//! An internal crate to support pin_project - **do not use directly**
// mem::take and #[non_exhaustive] requires Rust 1.40
// older compilers require explicit `extern crate`.
extern crate proc_macro;
use TokenStream;
use crate ProjKind;
/// An attribute that creates a projection type covering all the fields of
/// struct or enum.
///
/// This attribute creates a projection type according to the following rules:
///
/// * For the field that uses `#[pin]` attribute, makes the pinned reference to
/// the field.
/// * For the other fields, makes the unpinned reference to the field.
///
/// And the following methods are implemented on the original type:
///
/// ```rust
/// # #[rustversion::since(1.36)]
/// # fn dox() {
/// # use std::pin::Pin;
/// # type Projection<'a> = &'a ();
/// # type ProjectionRef<'a> = &'a ();
/// # trait Dox {
/// fn project(self: Pin<&mut Self>) -> Projection<'_>;
/// fn project_ref(self: Pin<&Self>) -> ProjectionRef<'_>;
/// # }
/// # }
/// ```
///
/// By passing an argument with the same name as the method to the attribute,
/// you can name the projection type returned from the method:
///
/// ```rust
/// use pin_project::pin_project;
/// use std::pin::Pin;
///
/// #[pin_project(project = EnumProj)]
/// enum Enum<T> {
/// Variant(#[pin] T),
/// }
///
/// fn func<T>(x: Pin<&mut Enum<T>>) {
/// match x.project() {
/// EnumProj::Variant(y) => {
/// let _: Pin<&mut T> = y;
/// }
/// }
/// }
/// ```
///
/// The visibility of the projected type and projection method is based on the
/// original type. However, if the visibility of the original type is `pub`, the
/// visibility of the projected type and the projection method is downgraded to
/// `pub(crate)`.
///
/// # Safety
///
/// This attribute is completely safe. In the absence of other `unsafe` code
/// *that you write*, it is impossible to cause [undefined
/// behavior][undefined-behavior] with this attribute.
///
/// This is accomplished by enforcing the four requirements for pin projection
/// stated in [the Rust documentation][pin-projection]:
///
/// 1. The struct must only be [`Unpin`] if all the structural fields are
/// [`Unpin`].
///
/// To enforce this, this attribute will automatically generate an [`Unpin`]
/// implementation for you, which will require that all structurally pinned
/// fields be [`Unpin`].
///
/// If you attempt to provide an [`Unpin`] impl, the blanket impl will then
/// apply to your type, causing a compile-time error due to the conflict with
/// the second impl.
///
/// If you wish to provide a manual [`Unpin`] impl, you can do so via the
/// [`UnsafeUnpin`][unsafe-unpin] argument.
///
/// 2. The destructor of the struct must not move structural fields out of its
/// argument.
///
/// To enforce this, this attribute will generate code like this:
///
/// ```rust
/// struct MyStruct {}
/// trait MyStructMustNotImplDrop {}
/// impl<T: Drop> MyStructMustNotImplDrop for T {}
/// impl MyStructMustNotImplDrop for MyStruct {}
/// ```
///
/// If you attempt to provide an [`Drop`] impl, the blanket impl will then
/// apply to your type, causing a compile-time error due to the conflict with
/// the second impl.
///
/// If you wish to provide a custom [`Drop`] impl, you can annotate an impl
/// with [`#[pinned_drop]`][pinned-drop]. This impl takes a pinned version of
/// your struct - that is, [`Pin`]`<&mut MyStruct>` where `MyStruct` is the
/// type of your struct.
///
/// You can call `project()` on this type as usual, along with any other
/// methods you have defined. Because your code is never provided with
/// a `&mut MyStruct`, it is impossible to move out of pin-projectable
/// fields in safe code in your destructor.
///
/// 3. You must make sure that you uphold the [`Drop`
/// guarantee][drop-guarantee]: once your struct is pinned, the memory that
/// contains the content is not overwritten or deallocated without calling
/// the content's destructors.
///
/// Safe code doesn't need to worry about this - the only wait to violate
/// this requirement is to manually deallocate memory (which is `unsafe`),
/// or to overwrite a field with something else.
/// Because your custom destructor takes [`Pin`]`<&mut MyStruct>`, it's
/// impossible to obtain a mutable reference to a pin-projected field in safe
/// code.
///
/// 4. You must not offer any other operations that could lead to data being
/// moved out of the structural fields when your type is pinned.
///
/// As with requirement 3, it is impossible for safe code to violate this.
/// This crate ensures that safe code can never obtain a mutable reference to
/// `#[pin]` fields, which prevents you from ever moving out of them in safe
/// code.
///
/// Pin projections are also incompatible with [`#[repr(packed)]`][repr-packed]
/// structs. Attempting to use this attribute on a
/// [`#[repr(packed)]`][repr-packed] struct results in a compile-time error.
///
/// # Examples
///
/// Using `#[pin_project]` will automatically create the appropriate
/// conditional [`Unpin`] implementation:
///
/// ```rust
/// use pin_project::pin_project;
/// use std::pin::Pin;
///
/// #[pin_project]
/// struct Struct<T, U> {
/// #[pin]
/// pinned: T,
/// unpinned: U,
/// }
///
/// impl<T, U> Struct<T, U> {
/// fn method(self: Pin<&mut Self>) {
/// let this = self.project();
/// let _: Pin<&mut T> = this.pinned; // Pinned reference to the field
/// let _: &mut U = this.unpinned; // Normal reference to the field
/// }
/// }
/// ```
///
/// If you want to call the `project()` method multiple times or later use the
/// original [`Pin`] type, it needs to use [`.as_mut()`][`Pin::as_mut`] to avoid
/// consuming the [`Pin`].
///
/// ## Supported Items
///
/// `#[pin_project]` can be used on structs and enums.
///
/// Structs:
///
/// ```rust
/// use pin_project::pin_project;
/// use std::pin::Pin;
///
/// #[pin_project]
/// struct Struct<T, U> {
/// #[pin]
/// pinned: T,
/// unpinned: U,
/// }
///
/// impl<T, U> Struct<T, U> {
/// fn method(self: Pin<&mut Self>) {
/// let this = self.project();
/// let _: Pin<&mut T> = this.pinned;
/// let _: &mut U = this.unpinned;
/// }
/// }
/// ```
///
/// Tuple structs:
///
/// ```rust
/// use pin_project::pin_project;
/// use std::pin::Pin;
///
/// #[pin_project]
/// struct TupleStruct<T, U>(#[pin] T, U);
///
/// impl<T, U> TupleStruct<T, U> {
/// fn method(self: Pin<&mut Self>) {
/// let this = self.project();
/// let _: Pin<&mut T> = this.0;
/// let _: &mut U = this.1;
/// }
/// }
/// ```
///
/// Enums:
///
/// `#[pin_project]` supports enums, but to use it, you need to name the
/// projection type returned from the method.
///
/// ```rust
/// use pin_project::pin_project;
/// use std::pin::Pin;
///
/// #[pin_project(project = EnumProj)]
/// enum Enum<T, U> {
/// Tuple(#[pin] T),
/// Struct { field: U },
/// Unit,
/// }
///
/// impl<T, U> Enum<T, U> {
/// fn method(self: Pin<&mut Self>) {
/// match self.project() {
/// EnumProj::Tuple(x) => {
/// let _: Pin<&mut T> = x;
/// }
/// EnumProj::Struct { field } => {
/// let _: &mut U = field;
/// }
/// EnumProj::Unit => {}
/// }
/// }
/// }
/// ```
///
/// ## `!Unpin`
///
/// If you want to ensure that [`Unpin`] is not implemented, use the `!Unpin`
/// argument to `#[pin_project]`.
///
/// ```rust
/// use pin_project::pin_project;
///
/// #[pin_project(!Unpin)]
/// struct Struct<T> {
/// field: T,
/// }
/// ```
///
/// You can also ensure `!Unpin` by using `#[pin]` attribute for
/// a [`PhantomPinned`] field.
///
/// ```rust
/// use pin_project::pin_project;
/// use std::marker::PhantomPinned;
///
/// #[pin_project]
/// struct Struct<T> {
/// field: T,
/// #[pin]
/// _pin: PhantomPinned,
/// }
/// ```
///
/// Note that using [`PhantomPinned`] without `#[pin]` attribute has no effect.
///
/// ## `UnsafeUnpin`
///
/// If you want to implement [`Unpin`] manually, you must use the `UnsafeUnpin`
/// argument to `#[pin_project]`.
///
/// ```rust
/// use pin_project::{pin_project, UnsafeUnpin};
///
/// #[pin_project(UnsafeUnpin)]
/// struct Struct<T, U> {
/// #[pin]
/// pinned: T,
/// unpinned: U,
/// }
///
/// unsafe impl<T: Unpin, U> UnsafeUnpin for Struct<T, U> {}
/// ```
///
/// Note the usage of the unsafe [`UnsafeUnpin`] trait, instead of the usual
/// [`Unpin`] trait. [`UnsafeUnpin`] behaves exactly like [`Unpin`], except that
/// is unsafe to implement. This unsafety comes from the fact that pin
/// projections are being used. If you implement [`UnsafeUnpin`], you must
/// ensure that it is only implemented when all pin-projected fields implement
/// [`Unpin`].
///
/// See [`UnsafeUnpin`] trait for more details.
///
/// ## `#[pinned_drop]`
///
/// In order to correctly implement pin projections, a type's [`Drop`] impl must
/// not move out of any structurally pinned fields. Unfortunately,
/// [`Drop::drop`] takes `&mut Self`, not [`Pin`]`<&mut Self>`.
///
/// To ensure that this requirement is upheld, the `#[pin_project]` attribute
/// will provide a [`Drop`] impl for you. This [`Drop`] impl will delegate to
/// an impl block annotated with `#[pinned_drop]` if you use the `PinnedDrop`
/// argument to `#[pin_project]`.
///
/// This impl block acts just like a normal [`Drop`] impl,
/// except for the following two:
///
/// * `drop` method takes [`Pin`]`<&mut Self>`
/// * Name of the trait is `PinnedDrop`.
///
/// ```rust
/// # use std::pin::Pin;
/// pub trait PinnedDrop {
/// fn drop(self: Pin<&mut Self>);
/// }
/// ```
///
/// `#[pin_project]` implements the actual [`Drop`] trait via `PinnedDrop` you
/// implemented. To drop a type that implements `PinnedDrop`, use the [`drop`]
/// function just like dropping a type that directly implements [`Drop`].
///
/// In particular, it will never be called more than once, just like
/// [`Drop::drop`].
///
/// For example:
///
/// ```rust
/// use pin_project::{pin_project, pinned_drop};
/// use std::{fmt::Debug, pin::Pin};
///
/// #[pin_project(PinnedDrop)]
/// struct Struct<T: Debug, U: Debug> {
/// #[pin]
/// pinned_field: T,
/// unpin_field: U,
/// }
///
/// #[pinned_drop]
/// impl<T: Debug, U: Debug> PinnedDrop for Struct<T, U> {
/// fn drop(self: Pin<&mut Self>) {
/// println!("Dropping pinned field: {:?}", self.pinned_field);
/// println!("Dropping unpin field: {:?}", self.unpin_field);
/// }
/// }
///
/// fn main() {
/// let _x = Struct { pinned_field: true, unpin_field: 40 };
/// }
/// ```
///
/// See also [`#[pinned_drop]`][`pinned_drop`] attribute.
///
/// ## `project_replace()`
///
/// In addition to the `project()` and `project_ref()` methods which are always
/// provided when you use the `#[pin_project]` attribute, there is a third
/// method, `project_replace()` which can be useful in some situations. It is
/// equivalent to [`Pin::set`], except that the unpinned fields are moved and
/// returned, instead of being dropped in-place.
///
/// ```rust
/// # use std::pin::Pin;
/// # type ProjectionOwned = ();
/// # trait Dox {
/// fn project_replace(self: Pin<&mut Self>, other: Self) -> ProjectionOwned;
/// # }
/// ```
///
/// The `ProjectionOwned` type is identical to the `Self` type, except that
/// all pinned fields have been replaced by equivalent [`PhantomData`] types.
///
/// This method is opt-in, because it is only supported for [`Sized`] types, and
/// because it is incompatible with the [`#[pinned_drop]`][pinned-drop]
/// attribute described above. It can be enabled by using
/// `#[pin_project(Replace)]`.
///
/// For example:
///
/// ```rust
/// use pin_project::pin_project;
///
/// #[pin_project(Replace, project_replace = EnumProjOwn)]
/// enum Enum<T, U> {
/// A {
/// #[pin]
/// pinned_field: T,
/// unpinned_field: U,
/// },
/// B,
/// }
///
/// let mut x = Box::pin(Enum::A { pinned_field: 42, unpinned_field: "hello" });
///
/// match x.as_mut().project_replace(Enum::B) {
/// EnumProjOwn::A { unpinned_field, .. } => assert_eq!(unpinned_field, "hello"),
/// EnumProjOwn::B => unreachable!(),
/// }
/// ```
///
/// The `project_replace` argument is necessary whenever destructuring
/// the return type of `project_replace()`, and work in exactly the same way as
/// the `project` and `project_ref` arguments.
///
/// [`PhantomData`]: core::marker::PhantomData
/// [`PhantomPinned`]: core::marker::PhantomPinned
/// [`Pin::as_mut`]: core::pin::Pin::as_mut
/// [`Pin::set`]: core::pin::Pin::set
/// [`Pin`]: core::pin::Pin
/// [`UnsafeUnpin`]: https://docs.rs/pin-project/0.4/pin_project/trait.UnsafeUnpin.html
/// [`pinned_drop`]: ./attr.pinned_drop.html
/// [drop-guarantee]: https://doc.rust-lang.org/nightly/std/pin/index.html#drop-guarantee
/// [pin-projection]: https://doc.rust-lang.org/nightly/std/pin/index.html#projections-and-structural-pinning
/// [pinned-drop]: ./attr.pin_project.html#pinned_drop
/// [repr-packed]: https://doc.rust-lang.org/nomicon/other-reprs.html#reprpacked
/// [undefined-behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
/// [unsafe-unpin]: ./attr.pin_project.html#unsafeunpin
/// An attribute for annotating an impl block that implements [`Drop`].
///
/// This attribute is only needed when you wish to provide a [`Drop`]
/// impl for your type.
///
/// This impl block acts just like a normal [`Drop`] impl,
/// except for the following two:
///
/// * `drop` method takes [`Pin`]`<&mut Self>`
/// * Name of the trait is `PinnedDrop`.
///
/// ```rust
/// # use std::pin::Pin;
/// pub trait PinnedDrop {
/// fn drop(self: Pin<&mut Self>);
/// }
/// ```
///
/// `#[pin_project]` implements the actual [`Drop`] trait via `PinnedDrop` you
/// implemented. To drop a type that implements `PinnedDrop`, use the [`drop`]
/// function just like dropping a type that directly implements [`Drop`].
///
/// In particular, it will never be called more than once, just like
/// [`Drop::drop`].
///
/// ## Example
///
/// ```rust
/// use pin_project::{pin_project, pinned_drop};
/// use std::pin::Pin;
///
/// #[pin_project(PinnedDrop)]
/// struct Foo {
/// #[pin]
/// field: u8,
/// }
///
/// #[pinned_drop]
/// impl PinnedDrop for Foo {
/// fn drop(self: Pin<&mut Self>) {
/// println!("Dropping: {}", self.field);
/// }
/// }
///
/// fn main() {
/// let _x = Foo { field: 50 };
/// }
/// ```
///
/// See also ["pinned-drop" section of `#[pin_project]` attribute][pinned-drop].
///
/// ## Why `#[pinned_drop]` attribute is needed?
///
/// Implementing `PinnedDrop::drop` is safe, but calling it is not safe.
// This is because destructors can be called multiple times in safe code and
/// [double dropping is unsound](https://github.com/rust-lang/rust/pull/62360).
///
/// Ideally, it would be desirable to be able to forbid manual calls in
/// the same way as [`Drop::drop`], but the library cannot do it. So, by using
/// macros and replacing them with private traits like the following, we prevent
/// users from calling `PinnedDrop::drop` in safe code.
///
/// ```rust
/// # use std::pin::Pin;
/// pub trait PinnedDrop {
/// unsafe fn drop(self: Pin<&mut Self>);
/// }
/// ```
///
/// This allows implementing [`Drop`] safely using `#[pinned_drop]`.
/// Also by using the [`drop`] function just like dropping a type that directly
/// implements [`Drop`], can drop safely a type that implements `PinnedDrop`.
///
/// [`Pin`]: core::pin::Pin
/// [pinned-drop]: ./attr.pin_project.html#pinned_drop
/// An attribute to provide way to refer to the projected type returned by
/// `project` method.
///
/// The following syntaxes are supported.
///
/// ## `let` bindings
///
/// *The attribute at the expression position is not stable, so you need to use
/// a dummy `#[project]` attribute for the function.*
///
/// ### Examples
///
/// ```rust
/// use pin_project::{pin_project, project};
/// use std::pin::Pin;
///
/// #[pin_project]
/// struct Foo<T, U> {
/// #[pin]
/// future: T,
/// field: U,
/// }
///
/// impl<T, U> Foo<T, U> {
/// #[project] // Nightly does not need a dummy attribute to the function.
/// fn baz(self: Pin<&mut Self>) {
/// #[project]
/// let Foo { future, field } = self.project();
///
/// let _: Pin<&mut T> = future;
/// let _: &mut U = field;
/// }
/// }
/// ```
///
/// ## `match` expressions
///
/// *The attribute at the expression position is not stable, so you need to use
/// a dummy `#[project]` attribute for the function.*
///
/// ### Examples
///
/// ```rust
/// use pin_project::{pin_project, project};
/// use std::pin::Pin;
///
/// #[pin_project]
/// enum Enum<A, B, C> {
/// Tuple(#[pin] A, B),
/// Struct { field: C },
/// Unit,
/// }
///
/// impl<A, B, C> Enum<A, B, C> {
/// #[project] // Nightly does not need a dummy attribute to the function.
/// fn baz(self: Pin<&mut Self>) {
/// #[project]
/// match self.project() {
/// Enum::Tuple(x, y) => {
/// let _: Pin<&mut A> = x;
/// let _: &mut B = y;
/// }
/// Enum::Struct { field } => {
/// let _: &mut C = field;
/// }
/// Enum::Unit => {}
/// }
/// }
/// }
/// ```
///
/// ## `impl` blocks
///
/// All methods and associated functions in `#[project] impl` block become
/// methods of the projected type. If you want to implement methods on the
/// original type, you need to create another (non-`#[project]`) `impl` block.
///
/// To call a method implemented in `#[project] impl` block, you need to first
/// get the projected-type with `let this = self.project();`.
///
/// ### Examples
///
/// ```rust
/// use pin_project::{pin_project, project};
/// use std::pin::Pin;
///
/// #[pin_project]
/// struct Foo<T, U> {
/// #[pin]
/// future: T,
/// field: U,
/// }
///
/// // impl for the original type
/// impl<T, U> Foo<T, U> {
/// fn bar(self: Pin<&mut Self>) {
/// self.project().baz()
/// }
/// }
///
/// // impl for the projected type
/// #[project]
/// impl<T, U> Foo<T, U> {
/// fn baz(self) {
/// let Self { future, field } = self;
///
/// let _: Pin<&mut T> = future;
/// let _: &mut U = field;
/// }
/// }
/// ```
///
/// ## `use` statements
///
/// ### Examples
///
/// ```rust
/// # mod dox {
/// use pin_project::pin_project;
///
/// #[pin_project]
/// struct Foo<A> {
/// #[pin]
/// field: A,
/// }
///
/// mod bar {
/// use super::Foo;
/// use pin_project::project;
/// use std::pin::Pin;
///
/// #[project]
/// use super::Foo;
///
/// #[project]
/// fn baz<A>(foo: Pin<&mut Foo<A>>) {
/// #[project]
/// let Foo { field } = foo.project();
/// let _: Pin<&mut A> = field;
/// }
/// }
/// # }
/// ```
/// An attribute to provide way to refer to the projected type returned by
/// `project_ref` method.
///
/// This is the same as [`#[project]`][`project`] attribute except it refers to
/// the projected type returned by the `project_ref` method.
///
/// See [`#[project]`][`project`] attribute for more details.
///
/// [`project`]: ./attr.project.html
/// An attribute to provide way to refer to the projected type returned by
/// `project_replace` method.
///
/// This is the same as [`#[project]`][`project`] attribute except it refers to
/// the projected type returned by the `project_replace` method.
///
/// See [`#[project]`][`project`] attribute for more details.
///
/// [`project`]: ./attr.project.html
/// An internal helper macro.