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Add the Provider api to core::any
Signed-off-by: Nick Cameron <nrc@ncameron.org>
This commit is contained in:
Nick Cameron
parent
6609c6734d
commit
bb5db85f74
@ -1,5 +1,9 @@
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//! This module implements the `Any` trait, which enables dynamic typing
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//! of any `'static` type through runtime reflection.
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//! This module contains the `Any` trait, which enables dynamic typing
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//! of any `'static` type through runtime reflection. It also contains the
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//! `Provider` trait and accompanying API, which enable trait objects to provide
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//! data based on typed requests, an alternate form of runtime reflection.
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//!
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//! # `Any` and `TypeId`
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//!
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//! `Any` itself can be used to get a `TypeId`, and has more features when used
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//! as a trait object. As `&dyn Any` (a borrowed trait object), it has the `is`
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@ -37,7 +41,7 @@
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//! assert_eq!(boxed_id, TypeId::of::<Box<dyn Any>>());
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//! ```
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//!
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//! # Examples
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//! ## Examples
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//!
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//! Consider a situation where we want to log out a value passed to a function.
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//! We know the value we're working on implements Debug, but we don't know its
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@ -81,11 +85,76 @@
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//! do_work(&my_i8);
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//! }
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//! ```
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//!
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//! # `Provider` and `Demand`
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//!
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//! `Provider` and the associated APIs support generic, type-driven access to data, and a mechanism
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//! for implementers to provide such data. The key parts of the interface are the `Provider`
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//! trait for objects which can provide data, and the [`request_value`] and [`request_ref`]
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//! functions for requesting data from an object which implements `Provider`. Note that end users
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//! should not call `request_*` directly, they are helper functions for intermediate implementers
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//! to use to implement a user-facing interface.
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//!
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//! Typically, a data provider is a trait object of a trait which extends `Provider`. A user will
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//! request data from the trait object by specifying the type.
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//!
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//! ## Data flow
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//!
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//! * A user requests an object, which is delegated to `request_value` or `request_ref`
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//! * `request_*` creates a `Demand` object and passes it to `Provider::provide`
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//! * The object provider's implementation of `Provider::provide` tries providing values of
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//! different types using `Demand::provide_*`. If the type matches the type requested by
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//! the user, it will be stored in the `Demand` object.
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//! * `request_*` unpacks the `Demand` object and returns any stored value to the user.
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//!
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//! ## Examples
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//!
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//! ```
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//! # #![allow(incomplete_features)]
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//! # #![feature(provide_any)]
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//! # #![feature(trait_upcasting)]
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//! use std::any::{Provider, Demand, request_ref};
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//!
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//! // Definition of MyTrait
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//! trait MyTrait: Provider {
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//! // ...
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//! }
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//!
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//! // Methods on `MyTrait` trait objects.
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//! impl dyn MyTrait + '_ {
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//! /// Common case: get a reference to a field of the error.
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//! pub fn get_context_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
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//! request_ref::<T>(self)
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//! }
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//! }
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//!
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//! // Downstream implementation of `MyTrait` and `Provider`.
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//! # struct SomeConcreteType { some_string: String }
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//! impl MyTrait for SomeConcreteType {
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//! // ...
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//! }
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//!
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//! impl Provider for SomeConcreteType {
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//! fn provide<'a>(&'a self, req: &mut Demand<'a>) {
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//! req.provide_ref::<String>(&self.some_string);
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//! }
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//! }
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//!
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//! // Downstream usage of `MyTrait`.
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//! fn use_my_trait(obj: &dyn MyTrait) {
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//! // Request a &String from obj.
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//! let _ = obj.get_context_ref::<String>().unwrap();
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//! }
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//! ```
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//!
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//! In this example, if the concrete type of `obj` in `use_my_trait` is `SomeConcreteType`, then
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//! the `get_context_ref` call will return a reference to `obj.some_string`.
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#![stable(feature = "rust1", since = "1.0.0")]
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use crate::fmt;
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use crate::intrinsics;
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use crate::mem::transmute;
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///////////////////////////////////////////////////////////////////////////////
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// Any trait
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@ -700,3 +769,201 @@ pub const fn type_name<T: ?Sized>() -> &'static str {
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pub const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str {
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type_name::<T>()
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}
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///////////////////////////////////////////////////////////////////////////////
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// Provider trait
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///////////////////////////////////////////////////////////////////////////////
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/// Trait implemented by a type which can dynamically provide values based on type.
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#[unstable(feature = "provide_any", issue = "none")]
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pub trait Provider {
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/// Object providers should implement this method to provide *all* values they are able to
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/// provide using `req`.
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#[unstable(feature = "provide_any", issue = "none")]
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fn provide<'a>(&'a self, req: &mut Demand<'a>);
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}
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/// Request a value from the `Provider`.
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#[unstable(feature = "provide_any", issue = "none")]
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pub fn request_value<'a, T: 'static>(provider: &'a dyn Provider) -> Option<T> {
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request_by_type_tag::<'a, tags::Value<T>>(provider)
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}
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/// Request a reference from the `Provider`.
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#[unstable(feature = "provide_any", issue = "none")]
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pub fn request_ref<'a, T: ?Sized + 'static>(provider: &'a dyn Provider) -> Option<&'a T> {
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request_by_type_tag::<'a, tags::Ref<tags::MaybeSizedValue<T>>>(provider)
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}
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/// Request a specific value by tag from the `Provider`.
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fn request_by_type_tag<'a, I>(provider: &'a dyn Provider) -> Option<I::Reified>
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where
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I: tags::Type<'a>,
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{
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let mut tagged = TaggedOption::<'a, I>(None);
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provider.provide(tagged.as_demand());
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tagged.0
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}
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///////////////////////////////////////////////////////////////////////////////
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// Demand and its methods
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///////////////////////////////////////////////////////////////////////////////
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/// A helper object for providing objects by type.
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///
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/// An object provider provides values by calling this type's provide methods.
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#[allow(missing_debug_implementations)]
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#[unstable(feature = "provide_any", issue = "none")]
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// SAFETY: `TaggedOption::as_demand` relies on this precise definition.
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#[repr(transparent)]
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pub struct Demand<'a>(dyn Erased<'a> + 'a);
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impl<'a> Demand<'a> {
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/// Provide a value or other type with only static lifetimes.
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#[unstable(feature = "provide_any", issue = "none")]
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pub fn provide_value<T, F>(&mut self, fulfil: F) -> &mut Self
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where
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T: 'static,
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F: FnOnce() -> T,
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{
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self.provide_with::<tags::Value<T>, F>(fulfil)
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}
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/// Provide a reference, note that the referee type must be bounded by `'static`, but may be unsized.
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#[unstable(feature = "provide_any", issue = "none")]
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pub fn provide_ref<T: ?Sized + 'static>(&mut self, value: &'a T) -> &mut Self {
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self.provide::<tags::Ref<tags::MaybeSizedValue<T>>>(value)
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}
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/// Provide a value with the given `Type` tag.
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fn provide<I>(&mut self, value: I::Reified) -> &mut Self
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where
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I: tags::Type<'a>,
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{
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if let Some(res @ TaggedOption(None)) = self.0.downcast_mut::<I>() {
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res.0 = Some(value);
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}
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self
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}
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/// Provide a value with the given `Type` tag, using a closure to prevent unnecessary work.
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fn provide_with<I, F>(&mut self, fulfil: F) -> &mut Self
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where
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I: tags::Type<'a>,
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F: FnOnce() -> I::Reified,
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{
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if let Some(res @ TaggedOption(None)) = self.0.downcast_mut::<I>() {
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res.0 = Some(fulfil());
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}
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self
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}
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}
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///////////////////////////////////////////////////////////////////////////////
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// Type tags
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///////////////////////////////////////////////////////////////////////////////
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mod tags {
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//! Type tags are used to identify a type using a separate value. This module includes type tags
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//! for some very common types.
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//!
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//! Many users of the provider APIs will not need to use type tags at all. But if you want to
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//! use them with more complex types (typically those including lifetime parameters), you will
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//! need to write your own tags.
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use crate::marker::PhantomData;
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/// This trait is implemented by specific tag types in order to allow
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/// describing a type which can be requested for a given lifetime `'a`.
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///
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/// A few example implementations for type-driven tags can be found in this
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/// module, although crates may also implement their own tags for more
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/// complex types with internal lifetimes.
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pub trait Type<'a>: Sized + 'static {
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/// The type of values which may be tagged by this tag for the given
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/// lifetime.
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type Reified: 'a;
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}
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/// Similar to the [`Type`] trait, but represents a type which may be unsized (i.e., has a
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/// `'Sized` bound). E.g., `str`.
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pub trait MaybeSizedType<'a>: Sized + 'static {
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type Reified: 'a + ?Sized;
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}
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impl<'a, T: Type<'a>> MaybeSizedType<'a> for T {
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type Reified = T::Reified;
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}
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/// Type-based tag for types bounded by `'static`, i.e., with no borrowed element.
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#[derive(Debug)]
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pub struct Value<T: 'static>(PhantomData<T>);
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impl<'a, T: 'static> Type<'a> for Value<T> {
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type Reified = T;
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}
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/// Type-based tag similar to [`Value`] but which may be unsized (i.e., has a `'Sized` bound).
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#[derive(Debug)]
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pub struct MaybeSizedValue<T: ?Sized + 'static>(PhantomData<T>);
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impl<'a, T: ?Sized + 'static> MaybeSizedType<'a> for MaybeSizedValue<T> {
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type Reified = T;
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}
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/// Type-based tag for `&'a T` types.
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#[derive(Debug)]
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pub struct Ref<I>(PhantomData<I>);
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impl<'a, I: MaybeSizedType<'a>> Type<'a> for Ref<I> {
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type Reified = &'a I::Reified;
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}
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}
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/// An `Option` with a type tag `I`.
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///
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/// Since this struct implements `Erased`, the type can be erased to make a dynamically typed
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/// option. The type can be checked dynamically using `Erased::tag_id` and since this is statically
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/// checked for the concrete type, there is some degree of type safety.
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#[repr(transparent)]
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struct TaggedOption<'a, I: tags::Type<'a>>(Option<I::Reified>);
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impl<'a, I: tags::Type<'a>> TaggedOption<'a, I> {
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fn as_demand(&mut self) -> &mut Demand<'a> {
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// SAFETY: transmuting `&mut (dyn Erased<'a> + 'a)` to `&mut Demand<'a>` is safe since
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// `Demand` is repr(transparent) and holds only a `dyn Erased<'a> + 'a`.
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unsafe { transmute(self as &mut (dyn Erased<'a> + 'a)) }
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}
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}
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/// Represents a type-erased but identifiable object.
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///
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/// This trait is exclusively implemented by the `TaggedOption` type.
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trait Erased<'a>: 'a {
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/// The `TypeId` of the erased type.
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fn tag_id(&self) -> TypeId;
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}
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impl<'a, I: tags::Type<'a>> Erased<'a> for TaggedOption<'a, I> {
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fn tag_id(&self) -> TypeId {
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TypeId::of::<I>()
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}
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}
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#[unstable(feature = "provide_any", issue = "none")]
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impl<'a> dyn Erased<'a> {
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/// Returns some reference to the dynamic value if it is tagged with `I`,
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/// or `None` if it isn't.
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#[inline]
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fn downcast_mut<I>(&mut self) -> Option<&mut TaggedOption<'a, I>>
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where
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I: tags::Type<'a>,
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{
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if self.tag_id() == TypeId::of::<I>() {
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// SAFETY: Just checked whether we're pointing to an I.
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Some(unsafe { &mut *(self as *mut Self as *mut TaggedOption<'a, I>) })
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} else {
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None
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}
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}
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}
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@ -130,3 +130,25 @@ fn distinct_type_names() {
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assert_ne!(type_name_of_val(Velocity), type_name_of_val(Velocity(0.0, -9.8)),);
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}
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// Test the `Provider` API.
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struct SomeConcreteType {
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some_string: String,
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}
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impl Provider for SomeConcreteType {
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fn provide<'a>(&'a self, req: &mut Demand<'a>) {
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req.provide_ref::<String>(&self.some_string)
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.provide_value::<String, _>(|| "bye".to_owned());
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}
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}
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#[test]
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fn test_provider() {
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let obj: &dyn Provider = &SomeConcreteType { some_string: "hello".to_owned() };
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assert_eq!(&**request_ref::<String>(obj).unwrap(), "hello");
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assert_eq!(&*request_value::<String>(obj).unwrap(), "bye");
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assert_eq!(request_value::<u8>(obj), None);
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}
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@ -97,6 +97,7 @@
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#![feature(const_slice_from_ref)]
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#![feature(waker_getters)]
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#![feature(slice_flatten)]
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#![feature(provide_any)]
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#![deny(unsafe_op_in_unsafe_fn)]
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extern crate test;
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