Remove unsafe modifier from Memory<T>.Span (#106085)

* Remove `unsafe` modifier from `Memory<T>.Span`

* Make `desiredStartIndex` an `int`

* Use `nuint` and add comment

* fixup!b6b93d972b5b16b345e53c8c07c074cdc85cad9a

---------

Co-authored-by: Stephen Toub <[email protected]>

Author: xtqqczze

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

@@ -263,7 +263,7 @@ public Memory<T> Slice(int start, int length)
         /// <summary>
         /// Returns a span from the memory.
         /// </summary>
-        public unsafe Span<T> Span
+        public Span<T> Span
         {
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             get
@@ -327,7 +327,9 @@ public unsafe Span<T> Span
                     // least to be in-bounds when compared with the original Memory<T> instance, so using the span won't
                     // AV the process.
 
+                    // We use 'nuint' because it gives us a free early zero-extension to 64 bits when running on a 64-bit platform.
                     nuint desiredStartIndex = (uint)_index & (uint)ReadOnlyMemory<T>.RemoveFlagsBitMask;
+
                     int desiredLength = _length;
 
 #if TARGET_64BIT
@@ -343,7 +345,7 @@ public unsafe Span<T> Span
                     }
 #endif
 
-                    refToReturn = ref Unsafe.Add(ref refToReturn, (IntPtr)(void*)desiredStartIndex);
+                    refToReturn = ref Unsafe.Add(ref refToReturn, desiredStartIndex);
                     lengthOfUnderlyingSpan = desiredLength;
                 }
 

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

@@ -192,7 +192,7 @@ public ReadOnlyMemory<T> Slice(int start, int length)
         /// <summary>
         /// Returns a span from the memory.
         /// </summary>
-        public unsafe ReadOnlySpan<T> Span
+        public ReadOnlySpan<T> Span
         {
             [MethodImpl(MethodImplOptions.AggressiveInlining)]
             get
@@ -249,7 +249,9 @@ public unsafe ReadOnlySpan<T> Span
                     // least to be in-bounds when compared with the original Memory<T> instance, so using the span won't
                     // AV the process.
 
+                    // We use 'nuint' because it gives us a free early zero-extension to 64 bits when running on a 64-bit platform.
                     nuint desiredStartIndex = (uint)_index & (uint)RemoveFlagsBitMask;
+
                     int desiredLength = _length;
 
 #if TARGET_64BIT
@@ -265,7 +267,7 @@ public unsafe ReadOnlySpan<T> Span
                     }
 #endif
 
-                    refToReturn = ref Unsafe.Add(ref refToReturn, (IntPtr)(void*)desiredStartIndex);
+                    refToReturn = ref Unsafe.Add(ref refToReturn, desiredStartIndex);
                     lengthOfUnderlyingSpan = desiredLength;
                 }