Use Unsafe.BitCastto cast Memory<T> and ReadOnlyMemory<T>

Author: xtqqczze

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

@@ -21,7 +21,7 @@ namespace System
     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.
+        // as code uses Unsafe.BitCast 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
@@ -187,7 +187,7 @@ 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);
+            Unsafe.BitCast<Memory<T>, ReadOnlyMemory<T>>(memory);
 
         /// <summary>
         /// Returns an empty <see cref="Memory{T}"/>

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

@@ -21,7 +21,7 @@ namespace System
     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.
+        // as code uses Unsafe.BitCast 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

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);
+            Unsafe.BitCast<ReadOnlyMemory<T>, Memory<T>>(memory);
 
         /// <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