[stdlib] add span property to CollectionOfOne

Author: glessard

stdlib/public/core/CollectionOfOne.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -22,6 +22,7 @@
 ///     let b = a + CollectionOfOne(toAdd)
 ///     // b == [1, 2, 3, 4, 100]
 @frozen // trivial-implementation
+@_addressableForDependencies
 public struct CollectionOfOne<Element> {
   @usableFromInline // trivial-implementation
   internal var _element: Element
@@ -158,6 +159,20 @@ extension CollectionOfOne: RandomAccessCollection, MutableCollection {
   }
 }
 
+extension CollectionOfOne {
+
+  @available(SwiftStdlib 6.2, *)
+  public var span: Span<Element> {
+    @lifetime(borrow self)
+    @_alwaysEmitIntoClient
+    get {
+      let pointer = UnsafePointer<Element>(Builtin.addressOfBorrow(self))
+      let span = Span(_unsafeStart: pointer, count: 1)
+      return _overrideLifetime(span, borrowing: self)
+    }
+  }
+}
+
 @_unavailableInEmbedded
 extension CollectionOfOne: CustomDebugStringConvertible {
   /// A textual representation of the collection, suitable for debugging.

stdlib/public/core/Span/RawSpan.swift

@@ -316,7 +316,8 @@ extension RawSpan {
     let pointer = span._pointer
     let rawSpan = unsafe RawSpan(
       _unchecked: pointer,
-      byteCount: span.count &* MemoryLayout<Element>.stride
+      byteCount: span.count == 1 ? MemoryLayout<Element>.size
+                 : span.count &* MemoryLayout<Element>.stride
     )
     self = unsafe _overrideLifetime(rawSpan, copying: span)
   }

test/stdlib/Span/InlineStorage.swift

@@ -0,0 +1,70 @@
+//===--- InlineStorage.swift ----------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+// RUN: %target-run-stdlib-swift
+
+// REQUIRES: executable_test
+
+import StdlibUnittest
+
+var suite = TestSuite("InlineTypesStorageProperty")
+defer { runAllTests() }
+
+suite.test("CollectionOfOne.span property")
+.skip(.custom(
+  { if #available(SwiftStdlib 6.2, *) { false } else { true } },
+  reason: "Requires Swift 6.2's standard library"
+))
+.code {
+  guard #available(SwiftStdlib 6.2, *) else { return }
+
+  var s = "A long string that is absolutely not smol at all."
+  let u = Array(s.utf8)
+  let c = CollectionOfOne(consume s)
+  s = ""
+  let span = c.span
+  expectEqual(span.count, 1)
+  let v = Array(span[0].utf8)
+  expectEqual(u, v)
+}
+
+suite.test("CollectionOfOne.span property (simple)")
+.skip(.custom(
+  { if #available(SwiftStdlib 6.2, *) { false } else { true } },
+  reason: "Requires Swift 6.2's standard library"
+))
+.code {
+  guard #available(SwiftStdlib 6.2, *) else { return }
+  
+  let c = CollectionOfOne(Int.random(in: 0..<100000))
+  let span = c.span
+  expectEqual(span.count, c.indices.count)
+  expectEqual(span[0], c[0])
+}
+
+struct Padded: BitwiseCopyable {
+  var storage: (Int64, Int8)
+}
+
+suite.test("CollectionOfOne.span stride test")
+.skip(.custom(
+  { if #available(SwiftStdlib 6.2, *) { false } else { true } },
+  reason: "Requires Swift 6.2's standard library"
+))
+.code {
+  guard #available(SwiftStdlib 6.2, *) else { return }
+
+  let c = CollectionOfOne(Padded(storage: (-1, 1)))
+  let span = c.span
+  let bytes = span.bytes
+  expectEqual(bytes.byteCount, MemoryLayout.size(ofValue: c))
+}