ReadOnlySpan and Span must be readonly structs. #23452
Description
ReadOnlySpan and Span do not have any mutable fields and should be readonly just from the principle of least privilege.
Also, being not readonly makes the code like the following an error:
Span<int> stackAllocated = stackalloc int[100];
Span<int> returnableSpan = new int[100];
// error, compiler thinks that CopyTo could just do 'this = other'
// there would be no danger if Span<T> was `readonly`
returnableSpan.CopyTo(stackAllocated);Also see: dotnet/roslyn#21911
For now I can assume that ReadOnlySpan and Span will be readonly and specialcase them to be readonly regardless of annotations.
That would fix dotnet/roslyn#21911 temporarily.
Depending on the compiler used they should be either
public readonly ref struct Span<T> { ... }or
[S.R.CS.ReadOnlyAttribute]
[S.R.CS.IsByRefLikeAttribute]
public struct Span<T> { ... }