Avoid Unsafe.As for Memory<T> and ReadOnlyMemory<T> conversion (#111023)

* Use `Unsafe.BitCast`to cast `Memory<T>` and `ReadOnlyMemory<T>`

* remove use of `Unsafe`

Author: xtqqczze

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

@@ -20,9 +20,6 @@ namespace System
     [DebuggerDisplay("{ToString(),raw}")]
     public readonly struct Memory<T> : IEquatable<Memory<T>>
     {
-        // NOTE: With the current implementation, Memory<T> and ReadOnlyMemory<T> must have the same layout,
-        // as code uses Unsafe.As to cast between them.
-
         // The highest order bit of _index is used to discern whether _object is a pre-pinned array.
         // (_index < 0) => _object is a pre-pinned array, so Pin() will not allocate a new GCHandle
         //       (else) => Pin() needs to allocate a new GCHandle to pin the object.
@@ -187,7 +184,7 @@ internal Memory(object? obj, int start, int length)
         /// Defines an implicit conversion of a <see cref="Memory{T}"/> to a <see cref="ReadOnlyMemory{T}"/>
         /// </summary>
         public static implicit operator ReadOnlyMemory<T>(Memory<T> memory) =>
-            Unsafe.As<Memory<T>, ReadOnlyMemory<T>>(ref memory);
+            new ReadOnlyMemory<T>(memory._object, memory._index, memory._length);
 
         /// <summary>
         /// Returns an empty <see cref="Memory{T}"/>

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

@@ -20,15 +20,12 @@ namespace System
     [DebuggerDisplay("{ToString(),raw}")]
     public readonly struct ReadOnlyMemory<T> : IEquatable<ReadOnlyMemory<T>>
     {
-        // NOTE: With the current implementation, Memory<T> and ReadOnlyMemory<T> must have the same layout,
-        // as code uses Unsafe.As to cast between them.
-
         // The highest order bit of _index is used to discern whether _object is a pre-pinned array.
         // (_index < 0) => _object is a pre-pinned array, so Pin() will not allocate a new GCHandle
         //       (else) => Pin() needs to allocate a new GCHandle to pin the object.
-        private readonly object? _object;
-        private readonly int _index;
-        private readonly int _length;
+        internal readonly object? _object;
+        internal readonly int _index;
+        internal readonly int _length;
 
         internal const int RemoveFlagsBitMask = 0x7FFFFFFF;
 

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs

@@ -71,7 +71,7 @@ ref Unsafe.As<T, byte>(ref GetReference(span)),
         /// as <see cref="Memory{T}"/> but only used for reading to store a <see cref="ReadOnlyMemory{T}"/>.
         /// </remarks>
         public static Memory<T> AsMemory<T>(ReadOnlyMemory<T> memory) =>
-            Unsafe.As<ReadOnlyMemory<T>, Memory<T>>(ref memory);
+            new Memory<T>(memory._object, memory._index, memory._length);
 
         /// <summary>
         /// Returns a reference to the 0th element of the Span. If the Span is empty, returns a reference to the location where the 0th element