Simplify RuntimeHelpers.GetSubArray

[jkotas]

The special casing of co-variant arrays was added in the original safe implementation to avoid exceptions from AsSpan (dotnet/coreclr#22331 (comment)). ArrayTypeMismatchException is no longer a concern with the new unsafe implementation. There is a subtle behavior change in the actual type for co-variant arrays of reference types. However, the new behavior matches Array.Resize and it is very unlikely for any code out there to depend on this co-variant arrays corner case.

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Issue Details

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The special casing of co-variant arrays was added in the original safe implementation to avoid exceptions from AsSpan (dotnet/coreclr#22331 (comment)). ArrayTypeMismatchException is no longer a concern with the new unsafe implementation. There is a subtle behavior change in the actual type for co-variant arrays of reference types. However, the new behavior matches Array.Resize and it is very unlikely for any code out there to depend on this co-variant arrays corner case.

Author:	jkotas
Assignees:	jkotas
Labels:	area-System.Runtime.CompilerServices
Milestone:	-

[jkotas]

My motivation for this change are AOT compatibility warnings caused by Array.CreateInstance. An alternative is to add suppression of the AOT compatibility warning (the warning is harmless - the array type is guaranteed to exist).

[stephentoub]

There is a subtle behavior change in the actual type for co-variant arrays of reference types. However, the new behavior matches Array.Resize and it is very unlikely for any code out there to depend on this co-variant arrays corner case.

The issue would be if I had code like:

static T[] Slice<T>(T[] array) => array[low..high];

class A : B { }

A[] array = new A[42];
A[] sliced = (A[])Slice<B>(array);

that would now fail to cast whereas previously it would succeed? Or conversely:

static T[] Slice<T>(T[] array) => array[low..high];

class A : B { }
class C : B { }

A[] array = new A[42];
B[] sliced = Slice<B>(array);
sliced[0] = new C();

that would now succeed whereas previously it would fail due to trying to store a C into an A[]?

[jkotas]

Correct.

[MichalStrehovsky]

Thank you!

[MichalStrehovsky]

Cc @LakshanF who will be looking at other AOT-warnings-triggering patterns in CoreLib.