Unnecessary defensive copies in `string` + implicit `object` operator

When string + something (not a string) is encountered, the compiler converts the something to a string using the ToString method, instead of boxing it. However, it does not currently recognise that defensive copies are unnecessary if the ToString method is marked readonly.

Version Used:

Sharplab.io Also reproduces with the compiler included in .NET SDK 8.0.100

Steps to Reproduce:

using System;
public class C
{
    public static string M1(in S1 s)
    {
        return "" + s; //defensive copy (unnecessary)
    }

    public static string M2(ref S1 s)
    {
        return "" + s; //no defensive copy
    }
}
public struct S1
{
    public readonly override string ToString() => "";
}

Link

Expected Behavior:

s.ToString() is emitted without a defensive copy of s in both cases, since the method is marked as readonly.

Actual Behavior:

s.ToString() is emitted with a defensive copy of s in M1.