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// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
#[cfg(feature = "alloc")]
use alloc::borrow::{Cow, ToOwned};
use core::mem;
/// The `Yokeable<'a>` trait is implemented on the `'static` version of any zero-copy type; for
/// example, `Cow<'static, T>` implements `Yokeable<'a>` (for all `'a`). One can use
/// `Yokeable::Output` on this trait to obtain the "lifetime'd" value of the `Cow<'static, T>`,
/// e.g. `<Cow<'static, T> as Yokeable<'a>'>::Output` is `Cow<'a, T>`.
///
/// A [`Yokeable`] type is essentially one with a covariant lifetime parameter,
/// matched to the parameter in the trait definition. The trait allows one to cast
/// the covariant lifetime to and from `'static`.
///
/// **Most of the time, if you need to implement [`Yokeable`], you should be able to use the safe
/// [`#[derive(Yokeable)]`](yoke_derive::Yokeable) custom derive.**
///
/// While Rust does not yet have GAT syntax, for the purpose of this documentation
/// we shall refer to "`Self` with a lifetime `'a`" with the syntax `Self<'a>`.
/// Self<'static> is a stand-in for the HKT Self<'_>: lifetime -> type.
///
/// With this terminology, [`Yokeable`] exposes ways to cast between `Self<'static>` and `Self<'a>` generically.
/// This is useful for turning covariant lifetimes to _dynamic_ lifetimes, where `'static` is
/// used as a way to "erase" the lifetime.
///
/// # Safety
///
/// This trait is safe to implement on types with a _covariant_ lifetime parameter, i.e. one where
/// [`Self::transform()`]'s body can simply be `{ self }`. This will occur when the lifetime
/// parameter is used within references, but not in the arguments of function pointers or in mutable
/// positions (either in `&mut` or via interior mutability)
///
/// This trait must be implemented on the `'static` version of such a type, e.g. one should
/// implement `Yokeable<'a>` (for all `'a`) on `Cow<'static, T>`.
///
/// There are further constraints on implementation safety on individual methods.
///
/// # Trait bounds
///
/// trait bounds on `Yokeable::Output`. For more information and for workarounds, see
/// [`crate::trait_hack`].
///
/// # Implementation example
///
/// Implementing this trait manually is unsafe. Where possible, you should use the safe
/// [`#[derive(Yokeable)]`](yoke_derive::Yokeable) custom derive instead. We include an example
/// in case you have your own zero-copy abstractions you wish to make yokeable.
///
/// ```rust
/// # use yoke::Yokeable;
/// # use std::borrow::Cow;
/// # use std::{mem, ptr};
/// struct Bar<'a> {
/// numbers: Cow<'a, [u8]>,
/// string: Cow<'a, str>,
/// owned: Vec<u8>,
/// }
///
/// unsafe impl<'a> Yokeable<'a> for Bar<'static> {
/// type Output = Bar<'a>;
/// fn transform(&'a self) -> &'a Bar<'a> {
/// // covariant lifetime cast, can be done safely
/// self
/// }
///
/// fn transform_owned(self) -> Bar<'a> {
/// // covariant lifetime cast, can be done safely
/// self
/// }
///
/// unsafe fn make(from: Bar<'a>) -> Self {
/// // We're just doing mem::transmute() here, however Rust is
/// // not smart enough to realize that Bar<'a> and Bar<'static> are of
/// // the same size, so instead we use transmute_copy
///
/// // This assert will be optimized out, but is included for additional
/// // peace of mind as we are using transmute_copy
/// debug_assert!(mem::size_of::<Bar<'a>>() == mem::size_of::<Self>());
/// let ptr: *const Self = (&from as *const Self::Output).cast();
/// mem::forget(from);
/// ptr::read(ptr)
/// }
///
/// fn transform_mut<F>(&'a mut self, f: F)
/// where
/// F: 'static + FnOnce(&'a mut Self::Output),
/// {
/// unsafe { f(mem::transmute::<&mut Self, &mut Self::Output>(self)) }
/// }
/// }
/// ```
pub unsafe trait Yokeable<'a>: 'static {
/// This type MUST be `Self` with the `'static` replaced with `'a`, i.e. `Self<'a>`
type Output: 'a;
/// This method must cast `self` between `&'a Self<'static>` and `&'a Self<'a>`.
///
/// # Implementation safety
///
/// If the invariants of [`Yokeable`] are being satisfied, the body of this method
/// should simply be `{ self }`, though it's acceptable to include additional assertions
/// if desired.
fn transform(&'a self) -> &'a Self::Output;
/// This method must cast `self` between `Self<'static>` and `Self<'a>`.
///
/// # Implementation safety
///
/// If the invariants of [`Yokeable`] are being satisfied, the body of this method
/// should simply be `{ self }`, though it's acceptable to include additional assertions
/// if desired.
fn transform_owned(self) -> Self::Output;
/// This method can be used to cast away `Self<'a>`'s lifetime.
///
/// # Safety
///
/// The returned value must be destroyed before the data `from` was borrowing from is.
///
/// # Implementation safety
///
/// A safe implementation of this method must be equivalent to a transmute between
/// `Self<'a>` and `Self<'static>`
unsafe fn make(from: Self::Output) -> Self;
/// This method must cast `self` between `&'a mut Self<'static>` and `&'a mut Self<'a>`,
/// and pass it to `f`.
///
/// # Implementation safety
///
/// A safe implementation of this method must be equivalent to a pointer cast/transmute between
/// `&mut Self<'a>` and `&mut Self<'static>` being passed to `f`
///
/// # Why is this safe?
///
/// Typically covariant lifetimes become invariant when hidden behind an `&mut`,
/// which is why the implementation of this method cannot just be `f(self)`.
/// The reason behind this is that while _reading_ a covariant lifetime that has been cast to a shorter
/// one is always safe (this is roughly the definition of a covariant lifetime), writing
/// may not necessarily be safe since you could write a smaller reference to it. For example,
/// the following code is unsound because it manages to stuff a `'a` lifetime into a `Cow<'static>`
///
/// ```rust,compile_fail
/// # use std::borrow::Cow;
/// # use yoke::Yokeable;
/// struct Foo {
/// str: String,
/// cow: Cow<'static, str>,
/// }
///
/// fn unsound<'a>(foo: &'a mut Foo) {
/// let a: &str = &foo.str;
/// foo.cow.transform_mut(|cow| *cow = Cow::Borrowed(a));
/// }
/// ```
///
/// However, this code will not compile because [`Yokeable::transform_mut()`] requires `F: 'static`.
/// This enforces that while `F` may mutate `Self<'a>`, it can only mutate it in a way that does
/// not insert additional references. For example, `F` may call `to_owned()` on a `Cow` and mutate it,
/// but it cannot insert a new _borrowed_ reference because it has nowhere to borrow _from_ --
/// `f` does not contain any borrowed references, and while we give it `Self<'a>` (which contains borrowed
/// data), that borrowed data is known to be valid
///
/// Note that the `for<'b>` is also necessary, otherwise the following code would compile:
///
/// ```rust,compile_fail
/// # use std::borrow::Cow;
/// # use yoke::Yokeable;
/// # use std::mem;
/// #
/// // also safely implements Yokeable<'a>
/// struct Bar<'a> {
/// num: u8,
/// cow: Cow<'a, u8>,
/// }
///
/// fn unsound<'a>(bar: &'a mut Bar<'static>) {
/// bar.transform_mut(move |bar| bar.cow = Cow::Borrowed(&bar.num));
/// }
/// #
/// # unsafe impl<'a> Yokeable<'a> for Bar<'static> {
/// # type Output = Bar<'a>;
/// # fn transform(&'a self) -> &'a Bar<'a> {
/// # self
/// # }
/// #
/// # fn transform_owned(self) -> Bar<'a> {
/// # // covariant lifetime cast, can be done safely
/// # self
/// # }
/// #
/// # unsafe fn make(from: Bar<'a>) -> Self {
/// # let ret = mem::transmute_copy(&from);
/// # mem::forget(from);
/// # ret
/// # }
/// #
/// # fn transform_mut<F>(&'a mut self, f: F)
/// # where
/// # F: 'static + FnOnce(&'a mut Self::Output),
/// # {
/// # unsafe { f(mem::transmute(self)) }
/// # }
/// # }
/// ```
///
/// which is unsound because `bar` could be moved later, and we do not want to be able to
/// self-insert references to it.
///
/// The `for<'b>` enforces this by stopping the author of the closure from matching up the input
/// `&'b Self::Output` lifetime with `'a` and borrowing directly from it.
///
/// Thus the only types of mutations allowed are ones that move around already-borrowed data, or
/// introduce new owned data:
///
/// ```rust
/// # use std::borrow::Cow;
/// # use yoke::Yokeable;
/// struct Foo {
/// str: String,
/// cow: Cow<'static, str>,
/// }
///
/// fn sound<'a>(foo: &'a mut Foo) {
/// foo.cow.transform_mut(move |cow| cow.to_mut().push('a'));
/// }
/// ```
fn transform_mut<F>(&'a mut self, f: F)
where
// be VERY CAREFUL changing this signature, it is very nuanced (see above)
F: 'static + for<'b> FnOnce(&'b mut Self::Output);
}
#[cfg(feature = "alloc")]
unsafe impl<'a, T: 'static + ToOwned + ?Sized> Yokeable<'a> for Cow<'static, T>
where
<T as ToOwned>::Owned: Sized,
{
type Output = Cow<'a, T>;
#[inline]
fn transform(&'a self) -> &'a Cow<'a, T> {
// Doesn't need unsafe: `'a` is covariant so this lifetime cast is always safe
self
}
#[inline]
fn transform_owned(self) -> Cow<'a, T> {
// Doesn't need unsafe: `'a` is covariant so this lifetime cast is always safe
self
}
#[inline]
unsafe fn make(from: Cow<'a, T>) -> Self {
// i hate this
// unfortunately Rust doesn't think `mem::transmute` is possible since it's not sure the sizes
// are the same
debug_assert!(mem::size_of::<Cow<'a, T>>() == mem::size_of::<Self>());
let ptr: *const Self = (&from as *const Self::Output).cast();
mem::forget(from);
core::ptr::read(ptr)
}
#[inline]
fn transform_mut<F>(&'a mut self, f: F)
where
F: 'static + for<'b> FnOnce(&'b mut Self::Output),
{
// Cast away the lifetime of Self
unsafe { f(mem::transmute::<&'a mut Self, &'a mut Self::Output>(self)) }
}
}
unsafe impl<'a, T: 'static + ?Sized> Yokeable<'a> for &'static T {
type Output = &'a T;
#[inline]
fn transform(&'a self) -> &'a &'a T {
// Doesn't need unsafe: `'a` is covariant so this lifetime cast is always safe
self
}
#[inline]
fn transform_owned(self) -> &'a T {
// Doesn't need unsafe: `'a` is covariant so this lifetime cast is always safe
self
}
#[inline]
unsafe fn make(from: &'a T) -> Self {
mem::transmute(from)
}
#[inline]
fn transform_mut<F>(&'a mut self, f: F)
where
F: 'static + for<'b> FnOnce(&'b mut Self::Output),
{
// Cast away the lifetime of Self
unsafe { f(mem::transmute::<&'a mut Self, &'a mut Self::Output>(self)) }
}
}
#[cfg(feature = "alloc")]
unsafe impl<'a, T: 'static> Yokeable<'a> for alloc::vec::Vec<T> {
type Output = alloc::vec::Vec<T>;
#[inline]
fn transform(&'a self) -> &'a alloc::vec::Vec<T> {
// Doesn't need unsafe: `'a` is covariant so this lifetime cast is always safe
self
}
#[inline]
fn transform_owned(self) -> alloc::vec::Vec<T> {
// Doesn't need unsafe: `'a` is covariant so this lifetime cast is always safe
self
}
#[inline]
unsafe fn make(from: alloc::vec::Vec<T>) -> Self {
from
}
#[inline]
fn transform_mut<F>(&'a mut self, f: F)
where
F: 'static + for<'b> FnOnce(&'b mut Self::Output),
{
// Doesn't need unsafe: `'a` is covariant so this lifetime cast is always safe
f(self)
}
}