[stdlib] add storage property to String.UTF8View

Author: glessard

stdlib/public/core/StringUTF8View.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
@@ -89,6 +89,7 @@ extension String {
   ///     print(String(s1.utf8.prefix(15))!)
   ///     // Prints "They call me 'B"
   @frozen
+  @_addressableForDependencies
   public struct UTF8View: Sendable {
     @usableFromInline
     internal var _guts: _StringGuts
@@ -317,6 +318,31 @@ extension String.UTF8View {
   }
 }
 
+extension String.UTF8View {
+
+  @available(SwiftStdlib 6.2, *)
+  public var span: Span<UTF8.CodeUnit> {
+    @lifetime(borrow self)
+    borrowing get {
+      let count = _guts.count
+      if _guts.isSmall {
+        let a = Builtin.addressOfBorrow(self)
+        let address = unsafe UnsafePointer<UTF8.CodeUnit>(a)
+        let span = unsafe Span(_unsafeStart: address, count: count)
+        return unsafe _overrideLifetime(span, borrowing: self)
+      }
+      else if _guts.isFastUTF8 {
+        let buffer = unsafe _guts._object.fastUTF8
+        _internalInvariant(count == buffer.count)
+        let span = unsafe Span(_unsafeElements: buffer)
+        return unsafe _overrideLifetime(span, borrowing: self)
+      }
+      // handle other Objective-C bridging cases here
+      fatalError("Some bridged Strings are not supported at this time")
+    }
+  }
+}
+
 // Index conversions
 extension String.UTF8View.Index {
   /// Creates an index in the given UTF-8 view that corresponds exactly to the

test/stdlib/Span/StringUTF8SpanProperty.swift

@@ -0,0 +1,52 @@
+//===--- StringUTF8StorageProperty.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(-enable-experimental-feature LifetimeDependence -enable-experimental-feature Span -enable-experimental-feature AddressableTypes)
+
+// REQUIRES: executable_test
+
+import StdlibUnittest
+
+var suite = TestSuite("StringUTF8StorageProperty")
+defer { runAllTests() }
+
+suite.test("Span from Small String")
+.require(.stdlib_6_2).code {
+  guard #available(SwiftStdlib 6.2, *) else { return }
+
+  let s = "A small string.".utf8
+  let u = Array(s)
+  let span = s.span
+
+  let count = span.count
+  expectEqual(count, s.count)
+
+  for i in span.indices {
+    expectEqual(span[i], u[i])
+  }
+}
+
+suite.test("Span from Large Native String")
+.require(.stdlib_6_2).code {
+  guard #available(SwiftStdlib 6.2, *) else { return }
+
+  let s = "A long string that is altogether not smol.".utf8
+  let u = Array(s)
+  let span = s.span
+
+  let count = span.count
+  expectEqual(count, s.count)
+
+  for i in span.indices {
+    expectEqual(span[i], u[i])
+  }
+}