Finish support for rank 1 mdarrays (#3610)

Three parts:

1. Update casting logic to allow casting SzArray to rank 1 MdArray (but
not the other way around)
2. Update MdArray rank 1 methods to be able to operate on SzArrays
3. Update array creation path to emulate the behavior where allocating
rank 1 MdArray with 0 lower bounds actually gives you an SzArray

Third bullet point makes this the most annoying, because we can't
reliably support `newobj instance void int32[0...]::.ctor(int32)`
without hitting the type loader. I was also considering adding an
optional field on Rank 1 MdArrays that lets you get to the SzArray's
EEType from it. It might be better.

Fixes #3331.

1 files changed, 10 insertions, 2 deletions

diff --git a/src/Runtime.Base/src/System/Runtime/TypeCast.cs b/src/Runtime.Base/src/System/Runtime/TypeCast.cs
index d7a549a4b..62dea8448 100644
--- a/src/Runtime.Base/src/System/Runtime/TypeCast.cs
+++ b/src/Runtime.Base/src/System/Runtime/TypeCast.cs
@@ -219,8 +219,16 @@ namespace System.Runtime
 
             // compare the array types structurally
 
-            if (pObjType->ParameterizedTypeShape == pTargetType->ParameterizedTypeShape &&
-                CastCache.AreTypesAssignableInternal(pObjType->RelatedParameterType, pTargetType->RelatedParameterType,
+            if (pObjType->ParameterizedTypeShape != pTargetType->ParameterizedTypeShape)
+            {
+                // If the shapes are different, there's one more case to check for: Casting SzArray to MdArray rank 1.
+                if (!pObjType->IsSzArray || pTargetType->ArrayRank != 1)
+                {
+                    return null;
+                }
+            }
+
+            if (CastCache.AreTypesAssignableInternal(pObjType->RelatedParameterType, pTargetType->RelatedParameterType,
                 AssignmentVariation.AllowSizeEquivalence))
             {
                 return obj;