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#![doc(html_root_url = "https://docs.rs/singleton-trait/0.4.0")]
#![no_std]
use core::cell::{Cell, UnsafeCell};
use core::marker::PhantomData;
use core::mem::ManuallyDrop;
/*******************/
/* Singleton trait */
/*******************/
/**
* This trait denotes a type which has at most one logical identity at all times.
* This is sufficient to make borrowing decisions based only on the type, without
* regards to value identity.
*
* Implementers of the trait must uphold this contract, that any point
* during the execution of the program, there is at most one accessible
* logical value of this type. Much like how re-borrowings can
* make a type inaccessible, it is allowed for there to be more than one
* such binding, if the others are inaccessible due to unique borrowing.
*
* Some examples would include:
* * A type with exactly one instance constructed during the main method
* * A type with only one static instance (including one wrapped behind e.g. a Mutex)
* * An uninhabited type
* * A type which is constructed with a unique lifetime brand
*
* Any type which has a public constructor cannot meet this,
* Some non-examples include ZSTs like ()
* and the Exists<T> struct in this crate (when T is not a Singleton itself)
* Any type which implements Clone
*/
pub unsafe trait Singleton {}
/*****************/
/* Blanket Impls */
/*****************/
// Anything which is backed by at least one T
// can be given an implementation
//
// This can be witnessed by a function
// S -> T where T is a singleton
// SAFETY:
// These types are all backed by exactly one, unique T
unsafe impl<T: Singleton> Singleton for Cell<T> {}
unsafe impl<T: Singleton> Singleton for UnsafeCell<T> {}
unsafe impl<T: Singleton> Singleton for [T; 1] {}
// Not many other interesting cases, but some like Box require alloc
// SAFETY:
// 1. Every mutable reference points to a value of T
// 2. No two mutable references alias
// 3. By the contract of Singleton for T, there is at most one logical value of T
unsafe impl<'a, T: Singleton> Singleton for &'a mut T {}
// SAFETY:
// Exists<T> witnesses strict ownership of a value of type T
unsafe impl<'a, T: Singleton> Singleton for Erased<T> {}
/*******************/
/* Single-threaded */
/*******************/
/**
* A type `T` implements SingleThread if at any time there
* is a single thread from which all values/references to
* values of this type may be accessed.
*
* Usually, it is sufficient that `T` is `!Sync` and `Singleton`.
*
* Since Sending `T` denies access in the original thread, this
* property is maintained regardless of Sendability.
*/
pub unsafe trait SingleThread { }
/*****************/
/* Blanket Impls */
/*****************/
// SAFETY:
// These cases are structurally Sync
unsafe impl<T: SingleThread> SingleThread for Cell<T> {}
unsafe impl<T: SingleThread> SingleThread for [T; 1] {}
// other interesting cases generally require alloc, or could be changed.
// SAFETY:
// From `&&mut T` we can produce `&T`, and by the contract for
// T: SingleThread, this is locked to one thread, so
// we can assume that `&&mut T` is also locked to one thread
unsafe impl<'a, T: SingleThread> SingleThread for &'a mut T {}
unsafe impl<'a, T: SingleThread> SingleThread for &'a T {}
// SAFETY:
// Exists<T> witnesses strict ownership of a value of type T
unsafe impl<'a, T: SingleThread> SingleThread for Erased<T> {}
/*********************/
/* Phantom existence */
/*********************/
/**
* The Erased struct witnesses the logical ownership of a value of type T
* while remaining zero-sized. This can be used for ghost proofs of soundness.
*
* Erased<T> should be thought of a zero-sized owner of T. It is useful for inclusion
* in data structures where the value might not be eliminated by the compiler.
* It likely is not necessary for short-lived data.
*
* NOTE: drop implementations will never be called, as Exists<T> guarantees the existence
* of a valid T, which might not be true if they were called. On the other hand, since
* it does not hold a T, it cannot drop T when it is itself dropped
*
* Secondly, keep in mind that while Erased<T> serves as evidence, it does not include
* sufficient provenance for Stacked Borrows or LLVM, and so it is not techinically sound
* to recover a reference `&T` from an `Exists<&T>` and `*mut T` or `&UnsafeCell<T>`
* even when T is Singleton, but this could be possible if T is zero-sized.
* Because of the missing provenance, creating a reference this way could invalidate
* the original reference on which this Exists instance was based.
*/
#[derive(Copy)]
pub struct Erased<T> {
_phantom: PhantomData<ManuallyDrop<T>>,
}
impl<T: Copy> Clone for Erased<T> {
#[inline(always)]
fn clone(&self) -> Self {
*self
}
}
impl<T> Erased<T> {
#[inline(always)]
pub const fn new(t: T) -> Self {
let _ = ManuallyDrop::new(t);
// SAFETY: we have taken ownership of a T value above
unsafe { Self::new_unchecked() }
}
/**
* This function constructs a value of Erased<T> without taking logical ownership of a T.
*
* # Safety
*
* Constructing this asserts that there is a value of type T which has been leaked, or
* in which it is guaranteed that the program behaves the same up to observation as if a
* zero-sized copy of T were being passed.
*
*/
#[inline(always)]
pub const unsafe fn new_unchecked() -> Self {
Erased {
_phantom: PhantomData,
}
}
/**
* Turns a &Erased<T> into an Erased<&T>
*
* This can be especially useful when storing
* the Erased<T> inside another wrapper type
*
* ```
* # use singleton_trait::Erased;
* use core::cell::RefCell;
* struct Token;
* let lock = RefCell::new(Erased::new(Token));
*
* let locked = lock.borrow();
* let _: Erased<&Token> = locked.borrow();
* ```
*/
#[inline(always)]
pub fn borrow(&self) -> Erased<&T> {
// Safety:
// the identity function is pure
unsafe { self.map_borrow(|r| r) }
}
/**
* Turns a &mut Erased<T> into an Erased<&mut T>
*
* This can be especially useful when storing
* the Erased<T> inside another wrapper type
*
* ```
* # use singleton_trait::Erased;
* use core::cell::RefCell;
* struct Token;
* let lock = RefCell::new(Erased::new(Token));
*
* let mut locked = lock.borrow_mut();
* let _: Erased<&mut Token> = locked.borrow_mut();
* ```
*/
#[inline(always)]
pub fn borrow_mut(&mut self) -> Erased<&mut T> {
// Safety:
// the identity function is pure
unsafe { self.map_borrow_mut(|r| r) }
}
/**
* Maps a function on the inside of the Erased field.
*
* # Safety
*
* Due to the strictness guarantees, the passed closure must not cause any visible
* side effects, including side effects caused by owning R
*/
#[inline(always)]
pub unsafe fn map<R, F: FnOnce(T) -> R>(self, _: impl Exists<F>) -> Erased<R> {
// Safety:
//
// By the contract for the passed function, this is equivalent to calling it on the value of type T
Erased::<R>::new_unchecked()
}
/**
* Maps a function on the borrow of the Erased field.
*
* # Safety
*
* Due to the strictness guarantees, the passed closure must not cause any visible
* side effects, including side effects caused by owning R
*/
#[inline(always)]
pub unsafe fn map_borrow<'a, R, F: FnOnce(&'a T) -> R>(
&'a self,
_: impl Exists<F>,
) -> Erased<R> {
// Safety:
//
// By the contract for the passed function, this is equivalent to calling it on the borrow of T
Erased::<R>::new_unchecked()
}
/**
* Maps a function on the mutable borrow of the Erased field.
*
* # Safety
*
* Due to the strictness guarantees, the passed closure must not cause any visible
* side effects, including side effects caused by owning R
*/
#[inline(always)]
pub unsafe fn map_borrow_mut<'a, R, F: FnOnce(&'a mut T) -> R>(
&'a mut self,
_: impl Exists<F>,
) -> Erased<R> {
// Safety:
//
// By the contract for the passed function, this is equivalent to calling it on the mutable borrow of T
Erased::<R>::new_unchecked()
}
/**
* Fallback function which converts this Erased value into an Exists
* implementer.
*
* This exists because we cannot allow general trait implementations
* due to coherence rules, but we can't be sufficiently flexible in
* trait implementations due to a lack of subtyping constraints or
* trait covariance, which would mean references would be too inflexible
*/
#[inline(always)]
pub fn exists(self) -> impl Exists<T> {
struct Internal<T> {
er: Erased<T>,
}
impl<T> Exists<T> for Internal<T> {
#[inline(always)]
fn erase(self) -> Erased<T> {
self.er
}
}
Internal { er: self }
}
}
impl<T> Erased<Erased<T>> {
/**
* An erased erased value can be flattened into a single erasure,
* since Erased<T> is notionally equivalent to T
*/
#[inline(always)]
pub fn flatten(self) -> Erased<T> {
// SAFETY:
//
// By existential induction since the constructor for Erased is pure
unsafe { Erased::<T>::new_unchecked() }
}
}
impl<'a, T> Erased<&'a mut T> {
/**
* Re-borrow this erased mutable borrow. Usually
* this is implicit in Rust, but not for general wrappers.
* This can be seen as a "clone" method for mutable borrows,
* as it allows you to retain the erased borrow after the
* end of the lifetime.
*
* The Exists trait can perform this conversion automatically.
*
* ```
* # use singleton_trait::Erased;
* struct Token;
* fn recursively(mut b: Erased<&mut Token>) {
* recursively(b.reborrow());
* recursively(b);
* }
* ```
*/
#[inline(always)]
pub fn reborrow<'b>(&'b mut self) -> Erased<&'b mut T> {
// SAFETY
//
// Refs and derefs on reference types are pure
unsafe { self.map_borrow_mut(|r: &'b mut &'a mut T| &mut **r) }
}
/**
* Borrow this erased mutable borrow as immutable.
* This is effectively a Deref method under Erased
*
* The Exists trait can perform this conversion automatically.
*
* ```
* # use singleton_trait::Erased;
* struct Lock;
* fn use_lock(write: Erased<&mut Lock>) {
* read_from(write.read());
* }
*
* fn read_from(read: Erased<&Lock>) { }
* ```
*/
#[inline(always)]
pub fn read<'b>(&'b self) -> Erased<&'b T> {
// SAFETY
//
// Refs and derefs on reference types are pure
unsafe { self.map_borrow(|r: &'b &'a mut T| &**r) }
}
/**
* Consume this erased mutable borrow to
* turn it into an immutable borrow.
*
* The Exists trait can perform this conversion automatically.
*
* See also `read`, which is often more useful.
* This method uses the full inner lifetime
* without needing to borrow.
* ```
* # use singleton_trait::Erased;
* struct Lock;
* struct MutWrapper<'a> (Erased<&'a mut Lock>);
* struct Wrapper<'a> (Erased<&'a Lock>);
* impl<'a> MutWrapper<'a> {
* pub fn into_read(self) -> Wrapper<'a> {
* Wrapper(self.0.into_shared())
* }
* }
*
* ```
*/
#[inline(always)]
pub fn into_shared(self) -> Erased<&'a T> {
// SAFETY
//
// Refs and derefs on reference types are pure
unsafe { self.map(|r: &'a mut T| &*r) }
}
}
impl<T> From<T> for Erased<T> {
#[inline(always)]
fn from(t: T) -> Self {
Self::new(t)
}
}
/**
* The Exists trait is intended to be used with `impl`, to denote
* an argument where the existence of a value is sufficient as an argument
*
*
*/
pub trait Exists<T: Sized> {
fn erase(self) -> Erased<T>;
}
impl<T> Exists<T> for T {
#[inline(always)]
fn erase(self) -> Erased<T> {
self.into()
}
}
impl<'a, 'b: 'a, T> Exists<&'a T> for Erased<&'b T> {
#[inline(always)]
fn erase(self) -> Erased<&'a T> {
self
}
}
impl<'a, 'b: 'a, T> Exists<&'a T> for Erased<&'b mut T> {
#[inline(always)]
fn erase(self) -> Erased<&'a T> {
self.into_shared()
}
}
impl<'a, 'b: 'a, T> Exists<&'a mut T> for Erased<&'b mut T> {
#[inline(always)]
fn erase(self) -> Erased<&'a mut T> {
// SAFETY: Deref on reference is pure
unsafe { self.map(|r: &'a mut T| &mut *r) }
}
}
impl<'a, 'b, T> Exists<&'a T> for &'a Erased<&'b T> {
#[inline(always)]
fn erase(self) -> Erased<&'a T> {
*self
}
}
impl<'a, 'b: 'a, T> Exists<&'a T> for &'a Erased<&'b mut T> {
#[inline(always)]
fn erase(self) -> Erased<&'a T> {
// SAFETY: Deref on reference is pure
self.read()
}
}
impl<'a, 'b: 'a, T> Exists<&'a mut T> for &'a mut Erased<&'b mut T> {
#[inline(always)]
fn erase(self) -> Erased<&'a mut T> {
// SAFETY: Deref on reference is pure
self.reborrow()
}
}
#[cfg(test)]
mod tests {
use crate::*;
// this test should fail to compile if we're missing impls
// that can coerce between different lifetimes
#[test]
fn test_impls() {
/* Invariant type parameter to strictly test subtyping */
struct NonCopy;
impl Drop for NonCopy {
fn drop(&mut self) {}
}
struct Guard<'a>(PhantomData<fn(&'a ()) -> &'a ()>, &'a NonCopy);
fn takes_ref<'a>(_: &Guard<'a>, _: impl Exists<&'a ()>) {}
fn takes_mut<'a>(_: &Guard<'a>, _: impl Exists<&'a mut ()>) {}
let nc = NonCopy;
let guard = Guard(PhantomData, &nc);
let er = Erased::new(&());
takes_ref(&guard, er);
let mut x = ();
let er = Erased::new(&mut x);
takes_ref(&guard, er);
let mut x = ();
let er = Erased::new(&mut x);
takes_mut(&guard, er);
}
}