stackalloc should be disallowed for structs with ref fields

[jkotas]

Version Used: Microsoft.Net.Compilers.Toolset 4.4.0-2.22379.18 (current daily build)

Steps to Reproduce:

1. Build

using System;
using System.Reflection;
using System.Runtime.InteropServices;

unsafe
{
    StructWithRefField* p = stackalloc StructWithRefField[10];

    MyClass o = new();    
    // This has GC hole. The stackallocated array of StructWithRefField is not reported to GC
    p[0].RefField = ref o.ByteField;
}

ref struct StructWithRefField
{
    public ref byte RefField;
}

class MyClass
{
    public byte ByteField;
}

sharplab is crashing on this repro.

Expected Behavior:

Build error. stackalloc should be disallowed for structs with ref fields.

Actual Behavior:

Builds successfully, the program has GC hole at runtime.

[jcouv]

Assigned to @jaredpar to adjust language rules/spec

[cston]

The spec includes the following:

• A ref struct which has a ref field is never considered unmanaged

This was missed in the implementation.

[cston]

Reopening since the change has been reverted - see #63367.

[MichalPetryka]

Is this issue maybe a duplicate of #62528?

[cston]

@MichalPetryka, this appears to be a distinct issue from #62528, although we should test both scenarios with any potential fix. Thanks.

[jcouv]

Fixed in #64064