API proposal: Make ImmutableArray readonly struct

[huoyaoyuan]

Background

It's intuitive to use ImmutableArray<T> as readonly fields, and wrap it into other readonly structs. Currently it's not marked readonly, and may cause hidden copies. Although being small itself, immutable arrays are aggresively optimised for performance. Hidden copy of only 1 pointer size can be reduced.

Proposed API

namespace System.Collections.Immutable
{
-    public struct ImmutableArray<T>
+    public readonly struct ImmutableArray<T>
}

Works to do

The only write usage of its field is ImmutableInterlocked. Given that the layout is the prerequisite of interlocked to work, they should be fine to use Unsafe.As<ImmutableArray<T>, T[]>.

Seems that the Unsafe class came later than readonly structs. I guess it's the reason not using it.

[saltukkos]

First, I was going to say, that JIT will optimize defensive copies anyway like in this case
sharplab

But after I change the example to use Clear instead of Add it just blew up my mind.
sharplab

So, we definitely need the readonly modifier.

[huoyaoyuan]

Going to tag the area owners @eiriktsarpalis @layomia because this seems to be forgotten.
Is this a concrete API proposal?

[eiriktsarpalis]

@huoyaoyuan We're currently working our way through a long backlog of untriaged collections issues, so we haven't had the chance to go through this one yet. At first glance this looks reasonable, however we would need to investigate a bit more thoroughly. Thanks.