A `ref T` field where `T` is `unmanaged` is itself treated as `unmanaged`

[DaZombieKiller]

Tested with a daily SDK build (7.0.100-preview.7.22358.13):

unsafe
{
    var byref = default(ByReference<int>);
    var byptr = &byref; // does not error
}

readonly ref struct ByReference<T>
{
    public readonly ref T Value;
}

Expected Behavior:
error CS0208: Cannot take the address of, get the size of, or declare a pointer to a managed type ('ByReference<int>')

Actual Behavior:
The code compiles successfully.

[DaZombieKiller]

Worth noting: assuming this is unintended behaviour, it is currently being relied upon by the reflection stack.

[RikkiGibson]

It feels like it would be reasonable to make people use https://docs.microsoft.com/en-us/dotnet/api/system.runtime.compilerservices.unsafe.aspointer?view=net-6.0 if they want to keep doing this.

[Sergio0694]

Note, for that they'd currently be forced to use a function pointer jump, which adds an extra indirection. As in:

unsafe
{
    var byref = default(ByReference<int>);
    var f1 = (delegate*<ref byte, ref byte>)&Unsafe.As<byte, byte>;
    var f2 = (delegate*<ref ByReference<int>, ref byte>)f1;
    
    void* byptr = Unsafe.AsPointer(ref f2(ref byref));
}

That said, it would work around the issue until ref struct-s in generics are allowed 😄

[MichalPetryka]

@jaredpar Did #63209 fix this too?

[cston]

Thanks for reporting this issue. It has been fixed in #64064.

The compiler now reports:

warning CS8500: This takes the address of, gets the size of, or declares a pointer to
    a managed type ('ByReference<int>')

cc @jcouv