Auto merge of #91970 - nrc:provide-any, r=scottmcm

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`

library/core/src/any.rs

@@ -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`
@@ -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
@@ -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")]
 
@@ -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
+        }
+    }
+}