pull request introduces new APIs for counting and replacing elements in spans by adding CountAny, ReplaceAny, and ReplaceAnyExcept methods. It also adds corresponding tests to validate the behavior of these new methods in both in-place and source-destination scenarios.

• Added tests for ReadOnlySpan.CountAny and Span.ReplaceAny/ReplaceAnyExcept in System.Memory.
• Implemented new overloads in MemoryExtensions.cs to support CountAny methods and source/destination based replace semantics.
• Updated the reference assembly to include method signatures for the new APIs.

Reviewed Changes

File	Description
src/libraries/System.Memory/tests/ReadOnlySpan/CountAny.cs	Adds tests for CountAny extension methods on ReadOnlySpan.
src/libraries/System.Memory/tests/Span/ReplaceAny.cs	Adds tests for in-place and source/destination overloads of ReplaceAny and ReplaceAnyExcept.
src/libraries/System.Private.CoreLib/src/System/MemoryExtensions.cs	Implements the new CountAny, ReplaceAny, and ReplaceAnyExcept extension methods.
src/libraries/System.Memory/ref/System.Memory.cs	Updates reference signatures for the newly added methods.

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[stephentoub]

/ba-g leg timed out

[IDisposable]

Is it worth checking for and preventing in .ReplaceAny an endless loop if the supplied newValue is contained in the values being replaced? Before starting the loop we could do a test like

if (values.Contains(newValue)) // throw something

For .ReplaceAnyExcept the test would be to ensure that the newValue is included in the values passed and throwing if not, so that we're not endlessly looping.

I don't know if this is the sort of thing we should even be putting in the library, but this is a preventable footgun, so maybe?

[MihaZupan]

The span is always sliced at pos + 1, so you're guaranteed to make progress even if the element you've just replaced would have also been a match. You shouldn't ever get into an endless loop.

Doing something like

const string InvalidChars = "!#?_";
span.ReplaceAny(InvalidChars, '_');

is valid, even if it would behave the same even if the user dropped _ from the values set.

[IDisposable]

RIght! I missed that, thanks.

[ericstj]

@MihaZupan @stephentoub do you think this API warrants a note in dotnet/core#9824?

[stephentoub]

They're small but useful helpers.