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#![warn(missing_docs)] #![warn(rust_2018_idioms)] //! # Better Any //! //! Rust RFC for `non_static_type_id` feature has been reverted. //! Which means in foreseeable future there will be no built-in way in rust to get type id for non-static type //! let alone safely use it to downcast to a particular type. //! //! This crate provides tools to do these things safely for types with single lifetime. //! Although looks like it is technically possible to extend this approach for multiple lifetimes, //! consistent api and derive macro would be much harder to create and use because of the necessity //! to properly handle lifetime relations. //! Feel free to create an issue if you have actual use case where you need this functionality for multiple lifetimes. //! //! Also it has better downcasting that allows you do downcast not just from `dyn Tid` (like `dyn Any`) but from //! any trait object that implements `Tid`. //! So there is no more need to extend your traits with` fn to_any(&self)-> &dyn Any` //! //! MSRV: `1.41.0-stable` //! //! ### Usage //! //! Basically in places where before you have used `dyn Any` you can use `dyn Tid<'a>` //! - If your type is generic you should derive `Tid` implementation for it with `Tid` derive macro. //! Then to retrieve back concrete type `<dyn Tid>::downcast_*` methods should be used. //! - If your type is not generic/implements Any you can create `dyn Any` and convert it to `dyn Tid`. //! Then to retrieve back concrete type `<dyn Tid>::downcast_any_*` methods should be used //! - If your type is not generic and local to your crate you also can derive `Tid` but then you need to be careful //! to use methods that corresponds to the way you create `dyn Tid` for that particular type. //! Otherwise downcasting will return `None`. //! //! If all your types can implement `Tid` to avoid confusion //! recommended way is to use first option even if some types implement `Any`. //! If there are some types that implement `Any` and can't implement `Tid` (i.e. types from other library), //! recommended way is to use second option for all types that implement `Any` to reduce confusion to minimum. //! //! ### Interoperability with Any //! //! Unfortunately you can't just use `Tid` everywhere because currently it is impossible //! to implement `Tid` for `T:Any` since it would conflict with any other possible `Tid` implementation. //! To overcome this limitation there is a `From` impl to go from `dyn Any` to `dyn Tid` //! But `Any` and `Tid` deliberately return different type ids because otherwise `Type<'a>` and `Type<'static>` //! would be indistinguishable and it would allow to go from `Type<'static>` to `Type<'a>` via `dyn Tid` //! which is obviously unsound for invariant and contravariant structs. //! //! Although if you are using `dyn Trait` where `Trait:Tid` all of this wouldn't work, //! and you are left with `Tid` only. //! //! ### Safety //! //! It is safe because created trait object preserve lifetime information, //! thus allowing us to safely downcast with proper lifetime. //! Otherwise internally it is plain old `Any`. use std::any::{Any, TypeId}; use std::marker::PhantomData; /// Attribute macro that makes your implementation of `TidAble` safe /// Use it when you can't use derive e.g. for trait object. /// /// ```rust /// # use better_any::{TidAble,impl_tid}; /// trait Trait<'a>{} /// #[impl_tid] /// impl<'a> TidAble<'a> for Box<dyn Trait<'a> + 'a>{} /// ``` pub use better_typeid_derive::impl_tid; /// Derive macro to implement traits from this crate /// /// It checks if it is safe to implement `Tid` for your struct /// Also it adds `:TidAble<'a>` bound on type parameters /// unless your type parameter already has **explicit** `'static` bound pub use better_typeid_derive::Tid; /// This trait indicates that you can substitute this type as a type parameter to /// another type so that resulting type could implement `Tid`. /// /// So if you don't have such generic types, just use `Tid` everywhere, /// you don't need to use this trait at all. /// /// Only this trait is actually being implemented on user side. /// Other traits are mostly just blanket implementations over X:TidAble<'a> /// /// Note that this trait interfere with object safety, so you shouldn't use it as a super trait /// if you are going to make a trait object. Formally it is still object safe, /// but you can't make a trait object from it without specifying internal associate type /// like: `dyn TidAble<'a,Static=SomeType>` which make this trait object effectively useless. /// /// Unsafe because safety of this crate relies on correctness of this trait implementation. /// There are several safe ways to implement it: /// - `#[derive(Tid)]` derive macro /// - `type_id` declarative macro /// - `impl_tid` attribute macro // we need to have associate type because it allows TypeIdAdjuster to be a private type // and allows to implement it for generic types // it has lifetime and depends on Tid because it would be practically useless as a standalone trait // because even though user would be able to get type id for more types, // any action based on it would be unsound without checking on lifetimes pub unsafe trait TidAble<'a>: Tid<'a> { /// Implementation detail #[doc(hidden)] type Static: ?Sized + Any; } /// Extension trait that contains actual downcasting methods. /// /// Use methods from this trait only if `dyn Tid` was created directly from `T` for this particular `T` pub trait TidExt<'a> { /// Returns true if type behind self is equal to the type of T. fn is<T: Tid<'a>>(&self) -> bool; /// Attempts to downcast self to `T` behind reference fn downcast_ref<'b, T: Tid<'a>>(&'b self) -> Option<&'b T>; /// Attempts to downcast self to `T` behind mutable reference fn downcast_mut<'b, T: Tid<'a>>(&'b mut self) -> Option<&'b mut T>; /// Attempts to downcast self to `T` behind `Rc` pointer fn downcast_rc<T: Tid<'a>>(self: Rc<Self>) -> Result<Rc<T>, Rc<Self>>; /// Attempts to downcast self to `T` behind `Arc` pointer fn downcast_arc<T: Tid<'a>>(self: Arc<Self>) -> Result<Arc<T>, Arc<Self>>; /// Attempts to downcast self to `T` behind `Box` pointer fn downcast_box<T: Tid<'a>>(self: Box<Self>) -> Result<Box<T>, Box<Self>>; } /// If X is Sized then any of those calls is optimized to no-op because both T and Self are known statically. /// Useful if you have generic code that you want to behave differently depending on which /// concrete type replaces type parameter. Usually there are better ways to do this like specialization, /// but sometimes it can be the only way. impl<'a, X: ?Sized + Tid<'a>> TidExt<'a> for X { fn is<T: Tid<'a>>(&self) -> bool { self.self_id() == T::id() } #[inline] fn downcast_ref<'b, T: Tid<'a>>(&'b self) -> Option<&'b T> { // Tid<'a> is implemented only for types with lifetime 'a // so we can safely cast type back because lifetime invariant is preserved. if self.self_id() == T::id() { Some(unsafe { &*(self as *const _ as *const T) }) } else { None } } #[inline] fn downcast_mut<'b, T: Tid<'a>>(&'b mut self) -> Option<&'b mut T> { // see downcast_ref if self.self_id() == T::id() { Some(unsafe { &mut *(self as *mut _ as *mut T) }) } else { None } } #[inline] fn downcast_rc<T: Tid<'a>>(self: Rc<Self>) -> Result<Rc<T>, Rc<Self>> { if T::id() == self.self_id() { unsafe { Ok(Rc::from_raw(Rc::into_raw(self) as *const _)) } } else { Err(self) } } #[inline] fn downcast_arc<T: Tid<'a>>(self: Arc<Self>) -> Result<Arc<T>, Arc<Self>> { if T::id() == self.self_id() { unsafe { Ok(Arc::from_raw(Arc::into_raw(self) as *const _)) } } else { Err(self) } } #[inline] fn downcast_box<T: Tid<'a>>(self: Box<Self>) -> Result<Box<T>, Box<Self>> { if T::id() == self.self_id() { unsafe { Ok(Box::from_raw(Box::into_raw(self) as *mut _)) } } else { Err(self) } } } /// This trait indicates that this type can be converted to /// trait object with typeid while preserving lifetime information. /// Extends `Any` functionality for types with single lifetime /// /// Use it only as a `dyn Tid` or as super trait when you need to create trait object. /// In all other places use `TidAble<'a>`. /// /// Lifetime here is necessary to make `dyn Tid<'a> + 'a` invariant over `'a`. pub unsafe trait Tid<'a>: 'a { /// Returns type id of the type of `self` /// /// Note that returned type id is guaranteed to be different from provided by `Any`. /// It is necessary for the creation of `dyn Tid` from `dyn Any` to be sound. fn self_id(&self) -> TypeId; /// Returns type id of this type fn id() -> TypeId where Self: Sized; } unsafe impl<'a, T: ?Sized + TidAble<'a>> Tid<'a> for T { #[inline] fn self_id(&self) -> TypeId { adjust_id::<T::Static>() } #[inline] fn id() -> TypeId where Self: Sized, { adjust_id::<T::Static>() } } // this exists just to make TypeIdAdjuster private so type id difference between // `dyn Any` and `dyn Tid` would be guaranteed #[inline(always)] fn adjust_id<T: ?Sized + Any>() -> TypeId { TypeId::of::<TypeIdAdjuster<T>>() } /// Returns type id of `T` /// /// Use it only if `Tid::id()` is not enough when `T` is not sized. #[inline] pub fn typeid_of<'a, T: ?Sized + TidAble<'a>>() -> TypeId { adjust_id::<T::Static>() } impl<'a> From<Box<dyn Any>> for Box<dyn Tid<'a> + 'a> { #[inline] fn from(f: Box<dyn Any>) -> Self { // it should be safe because both Any and Tid have single entry in vtable // so there is no need to rely on the function order stability of the different traits // also, despite particular trait object layout is not stable, it still should be // the same for different trait objects in the same compilation pass. //todo find out more: // technically i think vtable also has a drop entry so different order still can happen // in theory but practically it is either always first or always last so it // shouldn't influence order unsafe { core::mem::transmute(f) } } } impl<'a: 'b, 'b> From<&'b dyn Any> for &'b (dyn Tid<'a> + 'a) { #[inline] fn from(f: &'b dyn Any) -> Self { unsafe { core::mem::transmute(f) } } } impl<'a: 'b, 'b> From<&'b mut dyn Any> for &'b mut (dyn Tid<'a> + 'a) { #[inline] fn from(f: &'b mut dyn Any) -> Self { unsafe { core::mem::transmute(f) } } } // Reverse is possible only for 'static // because otherwise even though user can't access type with lifetime because of different type id // drop still can be called after the end of lifetime. impl Into<Box<dyn Any>> for Box<dyn Tid<'static>> { fn into(self) -> Box<dyn Any> { unsafe { core::mem::transmute(self) } } } //wrapper to distinguish type ids coming from `dyn Any` and `dyn Tid` struct TypeIdAdjuster<T: ?Sized>(PhantomData<T>); impl<'a> dyn Tid<'a> + 'a { /// Tries to downcast Self to `T` /// /// Use it only if `dyn Tid` was created from `dyn Any` for this particular `T` /// /// ```rust /// # use std::any::Any; /// # use better_any::{Tid, TidAble, TidExt}; /// #[derive(Tid)] /// struct S; /// /// let a = &S as &dyn Any; /// let from_any: &dyn Tid = a.into(); /// assert!(from_any.downcast_any_ref::<S>().is_some()); /// assert!(from_any.downcast_ref::<S>().is_none()); /// /// let direct = &S as &dyn Tid; /// assert!(direct.downcast_any_ref::<S>().is_none()); /// assert!(direct.downcast_ref::<S>().is_some()); /// ``` #[inline] pub fn downcast_any_ref<T: Any>(&self) -> Option<&T> { // this condition can be true if and only if dyn Tid was created from dyn Any // because otherwise TypeId would be from TypeIdAdjuster<T> which cant be equal to the one of T // thus it is safe to increase lifetime back to 'static if self.self_id() == TypeId::of::<T>() { Some(unsafe { &*(self as *const _ as *const T) }) } else { None } } /// Use it only if `dyn Tid` was created from `dyn Any` for this particular `T` #[inline] pub fn downcast_any_mut<T: Any>(&mut self) -> Option<&mut T> { // see downcast_any_ref if self.self_id() == TypeId::of::<T>() { Some(unsafe { &mut *(self as *mut _ as *mut T) }) } else { None } } } macro_rules! stdimpl { ($struct: tt) => { unsafe impl<'a, T: ?Sized + TidAble<'a>> TidAble<'a> for $struct<T> { type Static = $struct<T::Static>; } }; } use std::cell::*; use std::rc::*; use std::sync::*; stdimpl!(Box); stdimpl!(Rc); stdimpl!(RefCell); stdimpl!(Cell); stdimpl!(Arc); stdimpl!(Mutex); stdimpl!(RwLock); #[impl_tid] impl<'a, T> TidAble<'a> for Option<T> {} #[impl_tid] impl<'a, T> TidAble<'a> for Vec<T> {} #[impl_tid] impl<'a, T, E> TidAble<'a> for Result<T, E> {} #[impl_tid] impl<'a> TidAble<'a> for dyn Tid<'a> + 'a {} // the logic behind this implementations is to connect Any with Tid somehow // I would say that if T:Any there is no much need to implement Tid<'a> for T. // because Any functionality already exists and `dyn Any` can be converted to `dyn Tid`. // unfortunately there is no way to implement Tid<'a> for T:Any, // which make impl<'a, T: Tid<'a>> Tid<'a> for &'a T {} almost useless // because it wouldn't work even for &'a i32 // This way we don't require user to newtype wrapping simple references. // And more complex types are usually not used as a type parameters directly. unsafe impl<'a, T: Any> TidAble<'a> for &'a T { type Static = &'static T; } unsafe impl<'a, T: Any> TidAble<'a> for &'a mut T { type Static = &'static mut T; } // whole impl can be via this macro but it would require to use `paste` crate // which is already a proc macro, so there is no much reason to do force everything to declarative macro // /// Simple version of derive macro to not pull all proc macro dependencies in simple cases /// when all structs are not generic /// /// ```rust /// use better_any::type_id; /// struct S; /// type_id!(S); /// struct F<'a>(&'a str); /// type_id!(F<'a>); /// ``` #[macro_export] macro_rules! type_id { ($struct: ident) => { unsafe impl<'a> $crate::TidAble<'a> for $struct { type Static = $struct; } }; ($struct: ident < $lt: lifetime >) => { unsafe impl<'a> $crate::TidAble<'a> for $struct<'a> { type Static = $struct<'static>; } }; // ($struct: ident < $($type:ident),* >) => { // unsafe impl<'a,$($type:$crate::TidAble<'a>),*> $crate::TidAble<'a> for $struct<$($type,)*> { // type Static = HygieneUnique<$($type::Static),*>; // } // // pub struct HygieneUnique<$($type),*>($(core::marker::PhantomData<$type>),*); // }; } //todo: not sure if worth it. // If only both T and X could be ?Sized it would DRY From impl as well // on the other hand it might have problems with type inference // // unsafe trait Cast<T: Deref>: Deref { // unsafe fn cast(self) -> T; // } // // unsafe impl<T: ?Sized, X> Cast<Rc<X>> for Rc<T> { // unsafe fn cast(self) -> Rc<X> { // Rc::from_raw(Rc::into_raw(self) as *const _) // } // } // // unsafe impl<T: ?Sized, X> Cast<Arc<X>> for Arc<T> { // unsafe fn cast(self) -> Arc<X> { // Arc::from_raw(Arc::into_raw(self) as *const _) // } // } // // unsafe impl<T: ?Sized, X> Cast<Box<X>> for Box<T> { // unsafe fn cast(self) -> Box<X> { // Box::from_raw(Box::into_raw(self) as *mut _) // } // } // // // impl<'a, F: Cast<T, Target = dyn Any>, T: Deref<Target = dyn Tid<'a> + 'a>> From<F> for T { // // fn from(f: F) -> Self { // // unsafe { f.cast() } // // } // // } // // fn downcast<'a, F: Cast<T>, T:Deref>(f: F) -> Result<T, F> // where // F::Target: Tid<'a>, // T::Target: Tid<'a>, // { // if <F::Target>::id() == f.self_id() { // unsafe { Ok(f.cast()) } // } else { // Err(f) // } // }