[stdlib] add MutableSpan.mutableBytes

Author: glessard

stdlib/public/core/Span/MutableRawSpan.swift

@@ -102,16 +102,16 @@ extension MutableRawSpan {
   }
 
   @_alwaysEmitIntoClient
-  @lifetime(copy elements)
+  @lifetime(borrow elements)
   public init<Element: BitwiseCopyable>(
-    _elements elements: consuming MutableSpan<Element>
+    _elements elements: inout MutableSpan<Element>
   ) {
-    let bytes = unsafe UnsafeMutableRawBufferPointer(
-      start: elements._pointer,
-      count: elements.count &* MemoryLayout<Element>.stride
+    let (start, count) = unsafe (elements._pointer, elements._count)
+    let span = unsafe MutableRawSpan(
+      _unchecked: start,
+      byteCount: count &* MemoryLayout<Element>.stride
     )
-    let span = unsafe MutableRawSpan(_unsafeBytes: bytes)
-    self = unsafe _overrideLifetime(span, copying: elements)
+    self = unsafe _overrideLifetime(span, mutating: &elements)
   }
 }
 

stdlib/public/core/Span/MutableSpan.swift

@@ -241,6 +241,17 @@ extension MutableSpan where Element: BitwiseCopyable {
       RawSpan(_mutableSpan: self)
     }
   }
+
+  /// Construct a MutableRawSpan over the memory represented by this span
+  ///
+  /// - Returns: a MutableRawSpan over the memory represented by this span
+  @_alwaysEmitIntoClient
+  public var mutableBytes: MutableRawSpan {
+    @lifetime(borrow self)
+    mutating get {
+      MutableRawSpan(_elements: &self)
+    }
+  }
 }
 
 @available(SwiftStdlib 6.2, *)