Adds SpanSplitEnumerator

[bbartels]

Implements #934

TODO:

• Add SplitAny tests
• Add SearchValues tests
• Add documentation for public API

IEquatable<T> type constraint is necessary due to the type constraint existing on the System.MemoryExtensions.IndexOf<T>() overloads and on SearchValues<T>.

[dotnet-issue-labeler]

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.

[dotnet-issue-labeler]

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.

[stephentoub]

Can you make the existing string.Split test cases work for this as well, validating that the new Split produces the same splits as string.Split does?

[bbartels]

I may not be understanding completely, do you mean:

1. Find the test cases for string.Split
2. Move them into tests/ReadOnlySpan/Split.char.cs
   or just add an Assert to the custom AssertEquals method in this file that does an additional string.Split to ensure the result of the SpanSplitEnumerator and string.Split align?

[stephentoub]

just add an Assert to the custom AssertEquals method in this file that does an additional string.Split to ensure the result of the SpanSplitEnumerator and string.Split align?

This :)

[stephentoub]

Oh, wait, no, I misread what you wrote. I meant the equivalent of this:

runtime/src/libraries/System.Runtime/tests/System.Runtime.Tests/System/String.SplitTests.cs

Lines 9 to 10 in 101c0da

	// These tests validate both String.Split and MemoryExtensions.Split, as they have equivalent semantics, with the
	// former creating a new array to store the results and the latter writing the results into a supplied span.

Augmenting those tests to test these overloads as well, rather than duplicating a ton of tests.

[stephentoub]

What about T other than char?

[bbartels]

The PR was still in draft, so was still going to work on more exhaustive tests. What/How many other types do you think would be reasonable to test? Not sure if this is a good idea, but I could generalise the tests so it casts the incoming string of the existing tests to ReadOnlySpan<byte/long/etc>. Though of course this means you are realistically testing the same exact data, just with the types differing.

[stephentoub]

What/How many other types do you think would be reasonable to test?

Just basic validation of an assortment of other types, e.g. byte, int, a custom reference type, and a custom value type.

[stephentoub]

Is _startNext needed rather than just updating _span to be the remainder to be searched?

[bbartels]

As with https://github.com/dotnet/runtime/pull/104534/files#r1668846871 I believe this was PR feedback at the time. Happy to change it