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Auto merge of #91970 - nrc:provide-any, r=scottmcm
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Add the Provider api to core::any

This is an implementation of [RFC 3192](rust-lang/rfcs#3192) ~~(which is yet to be merged, thus why this is a draft PR)~~. It adds an API for type-driven requests and provision of data from trait objects. A primary use case is for the `Error` trait, though that is not implemented in this PR. The only major difference to the RFC is that the functionality is added to the `any` module, rather than being in a sibling `provide_any` module (as discussed in the RFC thread).

~~Still todo: improve documentation on items, including adding examples.~~

cc `@yaahc`
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bors committed Jun 10, 2022
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376 changes: 373 additions & 3 deletions core/src/any.rs
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@@ -1,5 +1,9 @@
//! This module implements the `Any` trait, which enables dynamic typing
//! of any `'static` type through runtime reflection.
//! This module contains the `Any` trait, which enables dynamic typing
//! of any `'static` type through runtime reflection. It also contains the
//! `Provider` trait and accompanying API, which enable trait objects to provide
//! data based on typed requests, an alternate form of runtime reflection.
//!
//! # `Any` and `TypeId`
//!
//! `Any` itself can be used to get a `TypeId`, and has more features when used
//! as a trait object. As `&dyn Any` (a borrowed trait object), it has the `is`
Expand Down Expand Up @@ -37,7 +41,7 @@
//! assert_eq!(boxed_id, TypeId::of::<Box<dyn Any>>());
//! ```
//!
//! # Examples
//! ## Examples
//!
//! Consider a situation where we want to log out a value passed to a function.
//! We know the value we're working on implements Debug, but we don't know its
Expand Down Expand Up @@ -81,6 +85,73 @@
//! do_work(&my_i8);
//! }
//! ```
//!
//! # `Provider` and `Demand`
//!
//! `Provider` and the associated APIs support generic, type-driven access to data, and a mechanism
//! for implementers to provide such data. The key parts of the interface are the `Provider`
//! trait for objects which can provide data, and the [`request_value`] and [`request_ref`]
//! functions for requesting data from an object which implements `Provider`. Generally, end users
//! should not call `request_*` directly, they are helper functions for intermediate implementers
//! to use to implement a user-facing interface. This is purely for the sake of ergonomics, there is
//! safety concern here; intermediate implementers can typically support methods rather than
//! free functions and use more specific names.
//!
//! Typically, a data provider is a trait object of a trait which extends `Provider`. A user will
//! request data from a trait object by specifying the type of the data.
//!
//! ## Data flow
//!
//! * A user requests an object of a specific type, which is delegated to `request_value` or
//! `request_ref`
//! * `request_*` creates a `Demand` object and passes it to `Provider::provide`
//! * The data provider's implementation of `Provider::provide` tries providing values of
//! different types using `Demand::provide_*`. If the type matches the type requested by
//! the user, the value will be stored in the `Demand` object.
//! * `request_*` unpacks the `Demand` object and returns any stored value to the user.
//!
//! ## Examples
//!
//! ```
//! # #![feature(provide_any)]
//! use std::any::{Provider, Demand, request_ref};
//!
//! // Definition of MyTrait, a data provider.
//! trait MyTrait: Provider {
//! // ...
//! }
//!
//! // Methods on `MyTrait` trait objects.
//! impl dyn MyTrait + '_ {
//! /// Get a reference to a field of the implementing struct.
//! pub fn get_context_by_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
//! request_ref::<T, _>(self)
//! }
//! }
//!
//! // Downstream implementation of `MyTrait` and `Provider`.
//! # struct SomeConcreteType { some_string: String }
//! impl MyTrait for SomeConcreteType {
//! // ...
//! }
//!
//! impl Provider for SomeConcreteType {
//! fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
//! // Provide a string reference. We could provide multiple values with
//! // different types here.
//! demand.provide_ref::<String>(&self.some_string);
//! }
//! }
//!
//! // Downstream usage of `MyTrait`.
//! fn use_my_trait(obj: &dyn MyTrait) {
//! // Request a &String from obj.
//! let _ = obj.get_context_by_ref::<String>().unwrap();
//! }
//! ```
//!
//! In this example, if the concrete type of `obj` in `use_my_trait` is `SomeConcreteType`, then
//! the `get_context_ref` call will return a reference to `obj.some_string` with type `&String`.

#![stable(feature = "rust1", since = "1.0.0")]

Expand Down Expand Up @@ -700,3 +771,302 @@ pub const fn type_name<T: ?Sized>() -> &'static str {
pub const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str {
type_name::<T>()
}

///////////////////////////////////////////////////////////////////////////////
// Provider trait
///////////////////////////////////////////////////////////////////////////////

/// Trait implemented by a type which can dynamically provide values based on type.
#[unstable(feature = "provide_any", issue = "96024")]
pub trait Provider {
/// Data providers should implement this method to provide *all* values they are able to
/// provide by using `demand`.
///
/// Note that the `provide_*` methods on `Demand` have short-circuit semantics: if an earlier
/// method has successfully provided a value, then later methods will not get an opportunity to
/// provide.
///
/// # Examples
///
/// Provides a reference to a field with type `String` as a `&str`, and a value of
/// type `i32`.
///
/// ```rust
/// # #![feature(provide_any)]
/// use std::any::{Provider, Demand};
/// # struct SomeConcreteType { field: String, num_field: i32 }
///
/// impl Provider for SomeConcreteType {
/// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
/// demand.provide_ref::<str>(&self.field)
/// .provide_value::<i32, _>(|| self.num_field);
/// }
/// }
/// ```
#[unstable(feature = "provide_any", issue = "96024")]
fn provide<'a>(&'a self, demand: &mut Demand<'a>);
}

/// Request a value from the `Provider`.
///
/// # Examples
///
/// Get a string value from a provider.
///
/// ```rust
/// # #![feature(provide_any)]
/// use std::any::{Provider, request_value};
///
/// fn get_string<P: Provider>(provider: &P) -> String {
/// request_value::<String, _>(provider).unwrap()
/// }
/// ```
#[unstable(feature = "provide_any", issue = "96024")]
pub fn request_value<'a, T, P>(provider: &'a P) -> Option<T>
where
T: 'static,
P: Provider + ?Sized,
{
request_by_type_tag::<'a, tags::Value<T>, P>(provider)
}

/// Request a reference from the `Provider`.
///
/// # Examples
///
/// Get a string reference from a provider.
///
/// ```rust
/// # #![feature(provide_any)]
/// use std::any::{Provider, request_ref};
///
/// fn get_str<P: Provider>(provider: &P) -> &str {
/// request_ref::<str, _>(provider).unwrap()
/// }
/// ```
#[unstable(feature = "provide_any", issue = "96024")]
pub fn request_ref<'a, T, P>(provider: &'a P) -> Option<&'a T>
where
T: 'static + ?Sized,
P: Provider + ?Sized,
{
request_by_type_tag::<'a, tags::Ref<tags::MaybeSizedValue<T>>, P>(provider)
}

/// Request a specific value by tag from the `Provider`.
fn request_by_type_tag<'a, I, P>(provider: &'a P) -> Option<I::Reified>
where
I: tags::Type<'a>,
P: Provider + ?Sized,
{
let mut tagged = TaggedOption::<'a, I>(None);
provider.provide(tagged.as_demand());
tagged.0
}

///////////////////////////////////////////////////////////////////////////////
// Demand and its methods
///////////////////////////////////////////////////////////////////////////////

/// A helper object for providing data by type.
///
/// A data provider provides values by calling this type's provide methods.
#[unstable(feature = "provide_any", issue = "96024")]
#[repr(transparent)]
pub struct Demand<'a>(dyn Erased<'a> + 'a);

impl<'a> Demand<'a> {
/// Create a new `&mut Demand` from a `&mut dyn Erased` trait object.
fn new<'b>(erased: &'b mut (dyn Erased<'a> + 'a)) -> &'b mut Demand<'a> {
// SAFETY: transmuting `&mut (dyn Erased<'a> + 'a)` to `&mut Demand<'a>` is safe since
// `Demand` is repr(transparent).
unsafe { &mut *(erased as *mut dyn Erased<'a> as *mut Demand<'a>) }
}

/// Provide a value or other type with only static lifetimes.
///
/// # Examples
///
/// Provides a `String` by cloning.
///
/// ```rust
/// # #![feature(provide_any)]
/// use std::any::{Provider, Demand};
/// # struct SomeConcreteType { field: String }
///
/// impl Provider for SomeConcreteType {
/// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
/// demand.provide_value::<String, _>(|| self.field.clone());
/// }
/// }
/// ```
#[unstable(feature = "provide_any", issue = "96024")]
pub fn provide_value<T, F>(&mut self, fulfil: F) -> &mut Self
where
T: 'static,
F: FnOnce() -> T,
{
self.provide_with::<tags::Value<T>, F>(fulfil)
}

/// Provide a reference, note that the referee type must be bounded by `'static`,
/// but may be unsized.
///
/// # Examples
///
/// Provides a reference to a field as a `&str`.
///
/// ```rust
/// # #![feature(provide_any)]
/// use std::any::{Provider, Demand};
/// # struct SomeConcreteType { field: String }
///
/// impl Provider for SomeConcreteType {
/// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
/// demand.provide_ref::<str>(&self.field);
/// }
/// }
/// ```
#[unstable(feature = "provide_any", issue = "96024")]
pub fn provide_ref<T: ?Sized + 'static>(&mut self, value: &'a T) -> &mut Self {
self.provide::<tags::Ref<tags::MaybeSizedValue<T>>>(value)
}

/// Provide a value with the given `Type` tag.
fn provide<I>(&mut self, value: I::Reified) -> &mut Self
where
I: tags::Type<'a>,
{
if let Some(res @ TaggedOption(None)) = self.0.downcast_mut::<I>() {
res.0 = Some(value);
}
self
}

/// Provide a value with the given `Type` tag, using a closure to prevent unnecessary work.
fn provide_with<I, F>(&mut self, fulfil: F) -> &mut Self
where
I: tags::Type<'a>,
F: FnOnce() -> I::Reified,
{
if let Some(res @ TaggedOption(None)) = self.0.downcast_mut::<I>() {
res.0 = Some(fulfil());
}
self
}
}

#[unstable(feature = "provide_any", issue = "96024")]
impl<'a> fmt::Debug for Demand<'a> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Demand").finish_non_exhaustive()
}
}

///////////////////////////////////////////////////////////////////////////////
// Type tags
///////////////////////////////////////////////////////////////////////////////

mod tags {
//! Type tags are used to identify a type using a separate value. This module includes type tags
//! for some very common types.
//!
//! Currently type tags are not exposed to the user. But in the future, if you want to use the
//! Provider API with more complex types (typically those including lifetime parameters), you
//! will need to write your own tags.

use crate::marker::PhantomData;

/// This trait is implemented by specific tag types in order to allow
/// describing a type which can be requested for a given lifetime `'a`.
///
/// A few example implementations for type-driven tags can be found in this
/// module, although crates may also implement their own tags for more
/// complex types with internal lifetimes.
pub trait Type<'a>: Sized + 'static {
/// The type of values which may be tagged by this tag for the given
/// lifetime.
type Reified: 'a;
}

/// Similar to the [`Type`] trait, but represents a type which may be unsized (i.e., has a
/// `?Sized` bound). E.g., `str`.
pub trait MaybeSizedType<'a>: Sized + 'static {
type Reified: 'a + ?Sized;
}

impl<'a, T: Type<'a>> MaybeSizedType<'a> for T {
type Reified = T::Reified;
}

/// Type-based tag for types bounded by `'static`, i.e., with no borrowed elements.
#[derive(Debug)]
pub struct Value<T: 'static>(PhantomData<T>);

impl<'a, T: 'static> Type<'a> for Value<T> {
type Reified = T;
}

/// Type-based tag similar to [`Value`] but which may be unsized (i.e., has a `'Sized` bound).
#[derive(Debug)]
pub struct MaybeSizedValue<T: ?Sized + 'static>(PhantomData<T>);

impl<'a, T: ?Sized + 'static> MaybeSizedType<'a> for MaybeSizedValue<T> {
type Reified = T;
}

/// Type-based tag for reference types (`&'a T`, where T is represented by
/// `<I as MaybeSizedType<'a>>::Reified`.
#[derive(Debug)]
pub struct Ref<I>(PhantomData<I>);

impl<'a, I: MaybeSizedType<'a>> Type<'a> for Ref<I> {
type Reified = &'a I::Reified;
}
}

/// An `Option` with a type tag `I`.
///
/// Since this struct implements `Erased`, the type can be erased to make a dynamically typed
/// option. The type can be checked dynamically using `Erased::tag_id` and since this is statically
/// checked for the concrete type, there is some degree of type safety.
#[repr(transparent)]
struct TaggedOption<'a, I: tags::Type<'a>>(Option<I::Reified>);

impl<'a, I: tags::Type<'a>> TaggedOption<'a, I> {
fn as_demand(&mut self) -> &mut Demand<'a> {
Demand::new(self as &mut (dyn Erased<'a> + 'a))
}
}

/// Represents a type-erased but identifiable object.
///
/// This trait is exclusively implemented by the `TaggedOption` type.
unsafe trait Erased<'a>: 'a {
/// The `TypeId` of the erased type.
fn tag_id(&self) -> TypeId;
}

unsafe impl<'a, I: tags::Type<'a>> Erased<'a> for TaggedOption<'a, I> {
fn tag_id(&self) -> TypeId {
TypeId::of::<I>()
}
}

#[unstable(feature = "provide_any", issue = "96024")]
impl<'a> dyn Erased<'a> + 'a {
/// Returns some reference to the dynamic value if it is tagged with `I`,
/// or `None` otherwise.
#[inline]
fn downcast_mut<I>(&mut self) -> Option<&mut TaggedOption<'a, I>>
where
I: tags::Type<'a>,
{
if self.tag_id() == TypeId::of::<I>() {
// SAFETY: Just checked whether we're pointing to an I.
Some(unsafe { &mut *(self as *mut Self).cast::<TaggedOption<'a, I>>() })
} else {
None
}
}
}

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