UnderscoredAttributes.md

# Underscored Attributes Reference

**WARNING:** This information is provided primarily for compiler and standard
library developers. Usage of these attributes outside of the Swift monorepo
is STRONGLY DISCOURAGED.

The Swift reference has a chapter discussing [stable attributes][Attributes].
This document is intended to serve as a counterpart describing underscored
attributes, whose semantics are subject to change and most likely need to
go through the Swift evolution process before being stabilized.

[Attributes]: https://docs.swift.org/swift-book/ReferenceManual/Attributes.html

The attributes are organized in alphabetical order.

## `@_alignment(numericValue)`

Allows controlling the alignment of a type.

The alignment value specified must be a power of two, and cannot be less
than the "natural" alignment of the type that would otherwise be used by
the Swift ABI. This attribute is intended for the SIMD types in the standard
library which use it to increase the alignment of their internal storage to at
least 16 bytes.

## `@_alwaysEmitConformanceMetadata`

Forces conformances of the attributed protocol to always have their Type Metadata get emitted into the binary and prevents it from being optimized away or stripped by the linker.

## `@_alwaysEmitIntoClient`

Forces the body of a function to be emitted into client code.

Note that this is distinct from `@inline(__always)`; it doesn't force inlining
at call-sites, it only means that the implementation is compiled into the
module which uses the code.

This means that `@_alwaysEmitIntoClient` definitions are _not_ part of the
defining module's ABI, so changing the implementation at a later stage
does not break ABI.

Most notably, default argument expressions are implicitly
`@_alwaysEmitIntoClient`, which means that adding a default argument to a
function which did not have one previously does not break ABI.

## `@_assemblyVision`

Forces emission of assembly vision remarks for a function or method, showing
where various runtime calls and performance impacting hazards are in the code
at source level after optimization.

Adding this attribute to a type leads to remarks being emitted for all methods.

## `@_backDeploy(before: ...)`

The spelling of `@backDeployed(before:)` prior to the acceptance of [SE-0376](https://github.com/apple/swift-evolution/blob/main/proposals/0376-function-back-deployment.md).

## `@_borrowed`

Indicates that the [conservative access pattern](/docs/Lexicon.md#access-pattern)
for some storage (a subscript or a property) should use the `_read` accessor
instead of `get`.

For more details, see the forum post on
[Value ownership when reading from a storage declaration](https://forums.swift.org/t/value-ownership-when-reading-from-a-storage-declaration/15076).

## `@_cdecl("cName")`

Similar to `@_silgen_name` but uses the C calling convention.

This attribute doesn't have very well-defined semantics. Type bridging is not
done, so the parameter and return types should correspond directly to types
accessible in C. In most cases, it is preferable to define a static method
on an `@objc` class instead of using `@_cdecl`.

For potential ideas on stabilization, see
[Formalizing `@cdecl`](https://forums.swift.org/t/formalizing-cdecl/40677).

## `@_disfavoredOverload`

Marks an overload that the type checker should try to avoid using. When the
expression type checker is considering overloads, it will prefer a solution
with fewer `@_disfavoredOverload` declarations over one with more of them.

Use `@_disfavoredOverload` to work around known bugs in the overload
resolution rules that cannot be immediately fixed without a source break.
Don't use it to adjust overload resolution rules that are otherwise sensible
but happen to produce undesirable results for your particular API; it will
likely be removed or made into a no-op eventually, and then you will be
stuck with an overload set that cannot be made to function in the way
you intend.

`@_disfavoredOverload` was first introduced to work around a bug in overload
resolution with `ExpressibleByXYZLiteral` types. The type checker strongly
prefers to give literals their default type (e.g. `Int` for
`ExpressibleByIntegerLiteral`, `String` for `ExpressibleByStringLiteral`,
etc.). If an API should prefer some other type, but accept the default too,
 marking the declaration taking the default type with `@_disfavoredOverload`
 gives the desired behavior:

```swift
extension LocalizedStringKey: ExpressibleByStringLiteral { ... }

extension Text {
  // We want `Text("foo")` to use this initializer:
  init(_ key: LocalizedStringKey) { ... }

  // But without @_disfavoredOverload, it would use this one instead,
  // because that lets it give the literal its default type:
  @_disfavoredOverload init<S: StringProtocol>(_ str: S) { ... }
}
```

## `@_documentation(metadata: ...)`

Adds "documentation metadata" to the symbol. The identifier in the attribute is
added to the symbol graph in the `"metadata"` field of the symbol. This can be
used to add an arbitrary grouping or other indicator to symbols for use in
documentation.

## `@_documentation(visibility: ...)`

Forces the symbol to be treated as the given access level when checking
visibility. This can be used to, for example, force a symbol with an underscored
name to appear in `public` symbol graphs, or treat an otherwise-`public` symbol
as being `internal` or `private` for the purposes of documentation, to hide it
from `public` docs.

This can also be applied to `@_exported import` statements to only include the
imported symbols in symbol graphs with the given minimum access level. For
example, applying `@_documentation(visibility: internal)` to an `@_exported
import` statement will hide the imported symbols from `public` symbol graphs and
documentation, but show them on `internal` symbol graphs and documentation.

## `@_dynamicReplacement(for: targetFunc(label:))`

Marks a function as the dynamic replacement for another `dynamic` function.
This is similar to method swizzling in other languages such as Objective-C,
except that the replacement happens at program start (or loading a shared
library), instead of at an arbitrary point in time.

For more details, see the forum post on
[dynamic method replacement](https://forums.swift.org/t/dynamic-method-replacement/16619).

## `@_eagerMove`

When applied to a value, indicates that the value's lifetime is _not_ lexical,
that releases of the value may be hoisted without respect to deinit barriers.

When applied to a type, indicates that all values which are _statically_
instances of that type are themselves `@_eagerMove` as above, unless overridden
with `@_noEagerMove`.

Aggregates all of whose fields are `@_eagerMove` or trivial are inferred to be
`@_eagerMove`.

Note that a value of an `@_eagerMove` type that is passed to a generic API (to a
parameter not annotated `@_eagerMove` will, in that generic function's context,
not be statically an instance of the `@_eagerMove` type.  As a result it will
have a lexical lifetime in that function.

## `@_effects(effectname)`

Tells the compiler that the implementation of the defined function is limited
to certain side effects. The attribute argument specifies the kind of side
effect limitations that apply to the function including any other functions
it calls. This is used to provide information to the optimizer that it can't
already infer from static analysis.

Changing the implementation in a way that violates the optimizer's assumptions
about the effects results in undefined behavior.

### `@_effects(readnone)`

Defines that the function does not have any observable memory reads or writes
or any other observable side effects.

This does not mean that the function cannot read or write memory at all.
For example, it’s allowed to allocate and write to local objects inside the
function. For example, the following `readnone` function allocates an array and
writes to the array buffer

```swift
@_effects(readnone)
func lookup(_ i: Int) -> Int {
  let a = [7, 3 ,6, 9]
  return a[i]
}
```

A function can be marked as readnone if two calls of the same function with the
same parameters can be simplified to one call (e.g. by the CSE optimization)
without changing the semantics of the program.
For example,

```swift
  let a = lookup(i)
  // some other code, including memory writes
  let b = lookup(i)
```
is equivalent to

```swift
  let a = lookup(i)
  // some other code, including memory writes
  let b = a
```

Some conclusions:

* A `readnone` function must not return a newly allocated class instance.

* A `readnone` function can return a newly allocated copy-on-write object, like
  an Array, because COW data types conceptually behave like value types.

* A `readnone` function must not release any parameter or any object indirectly
  referenced from a parameter.

* Any kind of observable side-effects are not allowed, like `print`, file IO, etc.

### `@_effects(readonly)`

Defines that the function does not have any observable memory writes or any
other observable side effects, beside reading of memory.

Similar to `readnone`, a `readonly` function is allowed to write to local objects.

A function can be marked as `readonly` if it’s save to eliminate a call to such
a function in case its return value is not used.
Example:

```swift
@_effects(readonly)
func lookup2(_ instance: SomeClass) -> Int {
  let a = [7, 3 ,6, 9]
  return a[instance.i]
}
```

It is legal to eliminate an unused call to this function:

```
_ = lookup2(i)  // can be completely eliminated
```

Note that it would not be legal to CSE two calls to this function, because
between those calls the member `i` of the class instance could be modified:

```swift
  let a = lookup2(instance)
  instance.i += 1
  let b = lookup2(instance)   // cannot be CSE'd with the first call
```

The same conclusions as for `readnone` also apply to `readonly`.

### `@_effects(releasenone)`

Defines that the function does not release any class instance.

This effect must be used with care.
There are several code patterns which release objects in a non-obvious way.
For example:

* A parameter which is passed to an “owned” argument (and not stored), like
  initializer arguments.

* Assignments, because they release the old value

* COW data types, e.g. Strings. Conceptually they are value types, but
  internally the keep a reference counted buffer.

* Class references deep inside a hierarchy of value types.

### `@_effects(readwrite)`

This effect is not used by the compiler.

### `@_effects(notEscaping <selection>)`

Tells the compiler that a function argument does not escape.
The _selection_ specifies which argument or which "projection" of an argument
does not escape. The _selection_ consists of the argument name or `self` and
an optional projection path.

The projection path consists of field names or one of the following wildcards:

* `class*`: selects any class field, including tail allocated elements
* `value**`: selects any number and any kind of struct, tuple or enum
             fields/payload-cases.
* `**`: selects any number of any fields

For example:

```swift
struct Inner {
  let i: Class
}
struct Str {
	let a: Inner
	let b: Class
}

@_effects(notEscaping s.b)    // s.b does not escape, but s.a.i can escape
func foo1(_ s: Str) { ... }

@_effects(notEscaping s.v**)  // s.b and s.a.i do not escape
func foo2(_ s: Str) { ... }

@_effects(notEscaping s.**)   // s.b, s.a.i and all transitively reachable
                              // references from there do not escape
func foo3(_ s: Str) { ... }
```

### `@_effects(escaping <from-selection> => <to-selection>)`

Defines that an argument escapes to another argument or to the return value,
but not otherwise.
The _to-selection_ can also refer to `return`.

For example:

```swift
@_effects(escapes s.b => return)
func foo1(_ s: Str) -> Class {
  return s.b
}

@_effects(escapes s.b => o.a.i)
func foo2(_ s: Str, o: inout Str) {
  o.a.i = s.b
}
```

### `@_effects(escaping <from-selection> -> <to-selection>)`

This variant of an escaping effect defines a "non-exclusive" escape. This means
that not only the _from-selection_, but also other values
can escape to the _to-selection_.

For example:

```swift
var g: Class

@_effects(escapes s.b -> return)
func foo1(_ s: Str, _ cond: Bool) -> Class {
  return cond ? s.b : g
}
```

## `@_exported`

Re-exports all declarations from an imported module.

This attribute is most commonly used by overlays.

```swift
// module M
public func f() {}

// module N
@_exported import M

// module P
import N
func g() {
  N.f() // OK
}
```

## `@_expose(<language>)`

Indicates that a particular declaration should be included
in the emitted specified foreign language binding.

When applied to a nominal type declaration, all of the
public declarations inside this type become exposed as well.

### `_expose(Cxx[, <"cxxName">])`

Indicates that a particular declaration should be included
in the generated C++ binding header.

The optional "cxxName" string will be used as the name of
the generated C++ declaration.

## `@_fixed_layout`

Same as `@frozen` but also works for classes.

With `@_fixed_layout` classes, vtable layout still happens dynamically, so
non-public virtual methods can be removed, new virtual methods can be added,
and existing virtual methods can be reordered.

## `@_hasInitialValue`

Marks that a property has an initializing expression.

This information is lost in the swiftinterface,
but it is required as it results in a symbol for the initializer
(if a class/struct `init` is inlined, it will call initializers
for properties that it doesn't initialize itself).
This information is necessary for correct TBD file generation.

## `@_hasMissingDesignatedInitializers`

Indicates that there may be designated initializers that are
not printed in the swiftinterface file for a particular class.

This attribute is needed for the initializer model to maintain correctness when
[library evolution](/docs/LibraryEvolution.rst) is enabled. This is because a
class may have non-public designated initializers, and Swift allows the
inheritance of convenience initializers if and only if the subclass overrides
(or has synthesized overrides) of every designated initializer in its
superclass. Consider the following code:

```swift
// Lib.swift
open class A {
  init(invisible: ()) {}

  public init(visible: ()) {}
  public convenience init(hi: ()) { self.init(invisible: ()) }
}

// Client.swift
class B : A {
  var x: String

  public override init(visible: ()) {
    self.x = "Garbage"
    super.init(visible: ())
  }
}
```

In this case, if `B` were allowed to inherit the convenience initializer
`A.init(invisible:)` then an instance created via `B(hi: ())` would fail
to initialize `B.x` resulting in a memory safety hole. What's worse is
there is no way to close this safety hole because the user cannot override
the invisible designated initializer because they lack sufficient visibility.

## `@_hasStorage`

Marks a property as being a stored property in a swiftinterface.

For `@frozen` types, the compiler needs to be able to tell whether a particular
property is stored or computed to correctly perform type layout.

```swift
@frozen struct S {
  @_hasStorage var x: Int { get set } // stored
  var y: Int { get set } // computed
}
```

## `@_implementationOnly`

Used to mark an imported module as an implementation detail.
This prevents types from that module being exposed in API
(types of public functions, constraints in public extension etc.)
and ABI (usage in `@inlinable` code).

## `@_spiOnly`

Marks an import to be used in SPI and implementation details only.
The import statement will be printed in the private swiftinterface only and
skipped in the public swiftinterface. Any use of imported types and decls in API
will be diagnosed.
Requires setting the frontend flag `-experimental-spi-only-imports`.

## `@_implements(ProtocolName, Requirement)`

An attribute that indicates that a function with one name satisfies
a protocol requirement with a different name. This is especially useful
when two protocols declare a requirement with the same name, but the
conforming type wishes to offer two separate implementations.

```swift
protocol P { func foo() }

protocol Q { func foo() }

struct S : P, Q {
  @_implements(P, foo())
  func foo_p() {}
  @_implements(Q, foo())
  func foo_q() {}
}
```

## `@_implicitSelfCapture`

Allows access to `self` inside a closure without explicitly capturing it,
even when `Self` is a reference type.

```swift
class C {
  func f() {}
  func g(_: @escaping () -> Void) {
    g({ f() }) // error: call to method 'f' in closure requires explicit use of 'self'
  }
  func h(@_implicitSelfCapture _: @escaping () -> Void) {
    h({ f() }) // ok
  }
}
```

## `@_inheritActorContext`

(Note that it is "inherit", not "inherits", unlike below.)

Marks that a `@Sendable async` closure argument should inherit the actor
context (i.e. what actor it should be run on) based on the declaration site
of the closure. This is different from the typical behavior, where the closure
may be runnable anywhere unless its type specifically declares that it will
run on a specific actor.

## `@_inheritsConvenienceInitializers`

An attribute that signals that a class declaration inherits its convenience
initializers from its superclass. This implies that all designated initializers
-- even those that may not be visible in a swiftinterface file -- are
overridden. This attribute is often printed alongside
`@_hasMissingDesignatedInitializers` in this case.

## `@inline(__always)`

Forces the function to be inlined.

If it's not possible to always inline the function, e.g. if it's a self-
recursive function, the attribute is ignored.

This attribute has no effect in debug builds.

## `@_lexicalLifetimes`

Applies lexical lifetime rules within a module built with lexical lifetimes
disabled.  Facilitates gradual migration.

In modules built with lexical lifetimes disabled but lexical borrow scopes
enabled--the behavior of `-enable-lexical-lifetimes=false`--all lexical markers
are stripped by the LexicalLifetimeEliminator pass.  Functions annotated with
this attribute keep their lexical markers, affecting the optimizations that run
on the function subsequently.

## `@_noEagerMove`

When applied to a value, indicates that the value's lifetime is lexical, that
releases of the value may not be hoisted over deinit barriers.  

This is the default behavior, unless the value's type is annotated
`@_eagerMove`, in which case this attribute overrides that type-level
annotation.

When applied to a type, indicates that all values which are instances of that
type are themselves `@_noEagerMove` as above.

This is the default behavior, unless the type annotated is an aggregate that
consists entirely of `@_eagerMove` or trivial values, in which case the
attribute overrides the inferred type-level annotation.

## `@_marker`

Indicates that a protocol is a marker protocol. Marker protocols represent some
meaningful property at compile-time but have no runtime representation.

For more details, see [](https://github.com/apple/swift-evolution/blob/main/proposals/0302-concurrent-value-and-concurrent-closures.md#marker-protocols), which introduces marker protocols.
At the moment, the language only has one marker protocol: `Sendable`.

Fun fact: Rust has a very similar concept called
[marker traits](https://doc.rust-lang.org/std/marker/index.html),
including one called `Send`,
which inspired the design of `Sendable`.

## `@_noAllocation`, `@_noLocks`

These attributes are performance annotations. If a function is annotated with
such an attribute, the compiler issues a diagnostic message if the function
calls a runtime function which allocates memory or locks, respectively.
The `@_noLocks` attribute implies `@_noAllocation` because a memory allocation
also locks.

## `@_noImplicitCopy`

Marks a var decl as a variable that must be copied explicitly using the builtin
function Builtin.copy.

## `@_nonEphemeral`

Marks a function parameter that cannot accept a temporary pointer produced from
an inout-to-pointer, array-to-pointer, or string-to-pointer conversion. Such a
parameter may only accept a pointer that is guaranteed to outlive the duration
of the function call.

Attempting to pass a temporary pointer to an `@_nonEphemeral` parameter will
produce a warning. This attribute is primarily used within the standard library
on the various `UnsafePointer` initializers to warn users about
the undefined behavior caused by using a temporary pointer conversion as an
argument:

```swift
func baz() {
  var x = 0

  // warning: Initialization of 'UnsafePointer<Int>' results in a dangling pointer
  let ptr = UnsafePointer(&x)

  // warning: Initialization of 'UnsafePointer<Int>' results in a dangling pointer
  let ptr2 = UnsafePointer([1, 2, 3])
}
```

The temporary pointer conversion produces a pointer that is only
guaranteed to be valid for the duration of the call to the initializer,
and becomes invalid once the call ends.
So the newly created `UnsafePointer` will be dangling.

One exception to this is that inout-to-pointer conversions
on static stored properties and global stored properties
produce non-ephemeral pointers, as long as they have no observers:

```swift
var global = 0

struct S {
  static var staticVar = 0
}

func baz() {
  let ptr = UnsafePointer(&global) // okay
  let ptr2 = UnsafePointer(&S.staticVar) // okay
}
```

Additionally, if they are of a tuple or struct type, their stored members
without observers may also be passed inout as non-ephemeral pointers.

For more details, see the educational note on
[temporary pointer usage](/userdocs/diagnostics/temporary-pointers.md).

## `@_nonoverride`

Marks a declaration that is not an override of another.
When the `-warn-implicit-overrides` flag is used, a warning is issued when a
protocol restates a requirement from another protocol it refines without
annotating the declaration with either `override` or `@_nonoverride`.

An `override` annotation causes the overriding declaration to be treated
identically to the overridden declaration; a conforming type can only
provide one implementation ("witness"). Restating a protocol requirement
and then marking it as an `override` is generally only needed to help
associated type inference, and many `override` annotations correlate
closely with ABI FIXMEs.

Meanwhile, `@_nonoverride` is the "opposite" of `override`, allowing two
protocol requirements to be treated independently; a conforming type can
provide a distinct witness for each requirement (for example, by using
`@_implements`). Use `@_nonoverride` when semantics differ between the
two requirements. For example, `BidirectionalCollection.index(_:offsetBy:)`
allows negative offsets, while `Collection.index(_:offsetBy:)` does not,
and therefore the former is marked `@_nonoverride`.

The `@_nonoverride` annotation can also be specified on class members in
addition to protocol members. Since it is the "opposite" of `override`, it can
be used to suppress "near-miss" diagnostics for declarations that are similar
to but not meant to override another declaration, and it can be used to
intentionally break the override chain, creating an overload instead of an
override.

This attribute and the corresponding `-warn-implicit-overrides` flag are
used when compiling the standard library and overlays.

## `@_nonSendable`

There is no clang attribute to add a Swift conformance to an imported type, but
there *is* a clang attribute to add a Swift attribute to an imported type. So
`@Sendable` (which is not normally allowed on types) is used from clang headers
to indicate that an unconstrained, fully available `Sendable` conformance should
be added to a given type, while `@_nonSendable` indicates that an unavailable
`Sendable` conformance should be added to it.

`@_nonSendable` can have no options after it, in which case it "beats"
`@Sendable` if both are applied to the same declaration, or it can have
`(_assumed)` after it, in which case `@Sendable` "beats" it.
`@_nonSendable(_assumed)` is intended to be used when mass-marking whole regions
of a header as non-`Sendable` so that you can make spot exceptions with
`@Sendable`.

## `@_objcImplementation(CategoryName)`

Declares an extension that defines an implementation for the Objective-C
category `CategoryName` on the class in question, or for the main `@interface`
if the argument list is omitted.

This attribute is used to write fully Objective-C-compatible implementations in
Swift. Normal Objective-C interop allows Objective-C clients to use instances of
the subclass, but not to subclass them, and uses a generated header that is not
meant to be read by humans. `@_objcImplementation`, on the other hand, creates
classes that are virtually indistinguishable from classes implemented in native 
Objective-C: they do not have a Swift vtable or any other Swift-specific
metadata, Swift does not use any special knowledge of the class's "Swiftiness" 
when using the class so ObjC runtime calls work correctly and they can even be 
subclassed by Objective-C code, and you write a header for the class by hand 
that looks exactly like an equivalent ObjC class. Clients should not notice if 
you replace a native Objective-C `@implementation Foo (Bar)` with a Swift 
`@_objcImplementation(Bar) extension Foo`.

You create a class with this feature very differently from normal ObjC interop:

1. Hand-write headers that declare the class's Objective-C interface, just as
   you would for a native Objective-C class. Since you're handwriting these
   headers, you can write them just as you would for an Objective-C class:
   splitting them across multiple files, grouping related declarations together,
   adding comments, declaring Swift behavior using C attributes or API notes,
   etc.
   
2. Import your headers into Swift using a bridging header or umbrella header so
   Swift can see them.

3. Implement your class using a mixture of `@implementation` declarations in
   `.m` files and `@_objcImplementation extension`s in `.swift` files. Each
   `@interface` should have exactly one corresponding implementation; don't try
   to implement some members of a single `@interface` in ObjC and others in
   Swift.

   * To implement the main `@interface` of a class in Swift, use
     `@_objcImplementation extension ClassName`.
     
   * To implement a category in Swift, use
     `@_objcImplementation(CategoryName) extension ClassName`.
     
The members of an `@_objcImplementation` extension should fall into one of
three categories:
   
* **Swift-only members** include any member marked `final`. These are not
  `@objc` or `dynamic` and are only callable from Swift. Use these for
  Swift-only APIs, random helper methods, etc. 
    
* **ObjC helper members** include any non-`final` member marked `fileprivate`
  or `private`. These are implicitly `@objc dynamic`. Use these for action
  methods, selector-based callbacks, and other situations where you need a
  helper method to be accessible from an Objective-C message.
  
* **Member implementations** include any other non-`final` member. These are
  implicitly `@objc dynamic` and must match a member declared in the
  Objective-C header. Use these to implement the APIs declared in your
  headers. Swift will emit an error if these don't match your headers.

Notes:

* We don't currently plan to support ObjC generics.

* Eventually, we want the main `@_objcImplementation` extension to be able to
  declare stored properties that aren't in the interface. We also want
  `final` stored properties to be allowed to be resilent Swift types, but
  it's not clear how to achieve that without boxing them in `__SwiftValue`
  (which we might do as a stopgap).
     
* We should think about ObjC "direct" members, but that would probably
  require a way to spell this in Swift. 

## `@_objc_non_lazy_realization`

Marks a class as being non-lazily (i.e. eagerly) [realized](/docs/Lexicon.md#realization).

This is used for declarations which may be statically referenced and wouldn't
go through the normal lazy realization paths. For example, the empty array
class must be non-lazily realized, because empty arrays are statically
allocated. Otherwise, passing the empty array object to other code without
triggering realization could allow for the unrealized empty array class to be
passed to ObjC runtime APIs which only operate on realized classes, resulting
in a crash.

## `@_optimize([none|size|speed])`

Controls the compiler's optimization mode. This attribute is analogous to the
command-line flags `-Onone`, `-Osize` and `-Ospeed` respectively, but limited
to a single function body.

`@_optimize(none)` is handy for diagnosing and reducing compiler bugs as well
as improving debugging in Release builds.

## `@_originallyDefinedIn(module: "ModuleName", availabilitySpec...)`

Marks a declaration as being originally defined in a different module,
changing the name mangling. This can be used to move declarations
from a module to one of the modules it imports without breaking clients.

Consider the following example where a framework ToasterKit needs
to move some APIs to a lower-level framework ToasterKitCore.
Here are the necessary changes:

1. Add a linker flag `-reexport_framework ToasterKitCore` for ToasterKit.
   This ensures all symbols defined in ToasterKitCore will be accessible during
   runtime via ToasterKit, so existing apps continue to run.
2. In ToasterKit, use `@_exported import ToasterKitCore`.
   This ensures existing source code that only imports ToasterKit continues to
   type-check.
3. Move the necessary declarations from ToasterKit to ToasterKitCore.
   The moved declaration should have two attributes:
   - `@available` indicating when the declaration was introduced in ToasterKit.
   - `@_originallyDefinedIn` indicating the original module and when the
     declaration was moved to ToasterKitCore.
   ```swift
   @available(toasterOS 42, *)
   @_originallyDefinedIn(module: "ToasterKit", toasterOS 57)
   enum Toast {
   case underdone
   case perfect
   case burnt
   }
   ```
4. Add Swift compiler flags `-Xfrontend -emit-ldadd-cfile-path -Xfrontend /tmp/t.c`
   to ToasterKitCore's build settings. Add the emitted `/tmp/t.c` file to
   ToasterKit's compilation.
   This ensures when an app is built for deployment targets prior to the symbols' move,
   the app will look for these symbols in ToasterKit instead of ToasterKitCore.

More generally, multiple availabilities can be specified, like so:

```swift
@available(toasterOS 42, bowlOS 54, mugOS 54, *)
@_originallyDefinedIn(module: "ToasterKit", toasterOS 57, bowlOS 69, mugOS 69)
enum Toast { ... }
```

## `@_private(sourceFile: "FileName.swift")`

Fully bypasses access control, allowing access to private declarations
in the imported module. The imported module needs to be compiled with
`-Xfrontend -enable-private-imports` for this to work.

## `@_semantics("uniquely.recognized.id")`

Allows the optimizer to make use of some key invariants in performance critical
data types, especially `Array`. Since the implementation of these data types
is written in Swift using unsafe APIs, without these attributes the optimizer
would need to make conservative assumptions.

Changing the implementation in a way that violates the optimizer's assumptions
about the semantics results in undefined behavior.

## `@_show_in_interface`

Shows underscored protocols from the standard library in the generated interface.

By default, SourceKit hides underscored protocols from the generated
swiftinterface (for all modules, not just the standard library), but this
attribute can be used to override that behavior for the standard library.

## `@_silgen_name("cName")`

Changes the symbol name for a function, similar to an ASM label in C,
except that the platform symbol mangling (leading underscore on Darwin)
is maintained.

Since this has label-like behavior, it may not correspond to any declaration;
if so, it is assumed that the function is implemented in C.

A function defined by `@_silgen_name` is assumed to use the Swift ABI.

For more details, see the
[Standard Library Programmer's Manual](https://github.com/apple/swift/blob/main/docs/StandardLibraryProgrammersManual.md#_silgen_name).

## `@_specialize(...)`

Forces generation of a specialized implementation for a generic declaration.

See [Generics.rst](/docs/archive/Generics.rst) for more details.

## `@_specializeExtension`

Allows extending `@usableFromInline` internal types from foreign modules.
Consider the following example involving two modules:

```swift
// Module A
@usableFromInline
internal struct S<T> { /* ... */ }

// Module B
import A

@_specializeExtension
extension S { // OK
  // add methods here
}

extension S /* or A.S */ { // error: cannot find 'S' in scope
}
```

This ability can be used to add specializations of existing methods
in downstream libraries when used in conjunction with `@_specialize`.

```swift
// Module A
@usableFromInline
internal struct S<T> {
  @inlinable
  internal func doIt() { /* body */ }
}

// Module B
import A

@_specializeExtension
extension S { // ok
  @_specialize(exported: true, target: doIt(), where T == Int)
  public func specializedDoIt() {}
}

// Module C
import A
import B

func f(_ s: S<Int>) {
  s.doIt() // will call specialized version of doIt() where T == Int from B
}
```

## `@_spi(spiName)`

Marks a declaration as SPI (System Programming Interface), instead of API.
Modules exposing SPI and using library evolution generate an additional
`.private.swiftinterface` file (with `-emit-private-module-interface-path`)
in addition to the usual `.swiftinterface` file. This private interface exposes
both API and SPI.

Clients can access SPI by marking the import as `@_spi(spiName) import Module`.
This design makes it easy to find out which clients are using certain SPIs by
doing a textual search.

## `@_spi_available(platform, version)`

Like `@available`, this attribute indicates a decl is available only as an SPI.
This implies several behavioral changes comparing to regular `@available`:
1. Type checker diagnoses when a client accidently exposes such a symbol in library APIs.
2. When emitting public interfaces, `@_spi_available` is printed as `@available(platform, unavailable)`.
3. ClangImporter imports ObjC macros `SPI_AVAILABLE` and `__SPI_AVAILABLE` to this attribute.

## `@_staticInitializeObjCMetadata`

Indicates that a static initializer should be emitted to register the
Objective-C metadata when the image is loaded, rather than on first use of the
Objective-C metadata.

This attribute is inferred for `NSCoding` classes that won't
have static Objective-C metadata or have an `@NSKeyedArchiveLegacy` attribute.

## `@_transparent`

Marks a function to be "macro-like", i.e., it is guaranteed to be inlined
in debug builds.

See [TransparentAttr.md](/docs/TransparentAttr.md) for more details.

## `@_typeEraser(Proto)`

Marks a concrete nominal type as one that implements type erasure for a
protocol `Proto`.

A type eraser has the following restrictions:

1. It must be a concrete nominal type.
2. It must not have more restrictive access than `Proto`.
3. It must conform to `Proto`.
4. It must have an initializer of the form `init<T: Proto>(erasing: T)`.
  - Other generic requirements are permitted as long as the `init` can always
    be called with a value of any type conforming to `Proto`.
  - The `init` cannot have more restrictive access than `Proto`.

This feature was designed to be used for compiler-driven type erasure for
dynamic replacement of functions with an opaque return type.

## `@_unavailableFromAsync`

Marks a synchronous API as being unavailable from asynchronous contexts. Direct
usage of annotated API from asynchronous contexts will result in a warning from
the compiler.

## `@_unsafeMainActor`, `@_unsafeSendable`

Marks a parameter's (function) type as `@MainActor` (`@Sendable`) in Swift 6 and
within Swift 5 code that has adopted concurrency, but non-`@MainActor`
(non-`@Sendable`) everywhere else.

See the forum post on [Concurrency in Swift 5 and 6](https://forums.swift.org/t/concurrency-in-swift-5-and-6/49337)
for more details.

## `@_unsafeInheritExecutor`

This `async` function uses the pre-SE-0338 semantics of unsafely inheriting the caller's executor.  This is an underscored feature because the right way of inheriting an executor is to pass in the required executor and switch to it.  Unfortunately, there are functions in the standard library which need to inherit their caller's executor but cannot change their ABI because they were not defined as `@_alwaysEmitIntoClient` in the initial release.

## `@_weakLinked`

Allows a declaration to be weakly-referenced, i.e., any references emitted by
client modules to the declaration's symbol will have weak linkage. This means
that client code will compile without the guarantee that the symbol will be
available at runtime. This requires a dynamic safety check (such as using
`dlsym (3)`); otherwise, accessing the symbol when it is unavailable leads
to a runtime crash.

This is an unsafe alternative to using `@available`, which is statically checked.
If the availability of a library symbol is newer than the deployment target of
the client, the symbol will be weakly linked, but checking for `@available` and
`#(un)available` ensures that a symbol is not accessed when it is unavailable.

## `_local`

A distributed actor can be marked as "known to be local" which allows avoiding 
the distributed actor isolation checks. This is used for things like `whenLocal`
where the actor passed to the closure is known-to-be-local, and similarly a 
`self` of obtained from an _isolated_ function inside a distributed actor is 
also guaranteed to be local by construction.