WIP: Add private {ReadOnly}Span ctor that takes a ref, index and length

src/libraries/System.Private.CoreLib/src/System/MemoryExtensions.cs

@@ -120,7 +120,7 @@ public static ReadOnlySpan<char> AsSpan(this string? text, int start)
             if ((uint)start > (uint)text.Length)
                 ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
 
-            return new ReadOnlySpan<char>(ref Unsafe.Add(ref text.GetRawStringData(), (nint)(uint)start /* force zero-extension */), text.Length - start);
+            return new ReadOnlySpan<char>(ref text.GetRawStringData(), start, text.Length - start);
         }
 
         /// <summary>
@@ -152,7 +152,7 @@ public static ReadOnlySpan<char> AsSpan(this string? text, int start, int length
                 ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.start);
 #endif
 
-            return new ReadOnlySpan<char>(ref Unsafe.Add(ref text.GetRawStringData(), (nint)(uint)start /* force zero-extension */), length);
+            return new ReadOnlySpan<char>(ref text.GetRawStringData(), start, length);
         }
 
         /// <summary>Creates a new <see cref="ReadOnlyMemory{T}"/> over the portion of the target string.</summary>

src/libraries/System.Private.CoreLib/src/System/ReadOnlySpan.cs

@@ -126,6 +126,18 @@ internal ReadOnlySpan(ref T reference, int length)
             _length = length;
         }
 
+        // Constructor for internal use only.
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal ReadOnlySpan(ref T reference, int start, int length)
+        {
+            Debug.Assert(start >= 0);
+            Debug.Assert(length >= 0);
+            Debug.Assert(length - start >= 0);
+
+            _reference = new ByReference<T>(ref Unsafe.Add(ref reference, (nint)(uint)start /* force zero-extension */));
+            _length = length;
+        }
+
         /// <summary>
         /// Returns the specified element of the read-only span.
         /// </summary>
@@ -345,7 +357,7 @@ public ReadOnlySpan<T> Slice(int start)
             if ((uint)start > (uint)_length)
                 ThrowHelper.ThrowArgumentOutOfRangeException();
 
-            return new ReadOnlySpan<T>(ref Unsafe.Add(ref _reference.Value, (nint)(uint)start /* force zero-extension */), _length - start);
+            return new ReadOnlySpan<T>(ref _reference.Value, start, _length - start);
         }
 
         /// <summary>
@@ -368,7 +380,7 @@ public ReadOnlySpan<T> Slice(int start, int length)
                 ThrowHelper.ThrowArgumentOutOfRangeException();
 #endif
 
-            return new ReadOnlySpan<T>(ref Unsafe.Add(ref _reference.Value, (nint)(uint)start /* force zero-extension */), length);
+            return new ReadOnlySpan<T>(ref _reference.Value, start, length);
         }
 
         /// <summary>

src/libraries/System.Private.CoreLib/src/System/Span.cs

@@ -131,6 +131,18 @@ internal Span(ref T reference, int length)
             _length = length;
         }
 
+        // Constructor for internal use only.
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal Span(ref T reference, int start, int length)
+        {
+            Debug.Assert(start >= 0);
+            Debug.Assert(length >= 0);
+            Debug.Assert(length - start >= 0);
+
+            _reference = new ByReference<T>(ref Unsafe.Add(ref reference, (nint)(uint)start /* force zero-extension */));
+            _length = length;
+        }
+
         /// <summary>
         /// Returns a reference to specified element of the Span.
         /// </summary>
@@ -392,7 +404,7 @@ public Span<T> Slice(int start)
             if ((uint)start > (uint)_length)
                 ThrowHelper.ThrowArgumentOutOfRangeException();
 
-            return new Span<T>(ref Unsafe.Add(ref _reference.Value, (nint)(uint)start /* force zero-extension */), _length - start);
+            return new Span<T>(ref _reference.Value, start, _length - start);
         }
 
         /// <summary>
@@ -420,7 +432,7 @@ public Span<T> Slice(int start, int length)
                 ThrowHelper.ThrowArgumentOutOfRangeException();
 #endif
 
-            return new Span<T>(ref Unsafe.Add(ref _reference.Value, (nint)(uint)start /* force zero-extension */), length);
+            return new Span<T>(ref _reference.Value, start, length);
         }
 
         /// <summary>

src/libraries/System.Private.CoreLib/src/System/String.Searching.cs

@@ -119,7 +119,7 @@ public int IndexOfAny(char[] anyOf, int startIndex, int count)
                 ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.count, ExceptionResource.ArgumentOutOfRange_Count);
             }
 
-            int result = new ReadOnlySpan<char>(ref Unsafe.Add(ref _firstChar, startIndex), count).IndexOfAny(anyOf);
+            int result = new ReadOnlySpan<char>(ref _firstChar, startIndex, count).IndexOfAny(anyOf);
 
             return result < 0 ? result : result + startIndex;
         }
@@ -337,7 +337,7 @@ public unsafe int LastIndexOfAny(char[] anyOf, int startIndex, int count)
             }
 
             int startSearchAt = startIndex + 1 - count;
-            int result = new ReadOnlySpan<char>(ref Unsafe.Add(ref _firstChar, startSearchAt), count).LastIndexOfAny(anyOf);
+            int result = new ReadOnlySpan<char>(ref _firstChar, startSearchAt, count).LastIndexOfAny(anyOf);
 
             return result < 0 ? result : result + startSearchAt;
         }

src/libraries/System.Private.CoreLib/src/System/String.cs

@@ -361,7 +361,7 @@ internal bool TryGetSpan(int startIndex, int count, out ReadOnlySpan<char> slice
             }
 #endif
 
-            slice = new ReadOnlySpan<char>(ref Unsafe.Add(ref _firstChar, (nint)(uint)startIndex /* force zero-extension */), count);
+            slice = new ReadOnlySpan<char>(ref _firstChar, startIndex, count);
             return true;
         }
 

src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs

@@ -2062,7 +2062,7 @@ private void AppendWithExpansion(ref char value, int valueCount)
             Debug.Assert(m_ChunkLength == 0, "A new block was not created.");
 
             // Copy the second chunk
-            new ReadOnlySpan<char>(ref Unsafe.Add(ref value, firstLength), restLength).CopyTo(m_ChunkChars);
+            new ReadOnlySpan<char>(ref value, firstLength, restLength).CopyTo(m_ChunkChars);
             m_ChunkLength = restLength;
 
             AssertInvariants();