stdlib: Build libswiftCompatibilitySpan.dylib compatibility shim

This dynamic library contains a copy of the standard library's
exported entry points for the Span and RawSpan types. This will
allow backward deployment of code that uses those new types.

Author: slavapestov

stdlib/public/core/Span/RawSpan.swift

@@ -10,6 +10,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if SPAN_COMPATIBILITY_STUB
+import Swift
+#endif
+
 /// `RawSpan` represents a contiguous region of memory
 /// which contains initialized bytes.
 ///

stdlib/public/core/Span/Span.swift

@@ -10,6 +10,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if SPAN_COMPATIBILITY_STUB
+import Swift
+#endif
+
 /// `Span<Element>` represents a contiguous region of memory
 /// which contains initialized instances of `Element`.
 ///

stdlib/toolchain/CMakeLists.txt

@@ -50,6 +50,7 @@ if(SWIFT_STDLIB_SUPPORT_BACK_DEPLOYMENT)
   add_subdirectory(Compatibility50)
   add_subdirectory(Compatibility51)
   add_subdirectory(Compatibility56)
+  add_subdirectory(CompatibilitySpan)
   add_subdirectory(CompatibilityDynamicReplacements)
   add_subdirectory(CompatibilityConcurrency)
   add_subdirectory(CompatibilityThreading)

stdlib/toolchain/CompatibilitySpan/CMakeLists.txt

@@ -0,0 +1,34 @@
+if(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
+
+  set(library_name "swiftCompatibilitySpan")
+
+  add_swift_target_library("${library_name}" ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB
+    FakeStdlib.swift
+    ../../public/core/Span/RawSpan.swift
+    ../../public/core/Span/Span.swift
+
+    TARGET_SDKS ${SWIFT_APPLE_PLATFORMS}
+
+    LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
+
+    SWIFT_COMPILE_FLAGS
+      ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
+      -parse-stdlib
+      -module-abi-name Swift
+      -DSPAN_COMPATIBILITY_STUB
+
+    DEPLOYMENT_VERSION_OSX ${COMPATIBILITY_MINIMUM_DEPLOYMENT_VERSION_OSX}
+    DEPLOYMENT_VERSION_IOS ${COMPATIBILITY_MINIMUM_DEPLOYMENT_VERSION_IOS}
+    DEPLOYMENT_VERSION_TVOS ${COMPATIBILITY_MINIMUM_DEPLOYMENT_VERSION_TVOS}
+    DEPLOYMENT_VERSION_WATCHOS ${COMPATIBILITY_MINIMUM_DEPLOYMENT_VERSION_WATCHOS}
+    DEPLOYMENT_VERSION_XROS ${COMPATIBILITY_MINIMUM_DEPLOYMENT_VERSION_XROS}
+    DEPLOYMENT_VERSION_MACCATALYST ${COMPATIBILITY_MINIMUM_DEPLOYMENT_VERSION_MACCATALYST}
+
+    MACCATALYST_BUILD_FLAVOR "zippered"
+
+    INSTALL_IN_COMPONENT compiler
+
+    LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}" -Xlinker -not_for_dyld_shared_cache
+
+    BACK_DEPLOYMENT_LIBRARY 6.2)
+endif()

stdlib/toolchain/CompatibilitySpan/FakeStdlib.swift

@@ -0,0 +1,79 @@
+//===--- FakeStdlib.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
+//
+//===----------------------------------------------------------------------===//
+//
+// A handful of standard library stubs to allow Span.swift and RawSpan.swift
+// to be compiled as part of the compatibility shim.
+//
+//===----------------------------------------------------------------------===//
+
+import Swift
+
+@_alwaysEmitIntoClient @_transparent
+internal func _precondition(
+  _ condition: @autoclosure () -> Bool, _ message: StaticString = StaticString(),
+  file: StaticString = #file, line: UInt = #line
+) {
+	fatalError()
+}
+
+@_alwaysEmitIntoClient @_transparent
+internal func _internalInvariantFailure(
+  _ message: StaticString = StaticString(),
+  file: StaticString = #file, line: UInt = #line
+) -> Never {
+	fatalError()
+}
+
+@unsafe
+@_unsafeNonescapableResult
+@_alwaysEmitIntoClient
+@_transparent
+@lifetime(borrow source)
+internal func _overrideLifetime<
+  T: ~Copyable & ~Escapable, U: ~Copyable & ~Escapable
+>(
+  _ dependent: consuming T, borrowing source: borrowing U
+) -> T {
+  dependent
+}
+
+@unsafe
+@_unsafeNonescapableResult
+@_alwaysEmitIntoClient
+@_transparent
+@lifetime(copy source)
+internal func _overrideLifetime<
+  T: ~Copyable & ~Escapable, U: ~Copyable & ~Escapable
+>(
+  _ dependent: consuming T, copying source: borrowing U
+) -> T {
+  dependent
+}
+
+extension Range {
+    @_alwaysEmitIntoClient
+	internal init(_uncheckedBounds bounds: (lower: Bound, upper: Bound)) {
+	    self.init(uncheckedBounds: bounds)
+	}
+}
+
+extension Optional {
+  @_alwaysEmitIntoClient
+  internal var _unsafelyUnwrappedUnchecked: Wrapped {
+    get {
+      if let x = self {
+        return x
+      }
+      _internalInvariantFailure("_unsafelyUnwrappedUnchecked of nil optional")
+    }
+  }
+}