-//! This module implements the `Any` trait, which enables dynamic typing
-//! of any `'static` type through runtime reflection.
+//! Utilities for dynamic typing or type 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`
//! 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
//! println!("String ({}): {}", as_string.len(), as_string);
//! }
//! None => {
-//! println!("{:?}", value);
+//! println!("{value:?}");
//! }
//! }
//! }
//! 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
+//! no 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_by_ref` call will return a reference to `obj.some_string` with type `&String`.
#![stable(feature = "rust1", since = "1.0.0")]
// unsafe traits and unsafe methods (i.e., `type_id` would still be safe to call,
// but we would likely want to indicate as such in documentation).
#[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "Any")]
pub trait Any: 'static {
/// Gets the `TypeId` of `self`.
///
}
impl dyn Any {
- /// Returns `true` if the boxed type is the same as `T`.
+ /// Returns `true` if the inner type is the same as `T`.
///
/// # Examples
///
t == concrete
}
- /// Returns some reference to the boxed value if it is of type `T`, or
+ /// Returns some reference to the inner value if it is of type `T`, or
/// `None` if it isn't.
///
/// # Examples
// SAFETY: just checked whether we are pointing to the correct type, and we can rely on
// that check for memory safety because we have implemented Any for all types; no other
// impls can exist as they would conflict with our impl.
- unsafe { Some(&*(self as *const dyn Any as *const T)) }
+ unsafe { Some(self.downcast_ref_unchecked()) }
} else {
None
}
}
- /// Returns some mutable reference to the boxed value if it is of type `T`, or
+ /// Returns some mutable reference to the inner value if it is of type `T`, or
/// `None` if it isn't.
///
/// # Examples
// SAFETY: just checked whether we are pointing to the correct type, and we can rely on
// that check for memory safety because we have implemented Any for all types; no other
// impls can exist as they would conflict with our impl.
- unsafe { Some(&mut *(self as *mut dyn Any as *mut T)) }
+ unsafe { Some(self.downcast_mut_unchecked()) }
} else {
None
}
}
+
+ /// Returns a reference to the inner value as type `dyn T`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// #![feature(downcast_unchecked)]
+ ///
+ /// use std::any::Any;
+ ///
+ /// let x: Box<dyn Any> = Box::new(1_usize);
+ ///
+ /// unsafe {
+ /// assert_eq!(*x.downcast_ref_unchecked::<usize>(), 1);
+ /// }
+ /// ```
+ ///
+ /// # Safety
+ ///
+ /// The contained value must be of type `T`. Calling this method
+ /// with the incorrect type is *undefined behavior*.
+ #[unstable(feature = "downcast_unchecked", issue = "90850")]
+ #[inline]
+ pub unsafe fn downcast_ref_unchecked<T: Any>(&self) -> &T {
+ debug_assert!(self.is::<T>());
+ // SAFETY: caller guarantees that T is the correct type
+ unsafe { &*(self as *const dyn Any as *const T) }
+ }
+
+ /// Returns a mutable reference to the inner value as type `dyn T`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// #![feature(downcast_unchecked)]
+ ///
+ /// use std::any::Any;
+ ///
+ /// let mut x: Box<dyn Any> = Box::new(1_usize);
+ ///
+ /// unsafe {
+ /// *x.downcast_mut_unchecked::<usize>() += 1;
+ /// }
+ ///
+ /// assert_eq!(*x.downcast_ref::<usize>().unwrap(), 2);
+ /// ```
+ ///
+ /// # Safety
+ ///
+ /// The contained value must be of type `T`. Calling this method
+ /// with the incorrect type is *undefined behavior*.
+ #[unstable(feature = "downcast_unchecked", issue = "90850")]
+ #[inline]
+ pub unsafe fn downcast_mut_unchecked<T: Any>(&mut self) -> &mut T {
+ debug_assert!(self.is::<T>());
+ // SAFETY: caller guarantees that T is the correct type
+ unsafe { &mut *(self as *mut dyn Any as *mut T) }
+ }
}
impl dyn Any + Send {
- /// Forwards to the method defined on the type `Any`.
+ /// Forwards to the method defined on the type `dyn Any`.
///
/// # Examples
///
<dyn Any>::is::<T>(self)
}
- /// Forwards to the method defined on the type `Any`.
+ /// Forwards to the method defined on the type `dyn Any`.
///
/// # Examples
///
<dyn Any>::downcast_ref::<T>(self)
}
- /// Forwards to the method defined on the type `Any`.
+ /// Forwards to the method defined on the type `dyn Any`.
///
/// # Examples
///
pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
<dyn Any>::downcast_mut::<T>(self)
}
+
+ /// Forwards to the method defined on the type `dyn Any`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// #![feature(downcast_unchecked)]
+ ///
+ /// use std::any::Any;
+ ///
+ /// let x: Box<dyn Any> = Box::new(1_usize);
+ ///
+ /// unsafe {
+ /// assert_eq!(*x.downcast_ref_unchecked::<usize>(), 1);
+ /// }
+ /// ```
+ ///
+ /// # Safety
+ ///
+ /// Same as the method on the type `dyn Any`.
+ #[unstable(feature = "downcast_unchecked", issue = "90850")]
+ #[inline]
+ pub unsafe fn downcast_ref_unchecked<T: Any>(&self) -> &T {
+ // SAFETY: guaranteed by caller
+ unsafe { <dyn Any>::downcast_ref_unchecked::<T>(self) }
+ }
+
+ /// Forwards to the method defined on the type `dyn Any`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// #![feature(downcast_unchecked)]
+ ///
+ /// use std::any::Any;
+ ///
+ /// let mut x: Box<dyn Any> = Box::new(1_usize);
+ ///
+ /// unsafe {
+ /// *x.downcast_mut_unchecked::<usize>() += 1;
+ /// }
+ ///
+ /// assert_eq!(*x.downcast_ref::<usize>().unwrap(), 2);
+ /// ```
+ ///
+ /// # Safety
+ ///
+ /// Same as the method on the type `dyn Any`.
+ #[unstable(feature = "downcast_unchecked", issue = "90850")]
+ #[inline]
+ pub unsafe fn downcast_mut_unchecked<T: Any>(&mut self) -> &mut T {
+ // SAFETY: guaranteed by caller
+ unsafe { <dyn Any>::downcast_mut_unchecked::<T>(self) }
+ }
}
impl dyn Any + Send + Sync {
pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
<dyn Any>::downcast_mut::<T>(self)
}
+
+ /// Forwards to the method defined on the type `Any`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// #![feature(downcast_unchecked)]
+ ///
+ /// use std::any::Any;
+ ///
+ /// let x: Box<dyn Any> = Box::new(1_usize);
+ ///
+ /// unsafe {
+ /// assert_eq!(*x.downcast_ref_unchecked::<usize>(), 1);
+ /// }
+ /// ```
+ #[unstable(feature = "downcast_unchecked", issue = "90850")]
+ #[inline]
+ pub unsafe fn downcast_ref_unchecked<T: Any>(&self) -> &T {
+ // SAFETY: guaranteed by caller
+ unsafe { <dyn Any>::downcast_ref_unchecked::<T>(self) }
+ }
+
+ /// Forwards to the method defined on the type `Any`.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// #![feature(downcast_unchecked)]
+ ///
+ /// use std::any::Any;
+ ///
+ /// let mut x: Box<dyn Any> = Box::new(1_usize);
+ ///
+ /// unsafe {
+ /// *x.downcast_mut_unchecked::<usize>() += 1;
+ /// }
+ ///
+ /// assert_eq!(*x.downcast_ref::<usize>().unwrap(), 2);
+ /// ```
+ #[unstable(feature = "downcast_unchecked", issue = "90850")]
+ #[inline]
+ pub unsafe fn downcast_mut_unchecked<T: Any>(&mut self) -> &mut T {
+ // SAFETY: guaranteed by caller
+ unsafe { <dyn Any>::downcast_mut_unchecked::<T>(self) }
+ }
}
///////////////////////////////////////////////////////////////////////////////
/// While `TypeId` implements `Hash`, `PartialOrd`, and `Ord`, it is worth
/// noting that the hashes and ordering will vary between Rust releases. Beware
/// of relying on them inside of your code!
-#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
+#[derive(Clone, Copy, Debug, Hash, Eq)]
+#[derive_const(PartialEq, PartialOrd, Ord)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct TypeId {
t: u64,
/// assert_eq!(is_string(&0), false);
/// assert_eq!(is_string(&"cookie monster".to_string()), true);
/// ```
+ #[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_type_id", issue = "77125")]
pub const fn of<T: ?Sized + 'static>() -> TypeId {
/// "core::option::Option<alloc::string::String>",
/// );
/// ```
+#[must_use]
#[stable(feature = "type_name", since = "1.38.0")]
#[rustc_const_unstable(feature = "const_type_name", issue = "63084")]
pub const fn type_name<T: ?Sized>() -> &'static str {
/// let y = 1.0;
/// println!("{}", type_name_of_val(&y));
/// ```
+#[must_use]
#[unstable(feature = "type_name_of_val", issue = "66359")]
#[rustc_const_unstable(feature = "const_type_name", issue = "63084")]
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(provider: &impl Provider) -> String {
+/// request_value::<String>(provider).unwrap()
+/// }
+/// ```
+#[unstable(feature = "provide_any", issue = "96024")]
+pub fn request_value<'a, T>(provider: &'a (impl Provider + ?Sized)) -> Option<T>
+where
+ T: 'static,
+{
+ request_by_type_tag::<'a, tags::Value<T>>(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(provider: &impl Provider) -> &str {
+/// request_ref::<str>(provider).unwrap()
+/// }
+/// ```
+#[unstable(feature = "provide_any", issue = "96024")]
+pub fn request_ref<'a, T>(provider: &'a (impl Provider + ?Sized)) -> Option<&'a T>
+where
+ T: 'static + ?Sized,
+{
+ request_by_type_tag::<'a, tags::Ref<tags::MaybeSizedValue<T>>>(provider)
+}
+
+/// Request a specific value by tag from the `Provider`.
+fn request_by_type_tag<'a, I>(provider: &'a (impl Provider + ?Sized)) -> Option<I::Reified>
+where
+ I: tags::Type<'a>,
+{
+ 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 an `u8`.
+ ///
+ /// ```rust
+ /// #![feature(provide_any)]
+ ///
+ /// use std::any::{Provider, Demand};
+ /// # struct SomeConcreteType { field: u8 }
+ ///
+ /// impl Provider for SomeConcreteType {
+ /// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
+ /// demand.provide_value::<u8>(self.field);
+ /// }
+ /// }
+ /// ```
+ #[unstable(feature = "provide_any", issue = "96024")]
+ pub fn provide_value<T>(&mut self, value: T) -> &mut Self
+ where
+ T: 'static,
+ {
+ self.provide::<tags::Value<T>>(value)
+ }
+
+ /// Provide a value or other type with only static lifetimes computed using a closure.
+ ///
+ /// # 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_with::<String>(|| self.field.clone());
+ /// }
+ /// }
+ /// ```
+ #[unstable(feature = "provide_any", issue = "96024")]
+ pub fn provide_value_with<T>(&mut self, fulfil: impl FnOnce() -> T) -> &mut Self
+ where
+ T: 'static,
+ {
+ self.provide_with::<tags::Value<T>>(fulfil)
+ }
+
+ /// Provide a reference. 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 reference computed using a closure. 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 { business: String, party: String }
+ /// # fn today_is_a_weekday() -> bool { true }
+ ///
+ /// impl Provider for SomeConcreteType {
+ /// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
+ /// demand.provide_ref_with::<str>(|| {
+ /// if today_is_a_weekday() {
+ /// &self.business
+ /// } else {
+ /// &self.party
+ /// }
+ /// });
+ /// }
+ /// }
+ /// ```
+ #[unstable(feature = "provide_any", issue = "96024")]
+ pub fn provide_ref_with<T: ?Sized + 'static>(
+ &mut self,
+ fulfil: impl FnOnce() -> &'a T,
+ ) -> &mut Self {
+ self.provide_with::<tags::Ref<tags::MaybeSizedValue<T>>>(fulfil)
+ }
+
+ /// 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>(&mut self, fulfil: impl FnOnce() -> I::Reified) -> &mut Self
+ where
+ I: tags::Type<'a>,
+ {
+ if let Some(res @ TaggedOption(None)) = self.0.downcast_mut::<I>() {
+ res.0 = Some(fulfil());
+ }
+ self
+ }
+
+ /// Check if the `Demand` would be satisfied if provided with a
+ /// value of the specified type. If the type does not match or has
+ /// already been provided, returns false.
+ ///
+ /// # Examples
+ ///
+ /// Check if an `u8` still needs to be provided and then provides
+ /// it.
+ ///
+ /// ```rust
+ /// #![feature(provide_any)]
+ ///
+ /// use std::any::{Provider, Demand};
+ ///
+ /// struct Parent(Option<u8>);
+ ///
+ /// impl Provider for Parent {
+ /// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
+ /// if let Some(v) = self.0 {
+ /// demand.provide_value::<u8>(v);
+ /// }
+ /// }
+ /// }
+ ///
+ /// struct Child {
+ /// parent: Parent,
+ /// }
+ ///
+ /// impl Child {
+ /// // Pretend that this takes a lot of resources to evaluate.
+ /// fn an_expensive_computation(&self) -> Option<u8> {
+ /// Some(99)
+ /// }
+ /// }
+ ///
+ /// impl Provider for Child {
+ /// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
+ /// // In general, we don't know if this call will provide
+ /// // an `u8` value or not...
+ /// self.parent.provide(demand);
+ ///
+ /// // ...so we check to see if the `u8` is needed before
+ /// // we run our expensive computation.
+ /// if demand.would_be_satisfied_by_value_of::<u8>() {
+ /// if let Some(v) = self.an_expensive_computation() {
+ /// demand.provide_value::<u8>(v);
+ /// }
+ /// }
+ ///
+ /// // The demand will be satisfied now, regardless of if
+ /// // the parent provided the value or we did.
+ /// assert!(!demand.would_be_satisfied_by_value_of::<u8>());
+ /// }
+ /// }
+ ///
+ /// let parent = Parent(Some(42));
+ /// let child = Child { parent };
+ /// assert_eq!(Some(42), std::any::request_value::<u8>(&child));
+ ///
+ /// let parent = Parent(None);
+ /// let child = Child { parent };
+ /// assert_eq!(Some(99), std::any::request_value::<u8>(&child));
+ /// ```
+ #[unstable(feature = "provide_any", issue = "96024")]
+ pub fn would_be_satisfied_by_value_of<T>(&self) -> bool
+ where
+ T: 'static,
+ {
+ self.would_be_satisfied_by::<tags::Value<T>>()
+ }
+
+ /// Check if the `Demand` would be satisfied if provided with a
+ /// reference to a value of the specified type. If the type does
+ /// not match or has already been provided, returns false.
+ ///
+ /// # Examples
+ ///
+ /// Check if a `&str` still needs to be provided and then provides
+ /// it.
+ ///
+ /// ```rust
+ /// #![feature(provide_any)]
+ ///
+ /// use std::any::{Provider, Demand};
+ ///
+ /// struct Parent(Option<String>);
+ ///
+ /// impl Provider for Parent {
+ /// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
+ /// if let Some(v) = &self.0 {
+ /// demand.provide_ref::<str>(v);
+ /// }
+ /// }
+ /// }
+ ///
+ /// struct Child {
+ /// parent: Parent,
+ /// name: String,
+ /// }
+ ///
+ /// impl Child {
+ /// // Pretend that this takes a lot of resources to evaluate.
+ /// fn an_expensive_computation(&self) -> Option<&str> {
+ /// Some(&self.name)
+ /// }
+ /// }
+ ///
+ /// impl Provider for Child {
+ /// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
+ /// // In general, we don't know if this call will provide
+ /// // a `str` reference or not...
+ /// self.parent.provide(demand);
+ ///
+ /// // ...so we check to see if the `&str` is needed before
+ /// // we run our expensive computation.
+ /// if demand.would_be_satisfied_by_ref_of::<str>() {
+ /// if let Some(v) = self.an_expensive_computation() {
+ /// demand.provide_ref::<str>(v);
+ /// }
+ /// }
+ ///
+ /// // The demand will be satisfied now, regardless of if
+ /// // the parent provided the reference or we did.
+ /// assert!(!demand.would_be_satisfied_by_ref_of::<str>());
+ /// }
+ /// }
+ ///
+ /// let parent = Parent(Some("parent".into()));
+ /// let child = Child { parent, name: "child".into() };
+ /// assert_eq!(Some("parent"), std::any::request_ref::<str>(&child));
+ ///
+ /// let parent = Parent(None);
+ /// let child = Child { parent, name: "child".into() };
+ /// assert_eq!(Some("child"), std::any::request_ref::<str>(&child));
+ /// ```
+ #[unstable(feature = "provide_any", issue = "96024")]
+ pub fn would_be_satisfied_by_ref_of<T>(&self) -> bool
+ where
+ T: ?Sized + 'static,
+ {
+ self.would_be_satisfied_by::<tags::Ref<tags::MaybeSizedValue<T>>>()
+ }
+
+ fn would_be_satisfied_by<I>(&self) -> bool
+ where
+ I: tags::Type<'a>,
+ {
+ matches!(self.0.downcast::<I>(), Some(TaggedOption(None)))
+ }
+}
+
+#[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<I>(&self) -> Option<&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 { &*(self as *const Self).cast::<TaggedOption<'a, I>>() })
+ } else {
+ None
+ }
+ }
+
+ /// Returns some mutable 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
+ }
+ }
+}
projects / rustc.git / blobdiff