Use ReadOnlySpan<T> properties instead of static array fields

[alrz]

The similar change in runtime: https://github.com/dotnet/runtime/pull/79461/files among others.

[alrz]

Is there any requirement that's blocking this change in roslyn?

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[alrz]

Love to see this coming through, there's already a PR for an analyzer at dotnet/roslyn-analyzers#5548
@CyrusNajmabadi any chance it could get merged?

It doesn't support 2d arrays at the moment of which there's a few in roslyn, but I think for 2d arrays we could initialize a flat span and adjust the indexer call to point to to the desired element.

[sharwell]

@alrz RuntimeHelpers.CreateSpan doesn't exist in a bunch of the frameworks we target. I would not expect this change to proceed unless it was clearly shown to have no negative performance impact on net472 and netstandard2.0 usage.