Ref locals before C# 7

[ds5678]

Input code

public struct StructWithStringField
{
	public string Id;
	public static void ModifyArray(StructWithStringField[] array)
	{
		int num = 0;
		while (num < 10)
		{
			array[0].Id = (++num).ToString();
		}
	}
}

Erroneous output

public struct StructWithStringField
{
	public string Id;
	public static void ModifyArray(StructWithStringField[] array)
	{
		int num = 0;
		while (num < 10)
		{
			ref StructWithStringField reference = ref array[0];
			int num2 = ++num;
			reference.Id = num2.ToString();
		}
	}
 }

This produces an error before C# 7:

CS1525: Unexpected symbol 'ref'

Details

• Product in use: pretty unit test
• Version in use: latest commit (aff9649)
• Any other relevant information to the issue, or your interest in contributing a fix.
  • I, of course, am willing to work on this.
  • I think I've seen other situations with similar ref local issues, but I don't have any examples at this time. I'm checking to see if I can find some.

[siegfriedpammer]

This is an ILInlining problem.

[ds5678]

I narrowed the problem down with tests that are more specific. However, I'm struggling to figure out how to resolve the problem.

public static string Test2(int value)
{
	return (++value).ToString();
}
public static string Test3(int value)
{
	M(++value);
	return value.ToString();
}
private static void M(in int value)
{
}

I did a lot of debugging, both in ILInlining and TransformAssignment. I could not find a clean way to solve the problem in ILInlining, and my modifications to TransformAssignment (#3472) feel like they're the wrong way to solve this. I could use some guidance.