Rewrite SpanHelpers.IndexOfNullByte/IndexOfNullCharacter to unmanaged pointers

[EgorBo]

1. It seems to be containing potential GC holes, see Use IndexOf in RsaPaddingProcessor.FillNonZeroBytes #81327 (comment) for potential future uses.
2. Currently, this function is only used by System.String's ctors accepting unmanaged pointers which then converted to managed ones.

It also will allow to avoid doing verbose Unsafe stuff like

if (uValue == Unsafe.AddByteOffset(ref searchSpace, offset + 1))

and do

if (uValue == searchSpace[offset + 1])

instead.

[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.

[ghost]

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Issue Details

---

1. It seems to be containing potential GC holes, see Use IndexOf in RsaPaddingProcessor.FillNonZeroBytes #81327 (comment) for potential future uses.
2. Currently, this function is only used by System.String's ctors accepting unmanaged pointers which then converted to managed ones.

It also will allow to avoid doing verbose Unsafe stuff like

if (uValue == Unsafe.AddByteOffset(ref searchSpace, offset + 1))

and do

if (uValue == searchSpace[offset + 1])

instead.

Author:	EgorBo
Assignees:	-
Labels:	area-System.Runtime, untriaged
Milestone:	-

[teo-tsirpanis]

Fixed by #81347 (GitHub does not recognize "Addresses").