From 5196b1ed74649c99f943fcc9fd8ba2bf4b448655 Mon Sep 17 00:00:00 2001
From: Guillaume Lessard <guillaume.lessard@apple.com>
Date: Tue, 29 Apr 2025 18:04:39 -0700
Subject: [PATCH 01/10] [SE-0456, -0467] Data+Span+MutableSpan

---
 CMakeLists.txt                                |   3 +-
 Package.swift                                 |   6 +-
 Sources/FoundationEssentials/Data/Data.swift  | 161 +++++++++++++++++-
 .../FoundationEssentialsTests/DataTests.swift | 134 +++++++++++++++
 4 files changed, 301 insertions(+), 3 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8c21d7e3c..8dfb31f0a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -111,7 +111,8 @@ list(APPEND _SwiftFoundation_versions
 list(APPEND _SwiftFoundation_availability_names
     "FoundationPreview"
     "FoundationPredicate"
-    "FoundationPredicateRegex")
+    "FoundationPredicateRegex"
+    "FoundationSpan")
 
 # The aligned availability for each name (in the same order)
 list(APPEND _SwiftFoundation_availability_releases
diff --git a/Package.swift b/Package.swift
index de020cf03..1d47bca33 100644
--- a/Package.swift
+++ b/Package.swift
@@ -9,7 +9,8 @@ import CompilerPluginSupport
 let availabilityTags: [_Availability] = [
     _Availability("FoundationPreview"), // Default FoundationPreview availability,
     _Availability("FoundationPredicate"), // Predicate relies on pack parameter runtime support
-    _Availability("FoundationPredicateRegex") // Predicate regexes rely on new stdlib APIs
+    _Availability("FoundationPredicateRegex"), // Predicate regexes rely on new stdlib APIs
+    _Availability("FoundationSpan"), // Availability of Span types
 ]
 let versionNumbers = ["0.1", "0.2", "0.3", "0.4", "6.0.2", "6.1", "6.2"]
 
@@ -134,6 +135,9 @@ let package = Package(
           ] + wasiLibcCSettings,
           swiftSettings: [
             .enableExperimentalFeature("VariadicGenerics"),
+            .enableExperimentalFeature("LifetimeDependence"),
+            .enableExperimentalFeature("AddressableTypes"),
+            .enableExperimentalFeature("BuiltinModule"),
             .enableExperimentalFeature("AccessLevelOnImport")
           ] + availabilityMacros + featureSettings,
           linkerSettings: [
diff --git a/Sources/FoundationEssentials/Data/Data.swift b/Sources/FoundationEssentials/Data/Data.swift
index 9f5bb7245..6f56179b9 100644
--- a/Sources/FoundationEssentials/Data/Data.swift
+++ b/Sources/FoundationEssentials/Data/Data.swift
@@ -49,6 +49,7 @@
 #endif
 
 internal import _FoundationCShims
+import Builtin
 
 #if canImport(Darwin)
 import Darwin
@@ -604,6 +605,7 @@ internal final class __DataStorage : @unchecked Sendable {
 
 @frozen
 @available(macOS 10.10, iOS 8.0, watchOS 2.0, tvOS 9.0, *)
+@_addressableForDependencies
 public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollection, RangeReplaceableCollection, MutableDataProtocol, ContiguousBytes, Sendable {
 
     public typealias Index = Int
@@ -2198,7 +2200,105 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
     public func withUnsafeBytes<ResultType>(_ body: (UnsafeRawBufferPointer) throws -> ResultType) rethrows -> ResultType {
         return try _representation.withUnsafeBytes(body)
     }
-    
+
+#if compiler(>=6.2)
+    @available(FoundationSpan 6.2, *)
+    public var bytes: RawSpan {
+        @lifetime(borrow self)
+        borrowing get {
+            let buffer: UnsafeRawBufferPointer
+            switch _representation {
+            case .empty:
+                buffer = UnsafeRawBufferPointer(_empty: ())
+            case .inline:
+                buffer = unsafe UnsafeRawBufferPointer(
+                  start: UnsafeRawPointer(Builtin.addressOfBorrow(self)),
+                  count: _representation.count
+                )
+            case .large(let slice):
+                buffer = unsafe UnsafeRawBufferPointer(
+                  start: slice.storage.mutableBytes?.advanced(by: slice.startIndex), count: slice.count
+                )
+            case .slice(let slice):
+                buffer = unsafe UnsafeRawBufferPointer(
+                  start: slice.storage.mutableBytes?.advanced(by: slice.startIndex), count: slice.count
+                )
+            }
+            let span = unsafe RawSpan(_unsafeBytes: buffer)
+            return unsafe _overrideLifetime(span, borrowing: self)
+        }
+    }
+
+    @available(FoundationSpan 6.2, *)
+    public var span: Span<UInt8> {
+        @lifetime(borrow self)
+        borrowing get {
+            let span = unsafe bytes._unsafeView(as: UInt8.self)
+            return _overrideLifetime(span, borrowing: self)
+        }
+    }
+
+    @available(FoundationSpan 6.2, *)
+    public var mutableBytes: MutableRawSpan {
+        @lifetime(&self)
+        mutating get {
+            let buffer: UnsafeMutableRawBufferPointer
+            switch _representation {
+            case .empty:
+                buffer = UnsafeMutableRawBufferPointer(_empty: ())
+            case .inline:
+                buffer = unsafe UnsafeMutableRawBufferPointer(
+                  start: UnsafeMutableRawPointer(Builtin.addressOfBorrow(self)),
+                  count: _representation.count
+                )
+            case .large(let slice):
+                buffer = unsafe UnsafeMutableRawBufferPointer(
+                  start: slice.storage.mutableBytes?.advanced(by: slice.startIndex), count: slice.count
+                )
+            case .slice(let slice):
+                buffer = unsafe UnsafeMutableRawBufferPointer(
+                  start: slice.storage.mutableBytes?.advanced(by: slice.startIndex), count: slice.count
+                )
+            }
+            let span = unsafe MutableRawSpan(_unsafeBytes: buffer)
+            return unsafe _overrideLifetime(span, mutating: &self)
+        }
+    }
+
+    @available(FoundationSpan 6.2, *)
+    public var mutableSpan: MutableSpan<UInt8> {
+        @lifetime(&self)
+        mutating get {
+#if false
+            var bytes = mutableBytes
+            let span = unsafe bytes._unsafeMutableView(as: UInt8.self)
+            return _overrideLifetime(span, mutating: &self)
+#else
+            let buffer: UnsafeMutableRawBufferPointer
+            switch _representation {
+            case .empty:
+                buffer = UnsafeMutableRawBufferPointer(_empty: ())
+            case .inline:
+                buffer = unsafe UnsafeMutableRawBufferPointer(
+                  start: UnsafeMutableRawPointer(Builtin.addressOfBorrow(self)),
+                  count: _representation.count
+                )
+            case .large(let slice):
+                buffer = unsafe UnsafeMutableRawBufferPointer(
+                  start: slice.storage.mutableBytes?.advanced(by: slice.startIndex), count: slice.count
+                )
+            case .slice(let slice):
+                buffer = unsafe UnsafeMutableRawBufferPointer(
+                  start: slice.storage.mutableBytes?.advanced(by: slice.startIndex), count: slice.count
+                )
+            }
+            let span = unsafe MutableSpan<UInt8>(_unsafeBytes: buffer)
+            return unsafe _overrideLifetime(span, mutating: &self)
+#endif
+        }
+    }
+#endif
+
     @_alwaysEmitIntoClient
     public func withContiguousStorageIfAvailable<ResultType>(_ body: (_ buffer: UnsafeBufferPointer<UInt8>) throws -> ResultType) rethrows -> ResultType? {
         return try _representation.withUnsafeBytes {
@@ -2870,3 +2970,62 @@ extension Data : Codable {
         }
     }
 }
+
+#if compiler(>=6.2)
+// TODO: remove once _overrideLifetime is public in the standard library
+
+/// Unsafely discard any lifetime dependency on the `dependent` argument. Return
+/// a value identical to `dependent` with a lifetime dependency on the caller's
+/// borrow scope of the `source` argument.
+@unsafe
+@_unsafeNonescapableResult
+@_alwaysEmitIntoClient
+@_transparent
+@lifetime(borrow source)
+internal func _overrideLifetime<
+  T: ~Copyable & ~Escapable, U: ~Copyable & ~Escapable
+>(
+  _ dependent: consuming T, borrowing source: borrowing U
+) -> T {
+  // TODO: Remove @_unsafeNonescapableResult. Instead, the unsafe dependence
+  // should be expressed by a builtin that is hidden within the function body.
+  dependent
+}
+
+/// Unsafely discard any lifetime dependency on the `dependent` argument. Return
+/// a value identical to `dependent` that inherits all lifetime dependencies from
+/// the `source` argument.
+@unsafe
+@_unsafeNonescapableResult
+@_alwaysEmitIntoClient
+@_transparent
+@lifetime(copy source)
+internal func _overrideLifetime<
+  T: ~Copyable & ~Escapable, U: ~Copyable & ~Escapable
+>(
+  _ dependent: consuming T, copying source: borrowing U
+) -> T {
+  // TODO: Remove @_unsafeNonescapableResult. Instead, the unsafe dependence
+  // should be expressed by a builtin that is hidden within the function body.
+  dependent
+}
+
+/// Unsafely discard any lifetime dependency on the `dependent` argument.
+/// Return a value identical to `dependent` with a lifetime dependency
+/// on the caller's exclusive borrow scope of the `source` argument.
+@unsafe
+@_unsafeNonescapableResult
+@_alwaysEmitIntoClient
+@_transparent
+@lifetime(&source)
+internal func _overrideLifetime<
+  T: ~Copyable & ~Escapable, U: ~Copyable & ~Escapable
+>(
+  _ dependent: consuming T,
+  mutating source: inout U
+) -> T {
+  // TODO: Remove @_unsafeNonescapableResult. Instead, the unsafe dependence
+  // should be expressed by a builtin that is hidden within the function body.
+  dependent
+}
+#endif
diff --git a/Tests/FoundationEssentialsTests/DataTests.swift b/Tests/FoundationEssentialsTests/DataTests.swift
index ef820dbab..7f8318aff 100644
--- a/Tests/FoundationEssentialsTests/DataTests.swift
+++ b/Tests/FoundationEssentialsTests/DataTests.swift
@@ -1634,6 +1634,140 @@ class DataTests : XCTestCase {
         // source.advanced(by: 5)
     }
 
+    @available(FoundationSpan 6.2, *)
+    func test_InlineDataSpan() throws {
+        var source = Data()
+        var span = source.span
+        XCTAssertTrue(span.isEmpty)
+
+        source.append(contentsOf: [1, 2, 3])
+        span = source.span
+        XCTAssertFalse(span.isEmpty)
+        XCTAssertEqual(span.count, source.count)
+        XCTAssertEqual(span[0], 1)
+    }
+
+    @available(FoundationSpan 6.2, *)
+    func test_InlineSliceDataSpan() throws {
+        let source = Data(0 ... .max)
+        let span = source.span
+        XCTAssertEqual(span.count, source.count)
+        XCTAssertEqual(span[span.indices.last!], .max)
+    }
+
+    @available(FoundationSpan 6.2, *)
+    func test_LargeSliceDataSpan() throws {
+#if _pointerBitWidth(_64)
+        let count = Int(Int32.max)
+#elseif _pointerBitWidth(_32)
+        let count = Int(Int16.max)
+#else
+        #error("This test needs updating")
+#endif
+
+        let source = Data(repeating: 0, count: count).dropFirst()
+        XCTAssertNotEqual(source.startIndex, 0)
+        let span = source.span
+        XCTAssertFalse(span.isEmpty)
+    }
+
+    @available(FoundationSpan 6.2, *)
+    func test_InlineDataMutableSpan() throws {
+        var source = Data()
+        var span = source.mutableSpan
+        XCTAssertTrue(span.isEmpty)
+
+        source.append(contentsOf: [1, 2, 3])
+        let count = source.count
+        span = source.mutableSpan
+        let i = try XCTUnwrap(span.indices.randomElement())
+        XCTAssertFalse(span.isEmpty)
+        XCTAssertEqual(span.count, count)
+        let v = UInt8.random(in: 10..<100)
+        span[i] = v
+        XCTAssertEqual(source[i], v)
+    }
+
+    @available(FoundationSpan 6.2, *)
+    func test_InlineSliceDataMutableSpan() throws {
+        var source = Data(0..<100)
+        let count = source.count
+        var span = source.mutableSpan
+        XCTAssertEqual(span.count, count)
+        let i = try XCTUnwrap(span.indices.randomElement())
+        span[i] = .max
+        XCTAssertEqual(source[i], .max)
+    }
+
+    @available(FoundationSpan 6.2, *)
+    func test_LargeSliceDataMutableSpan() throws {
+  #if _pointerBitWidth(_64)
+        var count = Int(Int32.max)
+  #elseif _pointerBitWidth(_32)
+        var count = Int(Int16.max)
+  #else
+        #error("This test needs updating")
+  #endif
+
+        var source = Data(repeating: 0, count: count).dropFirst()
+        XCTAssertNotEqual(source.startIndex, 0)
+        count = source.count
+        var span = source.mutableSpan
+        XCTAssertEqual(span.count, count)
+        let i = try XCTUnwrap(span.indices.dropFirst().randomElement())
+        span[i] = .max
+        XCTAssertEqual(source[i], 0)
+        XCTAssertEqual(source[i+1], .max)
+    }
+
+    @available(FoundationSpan 6.2, *)
+    func test_InlineDataMutableRawSpan() throws {
+        var source = Data()
+        var span = source.mutableBytes
+        XCTAssertTrue(span.isEmpty)
+
+        source.append(contentsOf: [1, 2, 3])
+        let count = source.count
+        span = source.mutableBytes
+        let i = try XCTUnwrap(span.byteOffsets.randomElement())
+        XCTAssertFalse(span.isEmpty)
+        XCTAssertEqual(span.byteCount, count)
+        let v = UInt8.random(in: 10..<100)
+        span.storeBytes(of: v, toByteOffset: i, as: UInt8.self)
+        XCTAssertEqual(source[i], v)
+    }
+
+    @available(FoundationSpan 6.2, *)
+    func test_InlineSliceDataMutableRawSpan() throws {
+        var source = Data(0..<100)
+        let count = source.count
+        var span = source.mutableBytes
+        XCTAssertEqual(span.byteCount, count)
+        let i = try XCTUnwrap(span.byteOffsets.randomElement())
+        span.storeBytes(of: -1, toByteOffset: i, as: Int8.self)
+        XCTAssertEqual(source[i], .max)
+    }
+
+    @available(FoundationSpan 6.2, *)
+    func test_LargeSliceDataMutableRawSpan() throws {
+#if _pointerBitWidth(_64)
+        var count = Int(Int32.max)
+#elseif _pointerBitWidth(_32)
+        var count = Int(Int16.max)
+#else
+        #error("This test needs updating")
+#endif
+
+        var source = Data(repeating: 0, count: count).dropFirst()
+        XCTAssertNotEqual(source.startIndex, 0)
+        count = source.count
+        var span = source.mutableBytes
+        XCTAssertEqual(span.byteCount, count)
+        let i = try XCTUnwrap(span.byteOffsets.dropFirst().randomElement())
+        span.storeBytes(of: -1, toByteOffset: i, as: Int8.self)
+        XCTAssertEqual(source[i], 0)
+        XCTAssertEqual(source[i+1], .max)
+    }
 
     #if false // FIXME: XCTest doesn't support crash tests yet rdar://20195010&22387653
     func test_bounding_failure_subdata() {

From 9857ce13f39623ebfca3b13d2bd49331c1958be1 Mon Sep 17 00:00:00 2001
From: Guillaume Lessard <guillaume.lessard@apple.com>
Date: Wed, 30 Apr 2025 14:51:14 -0700
Subject: [PATCH 02/10] Document compiler issue

---
 Sources/FoundationEssentials/Data/Data.swift | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Sources/FoundationEssentials/Data/Data.swift b/Sources/FoundationEssentials/Data/Data.swift
index 6f56179b9..32ca407e4 100644
--- a/Sources/FoundationEssentials/Data/Data.swift
+++ b/Sources/FoundationEssentials/Data/Data.swift
@@ -2269,7 +2269,7 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
     public var mutableSpan: MutableSpan<UInt8> {
         @lifetime(&self)
         mutating get {
-#if false
+#if false // see https://github.com/swiftlang/swift/issues/81218
             var bytes = mutableBytes
             let span = unsafe bytes._unsafeMutableView(as: UInt8.self)
             return _overrideLifetime(span, mutating: &self)

From efd6b6c23b07808b6320a497ac085d9feb3a7f52 Mon Sep 17 00:00:00 2001
From: Guillaume Lessard <guillaume.lessard@apple.com>
Date: Wed, 30 Apr 2025 14:51:41 -0700
Subject: [PATCH 03/10] fix use of non-public initializers

---
 Sources/FoundationEssentials/Data/Data.swift | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/Sources/FoundationEssentials/Data/Data.swift b/Sources/FoundationEssentials/Data/Data.swift
index 32ca407e4..65ff69721 100644
--- a/Sources/FoundationEssentials/Data/Data.swift
+++ b/Sources/FoundationEssentials/Data/Data.swift
@@ -2209,7 +2209,7 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
             let buffer: UnsafeRawBufferPointer
             switch _representation {
             case .empty:
-                buffer = UnsafeRawBufferPointer(_empty: ())
+                buffer = UnsafeRawBufferPointer(start: nil, count: 0)
             case .inline:
                 buffer = unsafe UnsafeRawBufferPointer(
                   start: UnsafeRawPointer(Builtin.addressOfBorrow(self)),
@@ -2245,7 +2245,7 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
             let buffer: UnsafeMutableRawBufferPointer
             switch _representation {
             case .empty:
-                buffer = UnsafeMutableRawBufferPointer(_empty: ())
+                buffer = UnsafeMutableRawBufferPointer(start: nil, count: 0)
             case .inline:
                 buffer = unsafe UnsafeMutableRawBufferPointer(
                   start: UnsafeMutableRawPointer(Builtin.addressOfBorrow(self)),
@@ -2277,7 +2277,7 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
             let buffer: UnsafeMutableRawBufferPointer
             switch _representation {
             case .empty:
-                buffer = UnsafeMutableRawBufferPointer(_empty: ())
+                buffer = UnsafeMutableRawBufferPointer(start: nil, count: 0)
             case .inline:
                 buffer = unsafe UnsafeMutableRawBufferPointer(
                   start: UnsafeMutableRawPointer(Builtin.addressOfBorrow(self)),

From 8a6a7d998825228868665e8539c8c0b5ec376d93 Mon Sep 17 00:00:00 2001
From: Guillaume Lessard <guillaume.lessard@apple.com>
Date: Fri, 23 May 2025 12:30:55 -0700
Subject: [PATCH 04/10] Address availability and compiler versions

---
 Package.swift                                 |  2 +-
 README.md                                     |  2 +-
 Sources/FoundationEssentials/Data/Data.swift  |  6 ++--
 .../FoundationEssentialsTests/DataTests.swift | 35 ++++++++++++-------
 4 files changed, 26 insertions(+), 19 deletions(-)

diff --git a/Package.swift b/Package.swift
index 1d47bca33..f23d69e3b 100644
--- a/Package.swift
+++ b/Package.swift
@@ -10,7 +10,7 @@ let availabilityTags: [_Availability] = [
     _Availability("FoundationPreview"), // Default FoundationPreview availability,
     _Availability("FoundationPredicate"), // Predicate relies on pack parameter runtime support
     _Availability("FoundationPredicateRegex"), // Predicate regexes rely on new stdlib APIs
-    _Availability("FoundationSpan"), // Availability of Span types
+    _Availability("FoundationSpan", availability: .future), // Availability of Span types
 ]
 let versionNumbers = ["0.1", "0.2", "0.3", "0.4", "6.0.2", "6.1", "6.2"]
 
diff --git a/README.md b/README.md
index 4cc54f77b..d871bf60b 100644
--- a/README.md
+++ b/README.md
@@ -18,7 +18,7 @@ On all other Swift platforms, `swift-foundation` is available as part of the too
 ## Building and Testing
 
 > [!NOTE]
-> Building swift-foundation requires the in-development Swift 6.0 toolchain. You can download the Swift 6.0 nightly toolchain from [the Swift website](https://swift.org/install).
+> Building swift-foundation requires the in-development Swift 6.2 toolchain. You can download the Swift 6.2 nightly toolchain from [the Swift website](https://swift.org/install).
 
 Before building Foundation, first ensure that you have a Swift toolchain installed. Next, check out the _Getting Started_ section of the [Foundation Build Process](Foundation_Build_Process.md#getting-started) guide for detailed steps on building and testing.
 
diff --git a/Sources/FoundationEssentials/Data/Data.swift b/Sources/FoundationEssentials/Data/Data.swift
index 65ff69721..efd1f5af2 100644
--- a/Sources/FoundationEssentials/Data/Data.swift
+++ b/Sources/FoundationEssentials/Data/Data.swift
@@ -605,7 +605,9 @@ internal final class __DataStorage : @unchecked Sendable {
 
 @frozen
 @available(macOS 10.10, iOS 8.0, watchOS 2.0, tvOS 9.0, *)
+#if compiler(>=6.2)
 @_addressableForDependencies
+#endif
 public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollection, RangeReplaceableCollection, MutableDataProtocol, ContiguousBytes, Sendable {
 
     public typealias Index = Int
@@ -2201,7 +2203,6 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
         return try _representation.withUnsafeBytes(body)
     }
 
-#if compiler(>=6.2)
     @available(FoundationSpan 6.2, *)
     public var bytes: RawSpan {
         @lifetime(borrow self)
@@ -2297,7 +2298,6 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
 #endif
         }
     }
-#endif
 
     @_alwaysEmitIntoClient
     public func withContiguousStorageIfAvailable<ResultType>(_ body: (_ buffer: UnsafeBufferPointer<UInt8>) throws -> ResultType) rethrows -> ResultType? {
@@ -2971,7 +2971,6 @@ extension Data : Codable {
     }
 }
 
-#if compiler(>=6.2)
 // TODO: remove once _overrideLifetime is public in the standard library
 
 /// Unsafely discard any lifetime dependency on the `dependent` argument. Return
@@ -3028,4 +3027,3 @@ internal func _overrideLifetime<
   // should be expressed by a builtin that is hidden within the function body.
   dependent
 }
-#endif
diff --git a/Tests/FoundationEssentialsTests/DataTests.swift b/Tests/FoundationEssentialsTests/DataTests.swift
index 7f8318aff..27b0f83bc 100644
--- a/Tests/FoundationEssentialsTests/DataTests.swift
+++ b/Tests/FoundationEssentialsTests/DataTests.swift
@@ -1634,8 +1634,9 @@ class DataTests : XCTestCase {
         // source.advanced(by: 5)
     }
 
-    @available(FoundationSpan 6.2, *)
     func test_InlineDataSpan() throws {
+        guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
+
         var source = Data()
         var span = source.span
         XCTAssertTrue(span.isEmpty)
@@ -1647,16 +1648,18 @@ class DataTests : XCTestCase {
         XCTAssertEqual(span[0], 1)
     }
 
-    @available(FoundationSpan 6.2, *)
     func test_InlineSliceDataSpan() throws {
+        guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
+
         let source = Data(0 ... .max)
         let span = source.span
         XCTAssertEqual(span.count, source.count)
         XCTAssertEqual(span[span.indices.last!], .max)
     }
 
-    @available(FoundationSpan 6.2, *)
     func test_LargeSliceDataSpan() throws {
+        guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
+
 #if _pointerBitWidth(_64)
         let count = Int(Int32.max)
 #elseif _pointerBitWidth(_32)
@@ -1671,8 +1674,9 @@ class DataTests : XCTestCase {
         XCTAssertFalse(span.isEmpty)
     }
 
-    @available(FoundationSpan 6.2, *)
     func test_InlineDataMutableSpan() throws {
+        guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
+
         var source = Data()
         var span = source.mutableSpan
         XCTAssertTrue(span.isEmpty)
@@ -1688,8 +1692,9 @@ class DataTests : XCTestCase {
         XCTAssertEqual(source[i], v)
     }
 
-    @available(FoundationSpan 6.2, *)
     func test_InlineSliceDataMutableSpan() throws {
+        guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
+
         var source = Data(0..<100)
         let count = source.count
         var span = source.mutableSpan
@@ -1699,15 +1704,16 @@ class DataTests : XCTestCase {
         XCTAssertEqual(source[i], .max)
     }
 
-    @available(FoundationSpan 6.2, *)
     func test_LargeSliceDataMutableSpan() throws {
-  #if _pointerBitWidth(_64)
+        guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
+
+#if _pointerBitWidth(_64)
         var count = Int(Int32.max)
-  #elseif _pointerBitWidth(_32)
+#elseif _pointerBitWidth(_32)
         var count = Int(Int16.max)
-  #else
+#else
         #error("This test needs updating")
-  #endif
+#endif
 
         var source = Data(repeating: 0, count: count).dropFirst()
         XCTAssertNotEqual(source.startIndex, 0)
@@ -1720,8 +1726,9 @@ class DataTests : XCTestCase {
         XCTAssertEqual(source[i+1], .max)
     }
 
-    @available(FoundationSpan 6.2, *)
     func test_InlineDataMutableRawSpan() throws {
+        guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
+
         var source = Data()
         var span = source.mutableBytes
         XCTAssertTrue(span.isEmpty)
@@ -1737,8 +1744,9 @@ class DataTests : XCTestCase {
         XCTAssertEqual(source[i], v)
     }
 
-    @available(FoundationSpan 6.2, *)
     func test_InlineSliceDataMutableRawSpan() throws {
+        guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
+
         var source = Data(0..<100)
         let count = source.count
         var span = source.mutableBytes
@@ -1748,8 +1756,9 @@ class DataTests : XCTestCase {
         XCTAssertEqual(source[i], .max)
     }
 
-    @available(FoundationSpan 6.2, *)
     func test_LargeSliceDataMutableRawSpan() throws {
+        guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
+
 #if _pointerBitWidth(_64)
         var count = Int(Int32.max)
 #elseif _pointerBitWidth(_32)

From a02e0b7f57c8d7c849968ee6c6f793212b9ae48a Mon Sep 17 00:00:00 2001
From: Guillaume Lessard <guillaume.lessard@apple.com>
Date: Fri, 23 May 2025 17:02:39 -0700
Subject: [PATCH 05/10] adjust cmake configuration

---
 CMakeLists.txt                              | 3 ++-
 Sources/FoundationEssentials/CMakeLists.txt | 3 +++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8dfb31f0a..12eb7612d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -118,7 +118,8 @@ list(APPEND _SwiftFoundation_availability_names
 list(APPEND _SwiftFoundation_availability_releases
     ${_SwiftFoundation_BaseAvailability}
     ${_SwiftFoundation_BaseAvailability}
-    ${_SwiftFoundation_BaseAvailability})
+    ${_SwiftFoundation_BaseAvailability}
+    ${_SwiftFoundation_FutureAvailability})
 
 foreach(version ${_SwiftFoundation_versions})
     foreach(name release IN ZIP_LISTS _SwiftFoundation_availability_names _SwiftFoundation_availability_releases)
diff --git a/Sources/FoundationEssentials/CMakeLists.txt b/Sources/FoundationEssentials/CMakeLists.txt
index afdbdd507..b88ee86d7 100644
--- a/Sources/FoundationEssentials/CMakeLists.txt
+++ b/Sources/FoundationEssentials/CMakeLists.txt
@@ -80,6 +80,9 @@ endif()
 
 target_compile_options(FoundationEssentials PRIVATE
     "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-experimental-feature -Xfrontend VariadicGenerics>"
+    "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-experimental-feature -Xfrontend LifetimeDependence>"
+    "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-experimental-feature -Xfrontend AddressableTypes>"
+    "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-experimental-feature -Xfrontend BuiltinModule>"
     "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-experimental-feature -Xfrontend AccessLevelOnImport>"
     "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-experimental-feature -Xfrontend StrictConcurrency>"
     "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend InferSendableFromCaptures>"

From 88e1cbdc3ebbda2baedbaeedad1d8a2dfea44187 Mon Sep 17 00:00:00 2001
From: Guillaume Lessard <guillaume.lessard@apple.com>
Date: Fri, 30 May 2025 11:15:40 -0700
Subject: [PATCH 06/10] temporarily disable Span and MutableSpan for Windows

The current Windows nightly is too old to be able to compile the expected syntax.
---
 Package.swift                                 | 19 ++++++++++++++++++-
 Sources/FoundationEssentials/Data/Data.swift  | 10 ++++------
 .../FoundationEssentialsTests/DataTests.swift | 18 ++++++++++++++++++
 3 files changed, 40 insertions(+), 7 deletions(-)

diff --git a/Package.swift b/Package.swift
index f23d69e3b..744a79207 100644
--- a/Package.swift
+++ b/Package.swift
@@ -136,6 +136,14 @@ let package = Package(
           swiftSettings: [
             .enableExperimentalFeature("VariadicGenerics"),
             .enableExperimentalFeature("LifetimeDependence"),
+            .enableExperimentalFeature(
+              "InoutLifetimeDependence",
+              .when(platforms: [.macOS, .iOS, .watchOS, .tvOS, .linux])
+            ),
+            .enableExperimentalFeature(
+              "LifetimeDependenceMutableAccessors",
+              .when(platforms: [.macOS, .iOS, .watchOS, .tvOS, .linux])
+            ),
             .enableExperimentalFeature("AddressableTypes"),
             .enableExperimentalFeature("BuiltinModule"),
             .enableExperimentalFeature("AccessLevelOnImport")
@@ -153,7 +161,16 @@ let package = Package(
             resources: [
                 .copy("Resources")
             ],
-            swiftSettings: availabilityMacros + featureSettings
+            swiftSettings: [
+              .enableExperimentalFeature(
+                "InoutLifetimeDependence",
+                .when(platforms: [.macOS, .iOS, .watchOS, .tvOS, .linux])
+              ),
+              .enableExperimentalFeature(
+                "LifetimeDependenceMutableAccessors",
+                .when(platforms: [.macOS, .iOS, .watchOS, .tvOS, .linux])
+              ),
+            ] + availabilityMacros + featureSettings
         ),
 
         // FoundationInternationalization
diff --git a/Sources/FoundationEssentials/Data/Data.swift b/Sources/FoundationEssentials/Data/Data.swift
index efd1f5af2..dca8ca372 100644
--- a/Sources/FoundationEssentials/Data/Data.swift
+++ b/Sources/FoundationEssentials/Data/Data.swift
@@ -2239,6 +2239,7 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
         }
     }
 
+#if compiler(>=5.9) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
     @available(FoundationSpan 6.2, *)
     public var mutableBytes: MutableRawSpan {
         @lifetime(&self)
@@ -2298,6 +2299,7 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
 #endif
         }
     }
+#endif // $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
 
     @_alwaysEmitIntoClient
     public func withContiguousStorageIfAvailable<ResultType>(_ body: (_ buffer: UnsafeBufferPointer<UInt8>) throws -> ResultType) rethrows -> ResultType? {
@@ -2986,8 +2988,6 @@ internal func _overrideLifetime<
 >(
   _ dependent: consuming T, borrowing source: borrowing U
 ) -> T {
-  // TODO: Remove @_unsafeNonescapableResult. Instead, the unsafe dependence
-  // should be expressed by a builtin that is hidden within the function body.
   dependent
 }
 
@@ -3004,11 +3004,10 @@ internal func _overrideLifetime<
 >(
   _ dependent: consuming T, copying source: borrowing U
 ) -> T {
-  // TODO: Remove @_unsafeNonescapableResult. Instead, the unsafe dependence
-  // should be expressed by a builtin that is hidden within the function body.
   dependent
 }
 
+#if compiler(>=5.9) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
 /// Unsafely discard any lifetime dependency on the `dependent` argument.
 /// Return a value identical to `dependent` with a lifetime dependency
 /// on the caller's exclusive borrow scope of the `source` argument.
@@ -3023,7 +3022,6 @@ internal func _overrideLifetime<
   _ dependent: consuming T,
   mutating source: inout U
 ) -> T {
-  // TODO: Remove @_unsafeNonescapableResult. Instead, the unsafe dependence
-  // should be expressed by a builtin that is hidden within the function body.
   dependent
 }
+#endif // $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
diff --git a/Tests/FoundationEssentialsTests/DataTests.swift b/Tests/FoundationEssentialsTests/DataTests.swift
index 27b0f83bc..0e73ad667 100644
--- a/Tests/FoundationEssentialsTests/DataTests.swift
+++ b/Tests/FoundationEssentialsTests/DataTests.swift
@@ -1637,6 +1637,7 @@ class DataTests : XCTestCase {
     func test_InlineDataSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
+#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data()
         var span = source.span
         XCTAssertTrue(span.isEmpty)
@@ -1646,15 +1647,18 @@ class DataTests : XCTestCase {
         XCTAssertFalse(span.isEmpty)
         XCTAssertEqual(span.count, source.count)
         XCTAssertEqual(span[0], 1)
+#endif
     }
 
     func test_InlineSliceDataSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
+#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         let source = Data(0 ... .max)
         let span = source.span
         XCTAssertEqual(span.count, source.count)
         XCTAssertEqual(span[span.indices.last!], .max)
+#endif
     }
 
     func test_LargeSliceDataSpan() throws {
@@ -1668,15 +1672,18 @@ class DataTests : XCTestCase {
         #error("This test needs updating")
 #endif
 
+#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         let source = Data(repeating: 0, count: count).dropFirst()
         XCTAssertNotEqual(source.startIndex, 0)
         let span = source.span
         XCTAssertFalse(span.isEmpty)
+#endif
     }
 
     func test_InlineDataMutableSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
+#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data()
         var span = source.mutableSpan
         XCTAssertTrue(span.isEmpty)
@@ -1690,11 +1697,13 @@ class DataTests : XCTestCase {
         let v = UInt8.random(in: 10..<100)
         span[i] = v
         XCTAssertEqual(source[i], v)
+#endif
     }
 
     func test_InlineSliceDataMutableSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
+#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data(0..<100)
         let count = source.count
         var span = source.mutableSpan
@@ -1702,6 +1711,7 @@ class DataTests : XCTestCase {
         let i = try XCTUnwrap(span.indices.randomElement())
         span[i] = .max
         XCTAssertEqual(source[i], .max)
+#endif
     }
 
     func test_LargeSliceDataMutableSpan() throws {
@@ -1715,6 +1725,7 @@ class DataTests : XCTestCase {
         #error("This test needs updating")
 #endif
 
+#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data(repeating: 0, count: count).dropFirst()
         XCTAssertNotEqual(source.startIndex, 0)
         count = source.count
@@ -1724,11 +1735,13 @@ class DataTests : XCTestCase {
         span[i] = .max
         XCTAssertEqual(source[i], 0)
         XCTAssertEqual(source[i+1], .max)
+#endif
     }
 
     func test_InlineDataMutableRawSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
+#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data()
         var span = source.mutableBytes
         XCTAssertTrue(span.isEmpty)
@@ -1742,11 +1755,13 @@ class DataTests : XCTestCase {
         let v = UInt8.random(in: 10..<100)
         span.storeBytes(of: v, toByteOffset: i, as: UInt8.self)
         XCTAssertEqual(source[i], v)
+#endif
     }
 
     func test_InlineSliceDataMutableRawSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
+#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data(0..<100)
         let count = source.count
         var span = source.mutableBytes
@@ -1754,6 +1769,7 @@ class DataTests : XCTestCase {
         let i = try XCTUnwrap(span.byteOffsets.randomElement())
         span.storeBytes(of: -1, toByteOffset: i, as: Int8.self)
         XCTAssertEqual(source[i], .max)
+#endif
     }
 
     func test_LargeSliceDataMutableRawSpan() throws {
@@ -1767,6 +1783,7 @@ class DataTests : XCTestCase {
         #error("This test needs updating")
 #endif
 
+#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data(repeating: 0, count: count).dropFirst()
         XCTAssertNotEqual(source.startIndex, 0)
         count = source.count
@@ -1776,6 +1793,7 @@ class DataTests : XCTestCase {
         span.storeBytes(of: -1, toByteOffset: i, as: Int8.self)
         XCTAssertEqual(source[i], 0)
         XCTAssertEqual(source[i+1], .max)
+#endif
     }
 
     #if false // FIXME: XCTest doesn't support crash tests yet rdar://20195010&22387653

From 9e8be796a19262efefc3023fc83e0e4ea1c1a111 Mon Sep 17 00:00:00 2001
From: Guillaume Lessard <guillaume.lessard@apple.com>
Date: Fri, 30 May 2025 18:08:34 -0700
Subject: [PATCH 07/10] update linux build flags

---
 Sources/FoundationEssentials/CMakeLists.txt | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Sources/FoundationEssentials/CMakeLists.txt b/Sources/FoundationEssentials/CMakeLists.txt
index b88ee86d7..79435c105 100644
--- a/Sources/FoundationEssentials/CMakeLists.txt
+++ b/Sources/FoundationEssentials/CMakeLists.txt
@@ -74,7 +74,9 @@ endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL Linux OR CMAKE_SYSTEM_NAME STREQUAL Android)
     target_compile_options(FoundationEssentials PRIVATE
-        "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -Xcc -Xfrontend -D_GNU_SOURCE>")
+        "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -Xcc -Xfrontend -D_GNU_SOURCE>"
+        "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-experimental-feature -Xfrontend InoutLifetimeDependence>"
+        "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-experimental-feature -Xfrontend LifetimeDependenceMutableAccessors>")
     list(APPEND CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE)
 endif()
 

From fa9f9fb00e24c7843dd9ad1d714d739521c262f9 Mon Sep 17 00:00:00 2001
From: Guillaume Lessard <guillaume.lessard@apple.com>
Date: Fri, 30 May 2025 19:19:14 -0700
Subject: [PATCH 08/10] shield lifetime-dependence syntax

---
 Package.swift                                | 1 +
 Sources/FoundationEssentials/Data/Data.swift | 5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/Package.swift b/Package.swift
index 744a79207..e9a3dec09 100644
--- a/Package.swift
+++ b/Package.swift
@@ -145,6 +145,7 @@ let package = Package(
               .when(platforms: [.macOS, .iOS, .watchOS, .tvOS, .linux])
             ),
             .enableExperimentalFeature("AddressableTypes"),
+            .enableExperimentalFeature("AllowUnsafeAttribute"),
             .enableExperimentalFeature("BuiltinModule"),
             .enableExperimentalFeature("AccessLevelOnImport")
           ] + availabilityMacros + featureSettings,
diff --git a/Sources/FoundationEssentials/Data/Data.swift b/Sources/FoundationEssentials/Data/Data.swift
index dca8ca372..bff800131 100644
--- a/Sources/FoundationEssentials/Data/Data.swift
+++ b/Sources/FoundationEssentials/Data/Data.swift
@@ -2203,6 +2203,7 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
         return try _representation.withUnsafeBytes(body)
     }
 
+#if compiler(>=6.2) && $LifetimeDependence
     @available(FoundationSpan 6.2, *)
     public var bytes: RawSpan {
         @lifetime(borrow self)
@@ -2238,6 +2239,7 @@ public struct Data : Equatable, Hashable, RandomAccessCollection, MutableCollect
             return _overrideLifetime(span, borrowing: self)
         }
     }
+#endif
 
 #if compiler(>=5.9) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
     @available(FoundationSpan 6.2, *)
@@ -2974,7 +2976,7 @@ extension Data : Codable {
 }
 
 // TODO: remove once _overrideLifetime is public in the standard library
-
+#if compiler(>=6.2) && $LifetimeDependence
 /// Unsafely discard any lifetime dependency on the `dependent` argument. Return
 /// a value identical to `dependent` with a lifetime dependency on the caller's
 /// borrow scope of the `source` argument.
@@ -3006,6 +3008,7 @@ internal func _overrideLifetime<
 ) -> T {
   dependent
 }
+#endif
 
 #if compiler(>=5.9) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
 /// Unsafely discard any lifetime dependency on the `dependent` argument.

From 9913bdd53c0fd1f7680990b6e3959aa1cd45a841 Mon Sep 17 00:00:00 2001
From: Guillaume Lessard <guillaume.lessard@apple.com>
Date: Mon, 2 Jun 2025 08:52:56 -0700
Subject: [PATCH 09/10] shield new types from testing module

---
 .../FoundationEssentialsTests/DataTests.swift  | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/Tests/FoundationEssentialsTests/DataTests.swift b/Tests/FoundationEssentialsTests/DataTests.swift
index 0e73ad667..cb4f308b0 100644
--- a/Tests/FoundationEssentialsTests/DataTests.swift
+++ b/Tests/FoundationEssentialsTests/DataTests.swift
@@ -1637,7 +1637,7 @@ class DataTests : XCTestCase {
     func test_InlineDataSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
-#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
+#if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data()
         var span = source.span
         XCTAssertTrue(span.isEmpty)
@@ -1653,7 +1653,7 @@ class DataTests : XCTestCase {
     func test_InlineSliceDataSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
-#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
+#if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         let source = Data(0 ... .max)
         let span = source.span
         XCTAssertEqual(span.count, source.count)
@@ -1672,7 +1672,7 @@ class DataTests : XCTestCase {
         #error("This test needs updating")
 #endif
 
-#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
+#if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         let source = Data(repeating: 0, count: count).dropFirst()
         XCTAssertNotEqual(source.startIndex, 0)
         let span = source.span
@@ -1683,7 +1683,7 @@ class DataTests : XCTestCase {
     func test_InlineDataMutableSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
-#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
+#if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data()
         var span = source.mutableSpan
         XCTAssertTrue(span.isEmpty)
@@ -1703,7 +1703,7 @@ class DataTests : XCTestCase {
     func test_InlineSliceDataMutableSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
-#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
+#if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data(0..<100)
         let count = source.count
         var span = source.mutableSpan
@@ -1725,7 +1725,7 @@ class DataTests : XCTestCase {
         #error("This test needs updating")
 #endif
 
-#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
+#if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data(repeating: 0, count: count).dropFirst()
         XCTAssertNotEqual(source.startIndex, 0)
         count = source.count
@@ -1741,7 +1741,7 @@ class DataTests : XCTestCase {
     func test_InlineDataMutableRawSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
-#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
+#if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data()
         var span = source.mutableBytes
         XCTAssertTrue(span.isEmpty)
@@ -1761,7 +1761,7 @@ class DataTests : XCTestCase {
     func test_InlineSliceDataMutableRawSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
-#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
+#if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data(0..<100)
         let count = source.count
         var span = source.mutableBytes
@@ -1783,7 +1783,7 @@ class DataTests : XCTestCase {
         #error("This test needs updating")
 #endif
 
-#if $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
+#if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data(repeating: 0, count: count).dropFirst()
         XCTAssertNotEqual(source.startIndex, 0)
         count = source.count

From 9ed87ab8f6c4f1aa91b708c7beb4c6f05f1ced0d Mon Sep 17 00:00:00 2001
From: Guillaume Lessard <guillaume.lessard@apple.com>
Date: Mon, 2 Jun 2025 15:32:50 -0700
Subject: [PATCH 10/10] compile out MutableSpan mutations when unsupported by
 deployment target

---
 Tests/FoundationEssentialsTests/DataTests.swift | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/Tests/FoundationEssentialsTests/DataTests.swift b/Tests/FoundationEssentialsTests/DataTests.swift
index cb4f308b0..b076b0677 100644
--- a/Tests/FoundationEssentialsTests/DataTests.swift
+++ b/Tests/FoundationEssentialsTests/DataTests.swift
@@ -1683,6 +1683,7 @@ class DataTests : XCTestCase {
     func test_InlineDataMutableSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
+#if !canImport(Darwin) || FOUNDATION_FRAMEWORK
 #if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data()
         var span = source.mutableSpan
@@ -1696,21 +1697,27 @@ class DataTests : XCTestCase {
         XCTAssertEqual(span.count, count)
         let v = UInt8.random(in: 10..<100)
         span[i] = v
+        var sub = span.extracting(i ..< i+1)
+        sub.update(repeating: v)
         XCTAssertEqual(source[i], v)
+#endif
 #endif
     }
 
     func test_InlineSliceDataMutableSpan() throws {
         guard #available(FoundationSpan 6.2, *) else { throw XCTSkip("Span not available") }
 
+#if !canImport(Darwin) || FOUNDATION_FRAMEWORK
 #if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data(0..<100)
         let count = source.count
         var span = source.mutableSpan
         XCTAssertEqual(span.count, count)
         let i = try XCTUnwrap(span.indices.randomElement())
-        span[i] = .max
+        var sub = span.extracting(i..<i+1)
+        sub.update(repeating: .max)
         XCTAssertEqual(source[i], .max)
+#endif
 #endif
     }
 
@@ -1725,6 +1732,7 @@ class DataTests : XCTestCase {
         #error("This test needs updating")
 #endif
 
+#if !canImport(Darwin) || FOUNDATION_FRAMEWORK
 #if compiler(>=6.2) && $InoutLifetimeDependence && $LifetimeDependenceMutableAccessors
         var source = Data(repeating: 0, count: count).dropFirst()
         XCTAssertNotEqual(source.startIndex, 0)
@@ -1735,6 +1743,7 @@ class DataTests : XCTestCase {
         span[i] = .max
         XCTAssertEqual(source[i], 0)
         XCTAssertEqual(source[i+1], .max)
+#endif
 #endif
     }
 
@@ -1753,7 +1762,8 @@ class DataTests : XCTestCase {
         XCTAssertFalse(span.isEmpty)
         XCTAssertEqual(span.byteCount, count)
         let v = UInt8.random(in: 10..<100)
-        span.storeBytes(of: v, toByteOffset: i, as: UInt8.self)
+        var sub = span.extracting(i..<i+1)
+        sub.storeBytes(of: v, as: UInt8.self)
         XCTAssertEqual(source[i], v)
 #endif
     }