Underlying CoreCLR support for new string slicing overloads (#15811)

* Underlying CoreCLR support for new string slicing overloads

https://github.com/dotnet/corefx/issues/25254

These add the underlying support for the fast versions of
these extension methods.

* Underlying CoreCLR support for new string slicing overloads

https://github.com/dotnet/corefx/issues/24072

These add the underlying support for the fast versions of
these extension methods.

Author: atsushikan

src/mscorlib/shared/System/Span.NonGeneric.cs

@@ -22,19 +22,51 @@ namespace System
     /// </summary>
     public static class Span
     {
-        /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target string.</summary>
+        /// <summary>Creates a new <see cref="ReadOnlyMemory{char}"/> over the portion of the target string.</summary>
         /// <param name="text">The target string.</param>
         /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null reference (Nothing in Visual Basic).</exception>
         public static ReadOnlyMemory<char> AsReadOnlyMemory(this string text)
         {
             if (text == null)
-            {
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
-            }
 
             return new ReadOnlyMemory<char>(text, 0, text.Length);
         }
 
+        /// <summary>Creates a new <see cref="ReadOnlyMemory{char}"/> over the portion of the target string.</summary>
+        /// <param name="text">The target string.</param>
+        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null reference (Nothing in Visual Basic).</exception>
+        /// <exception cref="System.ArgumentOutOfRangeException">
+        /// Thrown when the specified <paramref name="start"/> index is not in range (&lt;0 or &gt;text.Length).
+        /// </exception>
+        public static ReadOnlyMemory<char> AsReadOnlyMemory(this string text, int start)
+        {
+            if (text == null)
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+
+            if ((uint)start > (uint)text.Length)
+                ThrowHelper.ThrowArgumentOutOfRangeException();
+
+            return new ReadOnlyMemory<char>(text, start, text.Length - start);
+        }
+
+        /// <summary>Creates a new <see cref="ReadOnlyMemory{char}"/> over the portion of the target string.</summary>
+        /// <param name="text">The target string.</param>
+        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null reference (Nothing in Visual Basic).</exception>
+        /// <exception cref="System.ArgumentOutOfRangeException">
+        /// Thrown when the specified <paramref name="start"/> index or <paramref name="length"/> is not in range.
+        /// </exception>
+        public static ReadOnlyMemory<char> AsReadOnlyMemory(this string text, int start, int length)
+        {
+            if (text == null)
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+
+            if ((uint)start > (uint)text.Length || (uint)length > (uint)(text.Length - start))
+                ThrowHelper.ThrowArgumentOutOfRangeException();
+
+            return new ReadOnlyMemory<char>(text, start, length);
+        }
+
         /// <summary>Attempts to get the underlying <see cref="string"/> from a <see cref="ReadOnlyMemory{T}"/>.</summary>
         /// <param name="readOnlyMemory">The memory that may be wrapping a <see cref="string"/> object.</param>
         /// <param name="text">The string.</param>
@@ -166,6 +198,50 @@ public static ReadOnlySpan<char> AsReadOnlySpan(this string text)
             return new ReadOnlySpan<char>(ref text.GetRawStringData(), text.Length);
         }
 
+        /// <summary>
+        /// Creates a new readonly span over the portion of the target string.
+        /// </summary>
+        /// <param name="text">The target string.</param>
+        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null
+        /// reference (Nothing in Visual Basic).
+        /// </exception>
+        /// <exception cref="System.ArgumentOutOfRangeException">
+        /// Thrown when the specified <paramref name="start"/> index is not in range (&lt;0 or &gt;text.Length).
+        /// </exception>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static ReadOnlySpan<char> AsReadOnlySpan(this string text, int start)
+        {
+            if (text == null)
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+
+            if ((uint)start > (uint)text.Length)
+                ThrowHelper.ThrowArgumentOutOfRangeException();
+
+            return new ReadOnlySpan<char>(ref Unsafe.Add(ref text.GetRawStringData(), start), text.Length - start);
+        }
+
+        /// <summary>
+        /// Creates a new readonly span over the portion of the target string.
+        /// </summary>
+        /// <param name="text">The target string.</param>
+        /// <exception cref="System.ArgumentNullException">Thrown when <paramref name="text"/> is a null
+        /// reference (Nothing in Visual Basic).
+        /// </exception>
+        /// <exception cref="System.ArgumentOutOfRangeException">
+        /// Thrown when the specified <paramref name="start"/> index or <paramref name="length"/> is not in range.
+        /// </exception>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static ReadOnlySpan<char> AsReadOnlySpan(this string text, int start, int length)
+        {
+            if (text == null)
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.text);
+
+            if ((uint)start > (uint)text.Length || (uint)length > (uint)(text.Length - start))
+                ThrowHelper.ThrowArgumentOutOfRangeException();
+
+            return new ReadOnlySpan<char>(ref Unsafe.Add(ref text.GetRawStringData(), start), length);
+        }
+
         internal static unsafe void CopyTo<T>(ref T destination, ref T source, int elementsCount)
         {
             if (Unsafe.AreSame(ref destination, ref source))