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//! This crate provides convenient wrappers for dealing with `&mut Option<T>`. There are two main types, `OptionGuard` and `OptionGuardMut`: //! //! ## `OptionGuard` //! //! Using `EmptyOptionExt::steal` on an `&mut Option<T>` produces the `T` from the option as well as an `OptionGuard`. If `OptionGuard::restore` is not called before the `OptionGuard` is dropped, then a panic will occur. //! //! ### Examples //! //! Calling `guard.restore()` puts the stolen value back into the original option: //! //! ```rust //! use empty_option::EmptyOptionExt; //! //! // A mutable option, from which we shall steal a value! //! let mut thing = Some(5); //! //! // Scope so that when we do `guard.restore()`, the mutable borrow on `thing` will end. //! { //! // Steal the value - we now have the guard and also a concrete `T` from our `Option<T>`. //! let (guard, five) = thing.steal(); //! //! assert_eq!(five, 5); //! //! // Move the value back into `thing` - we're done. //! guard.restore(6); //! } //! //! // The value is returned by `guard.restore()`. //! assert_eq!(thing, Some(6)); //! ``` //! //! But, if the guard is dropped instead, a runtime panic results. //! //! ```rust,should_panic //! use empty_option::EmptyOptionExt; //! //! let mut thing = Some(5); //! //! let (_, _) = thing.steal(); //! //! // Never return the value! //! ``` //! //! Calling `.steal()` on a `None` immediately panics: //! //! ```rust,should_panic //! let mut thing = None; //! //! // Panics here! //! let (guard, _) = thing.steal(); //! //! guard.restore(5); //! ``` //! //! ## `OptionGuardMut` //! //! Using `EmptyOptionExt::steal_mut` on an `&mut Option<T>` produces an `OptionGuardMut`, which dereferences to a `T`. To get the inner value out, `OptionGuardMut::into_inner` can be called. On `Drop`, if the `OptionGuardMut` is not consumed with `OptionGuardMut::into_inner`, the value in the `OptionGuardMut` will be returned to the `Option` that it was borrowed from. //! //! ### Examples //! //! Take a value from an option, which is automatically returned: //! //! ```rust //! use empty_option::EmptyOptionExt; //! //! let mut thing = Some(5); //! //! { //! let mut stolen = thing.steal_mut(); //! //! assert_eq!(*stolen, 5); //! //! *stolen = 6; //! } //! //! assert_eq!(thing, Some(6)); //! ``` //! //! If the guard is consumed, the value is never returned. //! //! ```rust //! use empty_option::EmptyOptionExt; //! //! let mut thing = Some(5); //! //! { //! // Keep the thing! //! let stolen = thing.steal_mut().into_inner(); //! //! assert_eq!(stolen, 5); //! } //! //! assert_eq!(thing, None); //! //! ``` //! //! Calling `steal_mut` on a `None` immediately panics: //! //! ```rust,should_panic //! let mut thing: Option<i32> = None; //! //! // Panics here! //! thing.steal_mut(); //! ``` use std::mem; use std::ops::{Deref, DerefMut}; /// Extension trait providing nice method sugar for `steal` and `steal_mut`. pub trait EmptyOptionExt { type Inner; /// Take a value out of an option, providing a guard which panics if the value is not returned. /// Panics on `None`. fn steal(&mut self) -> (OptionGuard<Self::Inner>, Self::Inner); /// Take a value out of an option, providing a guard which returns the value unless consumed by /// `OptionGuardMut::into_inner`. Panics on `None`. fn steal_mut<'a>(&'a mut self) -> OptionGuardMut<'a, Self::Inner>; } /// An option which has had its value taken. On `Drop`, `OptionGuard` will panic - in order to /// prevent a panic, the stolen value must be moved back in with `OptionGuard::restore`. /// /// This is useful if you are using an `Option` because you have a value which you need to take, /// and then deal with by-value, but you want to preserve the invariant that your optional value is /// always present. /// /// # Examples /// /// Calling `guard.restore()` puts the stolen value back into the original option: /// /// ``` /// # use empty_option::EmptyOptionExt; /// // A mutable option, from which we shall steal a value! /// let mut thing = Some(5); /// /// // Scope so that when we do `guard.restore()`, the mutable borrow on `thing` will end. /// { /// // Steal the value - we now have the guard and also a concrete `T` from our `Option<T>`. /// let (guard, five) = thing.steal(); /// /// assert_eq!(five, 5); /// /// // Move the value back into `thing` - we're done. /// guard.restore(6); /// } /// /// // The value is returned by `guard.restore()`. /// assert_eq!(thing, Some(6)); /// ``` /// /// But, if the guard is dropped instead, a runtime panic results. /// /// ```should_panic /// # use empty_option::EmptyOptionExt; /// let mut thing = Some(5); /// /// let (_, _) = thing.steal(); /// /// // Never return the value! /// ``` /// /// Calling `.steal()` on a `None` immediately panics: /// /// ```rust,should_panic /// let mut thing = None; /// /// // Panics here! /// let (guard, _) = thing.steal(); /// /// guard.restore(5); /// ``` pub struct OptionGuard<'a, T: 'a> { opt: &'a mut Option<T>, } impl<'a, T> Drop for OptionGuard<'a, T> { fn drop(&mut self) { panic!("`Some` value was never restored to a victimized Option!"); } } impl<'a, T> OptionGuard<'a, T> { fn new(opt: &'a mut Option<T>) -> OptionGuard<'a, T> { OptionGuard { opt } } /// Restore a stolen value to an `Option`. pub fn restore(self, obj: T) { *self.opt = Some(obj); mem::forget(self); } } /// A value taken from an `Option<T>`. `OptionGuardMut<T>` dereferences to a `T`, and the inner `T` /// can be moved out with `OptionGuardMut::into_inner`. When dropped, the `OptionGuardMut` moves /// the taken value back into the `Option` it came from. /// /// # Examples /// /// Take a value from an option, which is automatically returned: /// /// ``` /// # use empty_option::EmptyOptionExt; /// let mut thing = Some(5); /// /// { /// let mut stolen = thing.steal_mut(); /// /// assert_eq!(*stolen, 5); /// /// *stolen = 6; /// } /// /// assert_eq!(thing, Some(6)); /// ``` /// /// If the guard is consumed, the value is never returned. /// /// ``` /// # use empty_option::EmptyOptionExt; /// let mut thing = Some(5); /// /// { /// // Keep the thing! /// let stolen = thing.steal_mut().into_inner(); /// /// assert_eq!(stolen, 5); /// } /// /// assert_eq!(thing, None); /// ``` /// /// Calling `steal_mut` on a `None` immediately panics: /// /// ```rust,should_panic /// let mut thing: Option<i32> = None; /// /// // Panics here! /// thing.steal_mut(); /// ``` pub struct OptionGuardMut<'a, T: 'a> { origin: &'a mut Option<T>, value: Option<T>, } impl<'a, T> Drop for OptionGuardMut<'a, T> { fn drop(&mut self) { *self.origin = self.value.take(); } } impl<'a, T> OptionGuardMut<'a, T> { /// Keep the value stolen from the `Option` and do not return it. pub fn into_inner(mut self) -> T { self.value.take().unwrap() } } impl<'a, T> Deref for OptionGuardMut<'a, T> { type Target = T; fn deref(&self) -> &T { self.value.as_ref().unwrap() } } impl<'a, T> DerefMut for OptionGuardMut<'a, T> { fn deref_mut(&mut self) -> &mut T { self.value.as_mut().unwrap() } } impl<T> EmptyOptionExt for Option<T> { type Inner = T; fn steal(&mut self) -> (OptionGuard<T>, T) { let value = self.take().expect("attempted to steal from None"); (OptionGuard::new(self), value) } fn steal_mut(&mut self) -> OptionGuardMut<T> { let value = Some(self.take().expect("attempted to steal from None")); OptionGuardMut { origin: self, value, } } } #[cfg(test)] mod tests { use super::*; #[test] fn catch_and_release() { let mut thing = Some(5); { let (guard, five) = thing.steal(); assert_eq!(five, 5); guard.restore(6); } assert_eq!(thing, Some(6)); } #[test] #[should_panic] fn catch_and_keep() { let mut thing = Some(5); let (_, _) = thing.steal(); // Never return the value! } #[test] #[should_panic] fn catch_from_none() { let mut thing = None; let (guard, _) = thing.steal(); guard.restore(5); } #[test] fn mut_and_release() { let mut thing = Some(5); { let mut stolen = thing.steal_mut(); assert_eq!(*stolen, 5); *stolen = 6; } assert_eq!(thing, Some(6)); } #[test] fn mut_and_keep() { let mut thing = Some(5); { // Keep the thing! let stolen = thing.steal_mut().into_inner(); assert_eq!(stolen, 5); } assert_eq!(thing, None); } #[test] #[should_panic] fn mut_from_none() { let mut thing: Option<i32> = None; thing.steal_mut(); } }