Add CoreLib helpers for MemoryExtensions.AsSpan/AsMemory(T[], int) (#16505)

* Add CoreLib helpers for MemoryExtensions.AsSpan/AsMemory(T[], int)

(Part of https://github.com/dotnet/corefx/issues/26894)

We intentionally don't have (T[], int] constructor overloads
for Span and Memory. So as not to incur unnecessary argument checks,
we implement this directly in CoreLib and will invoke it from
CoreFx for the fast-Span version.

* Expose an internal array-int constructor instead.

Signed-off-by: dotnet-bot-corefx-mirror <dotnet-bot@microsoft.com>

Author: atsushikan

src/Common/src/CoreLib/System/Memory.cs

@@ -58,6 +58,26 @@ public Memory(T[] array)
             _length = array.Length;
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal Memory(T[] array, int start)
+        {
+            if (array == null)
+            {
+                if (start != 0)
+                    ThrowHelper.ThrowArgumentOutOfRangeException();
+                this = default;
+                return; // returns default
+            }
+            if (default(T) == null && array.GetType() != typeof(T[]))
+                ThrowHelper.ThrowArrayTypeMismatchException();
+            if ((uint)start > (uint)array.Length)
+                ThrowHelper.ThrowArgumentOutOfRangeException();
+
+            _object = array;
+            _index = start;
+            _length = array.Length - start;
+        }
+
         /// <summary>
         /// Creates a new memory over the portion of the target array beginning
         /// at 'start' index and ending at 'end' index (exclusive).

src/Common/src/CoreLib/System/Span.NonGeneric.cs

@@ -341,6 +341,30 @@ internal static unsafe bool EndsWithOrdinalHelper(ReadOnlySpan<char> span, ReadO
             return OrdinalHelper(span.Slice(span.Length - value.Length), value, value.Length);
         }
 
+        /// <summary>
+        /// Helper method for MemoryExtensions.AsSpan(T[] array, int start).
+        /// </summary>
+        public static Span<T> AsSpan<T>(T[] array, int start)
+        {
+            if (array == null)
+            {
+                if (start != 0)
+                    ThrowHelper.ThrowArgumentOutOfRangeException();
+                return default;
+            }
+            if (default(T) == null && array.GetType() != typeof(T[]))
+                ThrowHelper.ThrowArrayTypeMismatchException();
+            if ((uint)start > (uint)array.Length)
+                ThrowHelper.ThrowArgumentOutOfRangeException();
+
+            return new Span<T>(ref Unsafe.Add(ref Unsafe.As<byte, T>(ref array.GetRawSzArrayData()), start), array.Length - start);
+        }
+
+        /// <summary>
+        /// Helper method for MemoryExtensions.AsMemory(T[] array, int start).
+        /// </summary>
+        public static Memory<T> AsMemory<T>(T[] array, int start) => new Memory<T>(array, start);
+
         /// <summary>Creates a new <see cref="ReadOnlyMemory{char}"/> over the portion of the target string.</summary>
         /// <param name="text">The target string.</param>
         /// <remarks>Returns default when <paramref name="text"/> is null.</remarks>