[stdlib] add UMBP.mutableSpan accessor

Author: glessard

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -583,6 +583,18 @@ extension Unsafe${Mutable}BufferPointer where Element: ~Copyable {
       return unsafe _overrideLifetime(span, borrowing: self)
     }
   }
+%if Mutable:
+
+  @unsafe
+  @available(SwiftStdlib 6.2, *)
+  public var mutableSpan: MutableSpan<Element> {
+    @lifetime(borrow self)
+    @_alwaysEmitIntoClient
+    get {
+      unsafe MutableSpan(_unsafeElements: self)
+    }
+  }
+%end
 }
 
 extension Unsafe${Mutable}BufferPointer {

test/stdlib/Span/MutableSpanTests.swift

@@ -720,3 +720,25 @@ suite.test("extracting suffixes")
     expectEqual(span.extracting(droppingFirst: 1).count, b.count)
   }
 }
+
+suite.test("MutableSpan from UnsafeMutableBufferPointer")
+.require(.stdlib_6_2).code {
+  guard #available(SwiftStdlib 6.2, *) else { return }
+
+  let capacity = 4
+  let b = UnsafeMutableBufferPointer<Int>.allocate(capacity: capacity)
+  defer {
+    b.deallocate()
+  }
+  _ = b.initialize(fromContentsOf: 0..<capacity)
+
+  var span = b.mutableSpan
+  expectEqual(span.count, capacity)
+
+  span.swapAt(0, 3)
+  span.swapAt(1, 2)
+
+  _ = consume span
+
+  expectTrue(b.elementsEqual((0..<capacity).reversed()))
+}