Add fixer when for params span overloads are used with Expression selected

[KathleenDollard]

Summary

Span and ReadOnlySpan overloads are preferred for params arguments, as described in the following code:

using System.Linq.Expressions;
internal class Program
{
    private static void Main(string[] args)
    {
        Expression<Func<string>> f = ()=> Test('a', 'b'); // before
        // error CS8640: Expression tree cannot contain value of ref struct or restricted type 'ReadOnlySpan'.
        // error CS9226: An expression tree may not contain an expanded form of non-array params collection parameter.
        Expression<Func<string>> f2 = ()=> Test(new[] { 'a', 'b' });
    }

    static string Test(params char[] characters) => "";
    static string Test(params ReadOnlySpan<char> characters) => "";
}

Background and Motivation

C# prefers the Span and ReadOnlySpan overloads to improve performance, which is desirable in almost all cases. However, there is no resourcing to update expression trees to handle ref struct issues like this one. As a result, users that have expression trees will receive the errors described above.

Proposed Feature

Add a fixer when this error occurs so the user is gently led to the explicit array overloads.

Alternative Designs

The C# LDM has considered whether to special case expression trees and decided not to do this at this time.