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//! A cell with the ability to mutate the value through an immutable reference when safe. //! //! # Comparison with `RefCell` //! //! `RefCell` goes for completely runtime checking, having `try_borrow`, `try_borrow_mut`, //! `borrow` and `borrow_mut` all taking `&self` and using custom reference types everywhere. //! //! `MuCell` (out of pity and the fact that “non-ascii idents are not fully supported” I did not //! name it `ΜCell` with the crate named `µcell`) makes much more use of true Rust borrow checking //! for a result that is more efficient and has no possibility of panicking. //! //! However, its purpose is not the same as `RefCell`; it is designed specifically for cases where //! something only *needs* an immutable reference, but where being able to safely take a mutable //! reference can improve efficiency. Say, for example, where it’s beneficial to be able to cache //! the result of a calculation, but you don’t really want to *need* to do that. //! //! The purpose of all of this is for an accessor for a `T` that can be made more efficient if it //! can have `&mut self`, but doesn’t strictly require it. For this reason, it’s often going to be //! paired with [`std::borrow::Cow`](http://doc.rust-lang.org/std/borrow/enum.Cow.html), e.g. //! `Cow<str>` or `Cow<[T]>`, producing `Borrowed` if you are able to mutate the value or `Owned` //! of the same data if not. //! //! # Examples //! //! This example covers most of the surface area of the library: //! //! ```rust //! # use mucell::MuCell; //! let mut cell = MuCell::new(vec![1, 2, 3]); //! //! // You can borrow from the cell mutably at no cost. //! cell.borrow_mut().push(4); //! //! // You can borrow immutably, too, and it’s very cheap. //! // (Rust’s standard borrow checking prevents you from doing //! // this while there’s a mutable reference taken out.) //! assert_eq!(&*cell.borrow(), &[1, 2, 3, 4]); //! //! // So long as there are no active borrows, //! // try_mutate can be used to mutate the value. //! assert!(cell.try_mutate(|x| x.push(5))); //! assert_eq!(&*cell.borrow(), &[1, 2, 3, 4, 5]); //! //! // But when there is an immutable borrow active, //! // try_mutate says no. //! let b = cell.borrow(); //! assert!(!cell.try_mutate(|_| unreachable!())); //! drop(b); //! //! // We can have many immutable borrows at a time, too. //! { //! let a = cell.borrow(); //! let b = cell.borrow(); //! let c = cell.borrow(); //! assert_eq!(&*a as *const Vec<i32>, &*b as *const Vec<i32>); //! } //! //! // Once they’re all cleared, try_mutate is happy again. //! assert!(cell.try_mutate(|x| x.push(6))); //! assert_eq!(&*cell.borrow(), &[1, 2, 3, 4, 5, 6]); //! ``` //! //! Look at the examples in the repository for some slightly more practical (though still //! typically contrived) examples. #![cfg_attr(feature = "no_std", no_std)] #![cfg_attr(feature = "const_fn", feature(const_fn))] #![warn(bad_style, unused, missing_docs)] #[cfg(not(feature = "no_std"))] extern crate std as core; #[cfg(not(feature = "no_std"))] use std::borrow::Cow; use core::cell::{Cell, UnsafeCell}; use core::cmp::Ordering; use core::fmt; use core::hash::{Hash, Hasher}; use core::ops::{Deref, DerefMut}; type BorrowFlag = usize; const UNUSED: BorrowFlag = 0; const WRITING: BorrowFlag = !0; /// A cell with the ability to mutate the value through an immutable reference when safe. pub struct MuCell<T: ?Sized> { borrow: Cell<BorrowFlag>, value: UnsafeCell<T>, } #[cfg(feature = "const_fn")] #[macro_use] mod _m { macro_rules! const_fn { ($(#[$m:meta])* pub const fn $name:ident($value:ident: $T:ty) -> $R:ty { $body:expr }) => { $(#[$m])* pub const fn $name($value: $T) -> $R { $body } } } } #[cfg(not(feature = "const_fn"))] #[macro_use] mod _m { macro_rules! const_fn { ($(#[$m:meta])* pub const fn $name:ident($value:ident: $T:ty) -> $R:ty { $body:expr }) => { $(#[$m])* pub fn $name($value: $T) -> $R { $body } } } } impl<T> MuCell<T> { const_fn! { #[doc = " Creates a `MuCell` containing `value`. # Examples ``` use mucell::MuCell; let c = MuCell::new(5); ```"] #[inline] pub const fn new(value: T) -> MuCell<T> { MuCell { value: UnsafeCell::new(value), borrow: Cell::new(UNUSED), } } } /// Consumes the `MuCell`, returning the wrapped value. /// /// # Examples /// /// ``` /// use mucell::MuCell; /// /// let c = MuCell::new(5); /// /// let five = c.into_inner(); /// ``` #[inline] pub fn into_inner(self) -> T { // Since this function takes `self` (the `RefCell`) by value, the // compiler statically verifies that it is not currently borrowed. // Therefore the following assertion is just a `debug_assert!`. debug_assert!(self.borrow.get() == UNUSED); unsafe { self.value.into_inner() } } } impl<T: ?Sized> MuCell<T> { // Explicitly not implemented from RefCell is borrow_state. // // - Returning `Writing` would indicate you are in a `try_mutate` block, and so calling // `borrow()` would panic, but you should definitely know that already. // - Returning `Reading` would indicate there are immutable borrows alive, and so calling // `try_mutate()` would return `false`, but there’s no real value in knowing that. // // In short, there just doesn’t seem much point in providing it. /// Immutably borrows the wrapped value. /// /// The borrow lasts until the returned `Ref` exits scope. /// Multiple immutable borrows can be taken out at the same time. /// /// # Panics /// /// Panics if called inside the `try_mutate()` mutator function. /// But that’s generally a nonsensical thing to do, anyway, so just be sensible and you’re OK. #[inline] pub fn borrow(&self) -> Ref<&T> { Ref { _borrow: BorrowRef::new(&self.borrow), _value: unsafe { &*self.value.get() }, } } /// Mutably borrows the wrapped value. /// /// Unlike `RefCell.borrow_mut`, this method lets Rust’s type system prevent aliasing /// and so cannot have anything go wrong. It is also, in consequence, completely free, /// unlike `RefCell` or `MuCell.borrow` which all have to keep track of borrows at runtime. #[inline] pub fn borrow_mut(&mut self) -> &mut T { unsafe { &mut *self.value.get() } } /// Mutate the contained object if possible. /// /// If any immutable references produced by calling `borrow()` are active, /// this will return false, not executing the function given. /// /// If there are no immutable references active, /// this will execute the mutator function and return true. /// /// The mutator function should not touch `self` (not that it would really /// make much sense to be touching it, anyway); most notably, you may not call `borrow` on /// `self` inside the mutator, which includes things like the `==` implementation which borrow /// the value briefly; while calling `try_mutate` inside it will just return false, calling /// `borrow` will panic. #[inline] pub fn try_mutate<F: FnOnce(&mut T)>(&self, mutator: F) -> bool { if self.borrow.get() == UNUSED { self.borrow.set(WRITING); mutator(unsafe { &mut *self.value.get() }); self.borrow.set(UNUSED); true } else { false } } // Not implemented at present from RefCell: as_unsafe_cell. I don’t see the point of it, // but it can easily be added at a later date if desired. } struct BorrowRef<'b> { _borrow: &'b Cell<BorrowFlag>, } impl<'b> BorrowRef<'b> { #[inline] fn new(borrow: &'b Cell<BorrowFlag>) -> BorrowRef<'b> { match borrow.get() { WRITING => panic!("borrow() called inside try_mutate"), b => { borrow.set(b + 1); BorrowRef { _borrow: borrow } }, } } } impl<'b> Drop for BorrowRef<'b> { #[inline] fn drop(&mut self) { let borrow = self._borrow.get(); debug_assert!(borrow != WRITING && borrow != UNUSED); self._borrow.set(borrow - 1); } } impl<'b> Clone for BorrowRef<'b> { #[inline] fn clone(&self) -> BorrowRef<'b> { // Since this BorrowRef exists, // we know the borrow flag is not set to WRITING. let borrow = self._borrow.get(); debug_assert!(borrow != WRITING && borrow != UNUSED); self._borrow.set(borrow + 1); BorrowRef { _borrow: self._borrow } } } /// An immutable reference to a `MuCell`. /// Normally you should dereference to get at the object, /// but after transformation with `Ref::map` or `Ref::filter_map` /// you might instead use `.into_inner()`. pub struct Ref<'b, T: 'b> { // FIXME #12808: strange name to try to avoid interfering with // field accesses of the contained type via Deref _value: T, _borrow: BorrowRef<'b> } impl<'b, T: Deref + 'b> Deref for Ref<'b, T> { type Target = T::Target; #[inline] fn deref(&self) -> &T::Target { &*self._value } } impl<'b, T: DerefMut + 'b> DerefMut for Ref<'b, T> { #[inline] fn deref_mut(&mut self) -> &mut T::Target { &mut *self._value } } impl<'b, T: Clone> Ref<'b, T> { /// Copies a `Ref`. /// /// The `MuCell` is already immutably borrowed, so this cannot fail. /// /// This is an associated function that needs to be used as /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere /// with the widespread use of `r.borrow().clone()` to clone the contents of /// a `MuCell`. #[inline] pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> { Ref { _value: orig._value.clone(), _borrow: orig._borrow.clone(), } } } impl<'b, T: 'static> Ref<'b, T> { /// Consumes the `Ref`, returning the wrapped value. /// /// The `'static` constraint on `T` is what makes this possible; there is no longer any need to /// keep the borrow alive, and so the `Ref` itself can be consumed while keeping the contained /// value. /// /// # Examples /// /// ``` /// use mucell::{MuCell, Ref}; /// use std::borrow::Cow; /// /// let c = MuCell::new("foo"); /// /// let r1: Ref<Cow<str>> = Ref::map(c.borrow(), |s| Cow::from(*s)); /// let r2: Ref<String> = Ref::map(r1, |s| s.into_owned()); /// let string: String = r2.into_inner(); /// ``` #[inline] pub fn into_inner(self) -> T { self._value } } #[cfg(not(feature = "no_std"))] impl<'b, T: ?Sized> Ref<'b, Cow<'b, T>> where T: ToOwned, T::Owned: 'static { /// Extracts the owned data. /// /// Copies the data if it is not already owned. /// /// This code is precisely equivalent to `Ref::map(self, |cow| cow.into_owned()).into_inner()` /// and is purely a convenience method because `Ref<Cow<T>>` is a common case. /// /// # Examples /// /// ``` /// use mucell::{MuCell, Ref}; /// use std::borrow::Cow; /// /// let c = MuCell::new("foo"); /// /// let r: Ref<Cow<str>> = Ref::map(c.borrow(), |s| Cow::from(*s)); /// let string: String = r.into_owned(); /// ``` #[inline] pub fn into_owned(self) -> T::Owned { Ref::map(self, |cow| cow.into_owned()).into_inner() } } impl<'b, T> Ref<'b, T> { /// Make a new `Ref` for a component of the borrowed data. /// /// The `MuCell` is already immutably borrowed, so this cannot fail. /// /// This is an associated function that needs to be used as `Ref::map(...)`. /// A method would interfere with methods of the same name on the contents /// of a `MuCell` used through `Deref`. /// /// # Memory unsafety /// /// This function is marked as unsafe because it is possible (though not the easiest /// thing) to subvert memory safety by storing a reference to the wrapped value. /// This is a deficiency which cannot be solved without Rust supporting HKT. /// It’d need something like `where F: (for<'f> FnOnce(T) -> U where T: 'f, U: 'f)`. /// /// The only class of transformation functions that can structurally be known to be safe in /// current Rust is those with no non-static environment; the `map` function embodies that /// constraint and should be used where possible instead of this function. /// /// # Example /// /// ``` /// use mucell::{MuCell, Ref}; /// /// let c = MuCell::new((5, 'b')); /// let b1: Ref<&(u32, char)> = c.borrow(); /// let b2: Ref<&u32> = Ref::map(b1, |t| &t.0); /// assert_eq!(*b2, 5) /// ``` #[inline] pub unsafe fn map_unsafe<U, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U> where F: FnOnce(T) -> U { Ref { _value: f(orig._value), _borrow: orig._borrow, } } /// This is a safe version of `map_unsafe`, /// imposing the constraint that `F` is `'static`. /// /// This is the only way to make it safe in a pre-HKT world: /// by preventing the closure from capturing any non-static environment. /// Anything beyond that will require caution to ensure safety. #[inline] pub fn map<U, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U> where F: FnOnce(T) -> U + 'static { unsafe { Ref::map_unsafe(orig, f) } } /// Make a new `Ref` for a optional component of the borrowed data, e.g. an /// enum variant. /// /// The `MuCell` is already immutably borrowed, so this cannot fail. /// /// This is an associated function that needs to be used as /// `Ref::filter_map(...)`. A method would interfere with methods of the /// same name on the contents of a `MuCell` used through `Deref`. /// /// # Memory unsafety /// /// This function is marked as unsafe because it is possible (though not the easiest /// thing) to subvert memory safety by storing a reference to the wrapped value. /// This is a deficiency which cannot be solved without Rust supporting HKT. /// It’d need something like `where F: (for<'f> FnOnce(T) -> Option<U> where T: 'f, U: 'f)`. /// /// The only class of transformation functions that can structurally be known to be safe in /// current Rust is those with no non-static environment; the `map` function embodies that /// constraint and should be used where possible instead of this function. /// /// # Example /// /// ``` /// use mucell::{MuCell, Ref}; /// /// let c = MuCell::new(Ok(5)); /// let b1: Ref<&Result<u32, ()>> = c.borrow(); /// let b2: Ref<&u32> = Ref::filter_map(b1, |o| o.as_ref().ok()).unwrap(); /// assert_eq!(*b2, 5) /// ``` #[inline] pub unsafe fn filter_map_unsafe<U, F>(orig: Ref<'b, T>, f: F) -> Option<Ref<'b, U>> where F: FnOnce(T) -> Option<U> { let borrow = orig._borrow; f(orig._value).map(move |new| Ref { _value: new, _borrow: borrow, }) } /// This is a safe version of `filter_map_unsafe`, /// imposing the constraint that `F` is `'static`. /// /// This is the only way to make it safe in a pre-HKT world: /// by preventing the closure from capturing any non-static environment. /// Anything beyond that will require caution to ensure safety. #[inline] pub fn filter_map<U, F>(orig: Ref<'b, T>, f: F) -> Option<Ref<'b, U>> where F: FnOnce(T) -> Option<U> + 'static { unsafe { Ref::filter_map_unsafe(orig, f) } } } unsafe impl<T: ?Sized> Send for MuCell<T> where T: Send {} impl<T: ?Sized + PartialEq> PartialEq for MuCell<T> { fn eq(&self, other: &MuCell<T>) -> bool { *self.borrow() == *other.borrow() } } impl<T: ?Sized + Eq> Eq for MuCell<T> { } impl<T: PartialOrd> PartialOrd for MuCell<T> { fn partial_cmp(&self, other: &MuCell<T>) -> Option<Ordering> { self.borrow().partial_cmp(&*other.borrow()) } } impl<T: Ord> Ord for MuCell<T> { fn cmp(&self, other: &MuCell<T>) -> Ordering { self.borrow().cmp(&*other.borrow()) } } impl<T: Default> Default for MuCell<T> { fn default() -> MuCell<T> { MuCell::new(Default::default()) } } impl<T: Clone> Clone for MuCell<T> { fn clone(&self) -> MuCell<T> { MuCell::new(self.borrow().clone()) } } macro_rules! impl_fmt { ($($trait_name:ident)*) => {$( impl<T: fmt::$trait_name> fmt::$trait_name for MuCell<T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { self.borrow().fmt(f) } } )*} } impl_fmt!(Display Debug Octal LowerHex UpperHex Pointer Binary LowerExp UpperExp); impl<T> Hash for MuCell<T> where T: Hash { fn hash<H: Hasher>(&self, state: &mut H) { self.borrow().hash(state) } } // RefCell doesn’t have PartialOrd, Ord, Hash or fmt::*. TODO: why not? #[test] #[should_panic] fn test_borrow_in_try_mutate() { let a = MuCell::new(()); a.try_mutate(|_| { let _ = a.borrow(); }); } #[test] fn test_try_mutate_in_try_mutate() { let a = MuCell::new(()); assert!(a.try_mutate(|_| assert!(!a.try_mutate(|_| unreachable!())))); } /// A demonstration of the subversion of memory safety using `map_unsafe`. #[test] fn unsafe_subversion_demo() { let cell = MuCell::new(0); let (borrow, mut x) = (cell.borrow(), Option::None); unsafe { Ref::map_unsafe(borrow, |a| x = Option::Some(a)); } assert_eq!(x, Option::Some(&0)); assert!(cell.try_mutate(|n| *n += 1)); assert_eq!(x, Option::Some(&1)); }