Avoid optimizing away ref array access

src/Compilers/CSharp/Portable/BoundTree/BoundNodes.xml

@@ -661,6 +661,15 @@
     <Field Name="Indices" Type="ImmutableArray&lt;BoundExpression&gt;"/>
   </Node>
 
+  <!--
+    Represents an array access that comes from the RHS of an unused ref (non-readonly) assignment.
+    Needed so we can emit with correct side effects (runtime element type check).
+  -->
+  <Node Name="BoundRefArrayAccess" Base="BoundExpression">
+    <Field Name="Type" Type="TypeSymbol?" Override="true" Null="always"/>
+    <Field Name="ArrayAccess" Type="BoundArrayAccess" Null="disallow"/>
+  </Node>
+
   <!--
   Represents an operation that is special in both IL and Expression trees -
   getting length of a one-dimensional 0-based array (vector)

src/Compilers/CSharp/Portable/CodeGen/EmitExpression.cs

@@ -154,6 +154,10 @@ private void EmitExpressionCore(BoundExpression expression, bool used)
                     EmitArrayElementLoad((BoundArrayAccess)expression, used);
                     break;
 
+                case BoundKind.RefArrayAccess:
+                    EmitArrayElementRefLoad((BoundRefArrayAccess)expression, used);
+                    break;
+
                 case BoundKind.ArrayLength:
                     EmitArrayLength((BoundArrayLength)expression, used);
                     break;
@@ -1099,6 +1103,17 @@ private void EmitArrayElementLoad(BoundArrayAccess arrayAccess, bool used)
             EmitPopIfUnused(used);
         }
 
+        private void EmitArrayElementRefLoad(BoundRefArrayAccess refArrayAccess, bool used)
+        {
+            if (used)
+            {
+                throw ExceptionUtilities.Unreachable();
+            }
+
+            EmitArrayElementAddress(refArrayAccess.ArrayAccess, AddressKind.Writeable);
+            _builder.EmitOpCode(ILOpCode.Pop);
+        }
+
         private void EmitFieldLoad(BoundFieldAccess fieldAccess, bool used)
         {
             var field = fieldAccess.FieldSymbol;

src/Compilers/CSharp/Portable/CodeGen/Optimizer.cs

@@ -2186,6 +2186,16 @@ public override BoundNode VisitAssignmentOperator(BoundAssignmentOperator node)
 
             if (isLast)
             {
+                if (node.IsRef &&
+                    !node.WasCompilerGenerated &&
+                    left.LocalSymbol.RefKind == RefKind.Ref &&
+                    right is BoundArrayAccess arrayAccess &&
+                    // Value types do not need runtime element type checks.
+                    !arrayAccess.Type.IsValueType)
+                {
+                    return new BoundRefArrayAccess(arrayAccess.Syntax, arrayAccess);
+                }
+
                 // assigned local is not used later => just emit the Right
                 return right;
             }