Incorrect rewrite using foreach variable as ref parameter

[jgardiol]

I tried running roslyn-linq-rewrite (built from source) on a big solution and found some issues. There's an issue with expressions capturing the foreach loop variable when it is a struct type (eg. when iterating over an IDictionary<K,V>, the loop variable is of type KeyValuePair<K,V> which is a struct)

I made a minimal code snippet reproducing the issue:

public void Bugged(IDictionary<string, string> dict, IList<string> list) {
    foreach (KeyValuePair<string, string> kv in dict) {
        if (list.Any(item => item == kv.Value)) {
            list.Remove(kv.Value);
        }
    }
}

This gets rewritten to

public void Bugged(IDictionary<string, string> dict, IList<string> list)
{
    foreach (KeyValuePair<string, string> kv in dict)
    {
        if (Bugged_ProceduralLinq1(list, ref kv))
        {
            list.Remove(kv.Value);
        }
    }
}

bool Bugged_ProceduralLinq1(System.Collections.Generic.IList<string> _linqitems, ref System.Collections.Generic.KeyValuePair<string, string> kv)
{
    if (_linqitems == null)
        throw new System.ArgumentNullException();
    foreach (var _linqitem in _linqitems)
    {
        if (_linqitem == kv.Value)
        {
            return true;
        }
    }

    return false;
}

Which produces a compilation error (CS1567)

[bernd5]

The problem is, that a foreach variable is "readonly". The subroutine is not allowed to change the struct.
So we must submit the reference via readonly ref which is called "in" now in CSharp 7.