If a Span<T> parameter is never written to by a method body, it's better for the parameter to be a ReadOnlySpan<T> rather than Span<T>. Doing so:

• Prevents inadvertent mistakes by preventing the method from erroneously writing to the memory (short of using unsafe code)
• Advertizes to the caller the semantics of the callee (that it won't be mutating the argument)
• Can have better performance for the caller, e.g. if the caller has an array of reference types, constructing a ReadOnlySpan<T> is cheaper than constructing a Span<T>, as the latter needs to perform a variance check and the former does not.

We could extend such an analyzer to locals as well. This can also handle {ReadOnly}Memory.

And we should probably allow it to be configurable around visibility, e.g. default to only applying to internal/private members but allow it to be applied to public, too.