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Auto merge of #113464 - waynr:remove-provider-trait, r=Amanieu

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core/any: remove Provider trait, rename Demand to Request

This touches on two WIP features:

* `error_generic_member_access`
  * tracking issue: https://github.com/rust-lang/rust/issues/99301
  * RFC (WIP): https://github.com/rust-lang/rfcs/pull/2895
* `provide_any`
  * tracking issue: https://github.com/rust-lang/rust/issues/96024
  * RFC: https://github.com/rust-lang/rfcs/pull/3192

The changes in this PR are intended to address libs meeting feedback summarized by `@Amanieu` in https://github.com/rust-lang/rust/issues/96024#issuecomment-1554773172

The specific items this PR addresses so far are:

> We feel that the names "demand" and "request" are somewhat synonymous and would like only one of those to be used for better consistency.

I went with `Request` here since it sounds nicer, but I'm mildly concerned that at first glance it could be confused with the use of the word in networking context.

> The Provider trait should be deleted and its functionality should be merged into Error. We are happy to only provide an API that is only usable with Error. If there is demand for other uses then this can be provided through an external crate.

The net impact this PR has is that examples which previously looked like
```
    core::any::request_ref::<String>(&err).unwramp()
```

now look like
```
    (&err as &dyn core::error::Error).request_value::<String>().unwrap()
```

These are methods that based on the type hint when called return an `Option<T>` of that type. I'll admit I don't fully understand how that's done, but it involves `core::any::tags::Type` and `core::any::TaggedOption`, neither of which are exposed in the public API, to construct a `Request` which is then passed to the `Error.provide` method.

Something that I'm curious about is whether or not they are essential to the use of `Request` types (prior to this PR referred to as `Demand`) and if so does the fact that they are kept private imply that `Request`s are only meant to be constructed privately within the standard library? That's what it looks like to me.

These methods ultimately call into code that looks like:
```
/// Request a specific value by tag from the `Error`.
fn request_by_type_tag<'a, I>(err: &'a (impl Error + ?Sized)) -> Option<I::Reified>
where
    I: tags::Type<'a>,
{
    let mut tagged = core::any::TaggedOption::<'a, I>(None);
    err.provide(tagged.as_request());
    tagged.0
}
```

As far as the `Request` API is concerned, one suggestion I would like to make is that the previous example should look more like this:
```
/// Request a specific value by tag from the `Error`.
fn request_by_type_tag<'a, I>(err: &'a (impl Error + ?Sized)) -> Option<I::Reified>
where
    I: tags::Type<'a>,
{
    let tagged_request = core::any::Request<I>::new_tagged();
    err.provide(tagged_request);
    tagged.0
}
```
This makes it possible for anyone to construct a `Request` for use in their own projects without exposing an implementation detail like `TaggedOption` in the API surface.

Otherwise noteworthy is that I had to add `pub(crate)` on both `core::any::TaggedOption` and `core::any::tags` since `Request`s now need to be constructed in the `core::error` module. I considered moving `TaggedOption` into the `core::error` module but again I figured it's an implementation detail of `Request` and belongs closer to that.

At the time I am opening this PR, I have not yet looked into the following bit of feedback:

> We took a look at the generated code and found that LLVM is unable to optimize multiple .provide_* calls into a switch table because each call fetches the type id from Erased::type_id separately each time and the compiler doesn't know that these calls all return the same value. This should be fixed.

This is what I'll focus on next while waiting for feedback on the progress so far. I suspect that learning more about the type IDs will help me understand the need for `TaggedOption` a little better.
This commit is contained in:
bors 2023-08-14 18:18:03 +00:00
commit 4cea2bc339
10 changed files with 736 additions and 739 deletions
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1
library/alloc/src/lib.rs
View File

@ -138,7 +138,6 @@
#![feature(maybe_uninit_uninit_array_transpose)]
#![feature(pattern)]
#![feature(pointer_byte_offsets)]
#![feature(provide_any)]
#![feature(ptr_internals)]
#![feature(ptr_metadata)]
#![feature(ptr_sub_ptr)]

2
library/alloc/src/sync.rs
View File

@ -3575,7 +3575,7 @@ fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
core::error::Error::source(&**self)
}
fn provide<'a>(&'a self, req: &mut core::any::Demand<'a>) {
fn provide<'a>(&'a self, req: &mut core::error::Request<'a>) {
core::error::Error::provide(&**self, req);
}
}

587
library/core/src/any.rs
View File

@ -83,72 +83,6 @@
//! }
//! ```
//!
//! # `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")]
@ -798,524 +732,3 @@ 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(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")]
#[cfg_attr(not(doc), repr(transparent))] // work around https://github.com/rust-lang/rust/issues/90435
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
}
}
}

735
library/core/src/error.rs
View File

@ -4,8 +4,8 @@
#[cfg(test)]
mod tests;
use crate::any::{Demand, Provider, TypeId};
use crate::fmt::{Debug, Display};
use crate::any::TypeId;
use crate::fmt::{Debug, Display, Formatter, Result};
/// `Error` is a trait representing the basic expectations for error values,
/// i.e., values of type `E` in [`Result<T, E>`].
@ -123,16 +123,20 @@ fn cause(&self) -> Option<&dyn Error> {
/// Provides type based access to context intended for error reports.
///
/// Used in conjunction with [`Demand::provide_value`] and [`Demand::provide_ref`] to extract
/// Used in conjunction with [`Request::provide_value`] and [`Request::provide_ref`] to extract
/// references to member variables from `dyn Error` trait objects.
///
/// # Example
///
/// ```rust
/// #![feature(provide_any)]
/// #![feature(error_generic_member_access)]
/// use core::fmt;
/// use core::any::Demand;
/// use core::error::{request_ref, Request};
///
/// #[derive(Debug)]
/// enum MyLittleTeaPot {
/// Empty,
/// }
///
/// #[derive(Debug)]
/// struct MyBacktrace {
@ -147,21 +151,7 @@ fn cause(&self) -> Option<&dyn Error> {
/// }
///
/// #[derive(Debug)]
/// struct SourceError {
/// // ...
/// }
///
/// impl fmt::Display for SourceError {
/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
/// write!(f, "Example Source Error")
/// }
/// }
///
/// impl std::error::Error for SourceError {}
///
/// #[derive(Debug)]
/// struct Error {
/// source: SourceError,
/// backtrace: MyBacktrace,
/// }
///
@ -172,38 +162,26 @@ fn cause(&self) -> Option<&dyn Error> {
/// }
///
/// impl std::error::Error for Error {
/// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
/// demand
/// .provide_ref::<MyBacktrace>(&self.backtrace)
/// .provide_ref::<dyn std::error::Error + 'static>(&self.source);
/// fn provide<'a>(&'a self, request: &mut Request<'a>) {
/// request
/// .provide_ref::<MyBacktrace>(&self.backtrace);
/// }
/// }
///
/// fn main() {
/// let backtrace = MyBacktrace::new();
/// let source = SourceError {};
/// let error = Error { source, backtrace };
/// let error = Error { backtrace };
/// let dyn_error = &error as &dyn std::error::Error;
/// let backtrace_ref = dyn_error.request_ref::<MyBacktrace>().unwrap();
/// let backtrace_ref = request_ref::<MyBacktrace>(dyn_error).unwrap();
///
/// assert!(core::ptr::eq(&error.backtrace, backtrace_ref));
/// assert!(request_ref::<MyLittleTeaPot>(dyn_error).is_none());
/// }
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
#[allow(unused_variables)]
fn provide<'a>(&'a self, demand: &mut Demand<'a>) {}
fn provide<'a>(&'a self, request: &mut Request<'a>) {}
}
#[unstable(feature = "error_generic_member_access", issue = "99301")]
impl<E> Provider for E
where
E: Error + ?Sized,
{
fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
self.provide(demand)
}
}
mod private {
// This is a hack to prevent `type_id` from being overridden by `Error`
// implementations, since that can enable unsound downcasting.
@ -215,20 +193,6 @@ mod private {
#[unstable(feature = "never_type", issue = "35121")]
impl Error for ! {}
impl<'a> dyn Error + 'a {
/// Request a reference of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_ref<T: ?Sized + 'static>(&'a self) -> Option<&'a T> {
core::any::request_ref(self)
}
/// Request a value of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_value<T: 'static>(&'a self) -> Option<T> {
core::any::request_value(self)
}
}
// Copied from `any.rs`.
impl dyn Error + 'static {
/// Returns `true` if the inner type is the same as `T`.
@ -293,18 +257,6 @@ pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
<dyn Error + 'static>::downcast_mut::<T>(self)
}
/// Request a reference of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
<dyn Error>::request_ref(self)
}
/// Request a value of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_value<T: 'static>(&self) -> Option<T> {
<dyn Error>::request_value(self)
}
}
impl dyn Error + 'static + Send + Sync {
@ -328,18 +280,6 @@ pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
<dyn Error + 'static>::downcast_mut::<T>(self)
}
/// Request a reference of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_ref<T: ?Sized + 'static>(&self) -> Option<&T> {
<dyn Error>::request_ref(self)
}
/// Request a value of type `T` as context about this error.
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_value<T: 'static>(&self) -> Option<T> {
<dyn Error>::request_value(self)
}
}
impl dyn Error {
@ -412,6 +352,645 @@ pub fn sources(&self) -> Source<'_> {
}
}
/// Request a value of type `T` from the given `impl Error`.
///
/// # Examples
///
/// Get a string value from an error.
///
/// ```rust
/// # #![feature(error_generic_member_access)]
/// use std::error::Error;
/// use core::error::request_value;
///
/// fn get_string(err: &impl Error) -> String {
/// request_value::<String>(err).unwrap()
/// }
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_value<'a, T>(err: &'a (impl Error + ?Sized)) -> Option<T>
where
T: 'static,
{
request_by_type_tag::<'a, tags::Value<T>>(err)
}
/// Request a reference of type `T` from the given `impl Error`.
///
/// # Examples
///
/// Get a string reference from an error.
///
/// ```rust
/// # #![feature(error_generic_member_access)]
/// use core::error::Error;
/// use core::error::request_ref;
///
/// fn get_str(err: &impl Error) -> &str {
/// request_ref::<str>(err).unwrap()
/// }
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub fn request_ref<'a, T>(err: &'a (impl Error + ?Sized)) -> Option<&'a T>
where
T: 'static + ?Sized,
{
request_by_type_tag::<'a, tags::Ref<tags::MaybeSizedValue<T>>>(err)
}
/// Request a specific value by tag from the `Error`.
fn request_by_type_tag<'a, I>(err: &'a (impl Error + ?Sized)) -> Option<I::Reified>
where
I: tags::Type<'a>,
{
let mut tagged = TaggedOption::<'a, I>(None);
err.provide(tagged.as_request());
tagged.0
}
///////////////////////////////////////////////////////////////////////////////
// Request and its methods
///////////////////////////////////////////////////////////////////////////////
/// `Request` supports generic, type-driven access to data. It's use is currently restricted to the
/// standard library in cases where trait authors wish to allow trait implementors to share generic
/// information across trait boundaries. The motivating and prototypical use case is
/// `core::error::Error` which would otherwise require a method per concrete type (eg.
/// `std::backtrace::Backtrace` instance that implementors want to expose to users).
///
/// # Data flow
///
/// To describe the intended data flow for Request objects, let's consider two conceptual users
/// separated by API boundaries:
///
/// * Consumer - the consumer requests objects using a Request instance; eg a crate that offers
/// fancy `Error`/`Result` reporting to users wants to request a Backtrace from a given `dyn Error`.
///
/// * Producer - the producer provides objects when requested via Request; eg. a library with an
/// an `Error` implementation that automatically captures backtraces at the time instances are
/// created.
///
/// The consumer only needs to know where to submit their request and are expected to handle the
/// request not being fulfilled by the use of `Option<T>` in the responses offered by the producer.
///
/// * A Producer initializes the value of one of its fields of a specific type. (or is otherwise
/// prepared to generate a value requested). eg, `backtrace::Backtrace` or
/// `std::backtrace::Backtrace`
/// * A Consumer requests an object of a specific type (say `std::backtrace::Backtrace). In the case
/// of a `dyn Error` trait object (the Producer), there are methods called `request_ref` and
/// `request_value` are available to simplify obtaining an ``Option<T>`` for a given type. * The
/// Producer, when requested, populates the given Request object which is given as a mutable
/// reference.
/// * The Consumer extracts a value or reference to the requested type from the `Request` object
/// wrapped in an `Option<T>`; in the case of `dyn Error` the aforementioned `request_ref` and `
/// request_value` methods mean that `dyn Error` users don't have to deal with the `Request` type at
/// all (but `Error` implementors do). The `None` case of the `Option` suggests only that the
/// Producer cannot currently offer an instance of the requested type, not it can't or never will.
///
/// # Examples
///
/// The best way to demonstrate this is using an example implementation of `Error`'s `provide` trait
/// method:
///
/// ```
/// #![feature(error_generic_member_access)]
/// use core::fmt;
/// use core::error::Request;
/// use core::error::request_ref;
///
/// #[derive(Debug)]
/// enum MyLittleTeaPot {
/// Empty,
/// }
///
/// #[derive(Debug)]
/// struct MyBacktrace {
/// // ...
/// }
///
/// impl MyBacktrace {
/// fn new() -> MyBacktrace {
/// // ...
/// # MyBacktrace {}
/// }
/// }
///
/// #[derive(Debug)]
/// struct Error {
/// backtrace: MyBacktrace,
/// }
///
/// impl fmt::Display for Error {
/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
/// write!(f, "Example Error")
/// }
/// }
///
/// impl std::error::Error for Error {
/// fn provide<'a>(&'a self, request: &mut Request<'a>) {
/// request
/// .provide_ref::<MyBacktrace>(&self.backtrace);
/// }
/// }
///
/// fn main() {
/// let backtrace = MyBacktrace::new();
/// let error = Error { backtrace };
/// let dyn_error = &error as &dyn std::error::Error;
/// let backtrace_ref = request_ref::<MyBacktrace>(dyn_error).unwrap();
///
/// assert!(core::ptr::eq(&error.backtrace, backtrace_ref));
/// assert!(request_ref::<MyLittleTeaPot>(dyn_error).is_none());
/// }
/// ```
///
#[unstable(feature = "error_generic_member_access", issue = "99301")]
#[cfg_attr(not(doc), repr(transparent))] // work around https://github.com/rust-lang/rust/issues/90435
pub struct Request<'a>(dyn Erased<'a> + 'a);
impl<'a> Request<'a> {
/// Create a new `&mut Request` from a `&mut dyn Erased` trait object.
fn new<'b>(erased: &'b mut (dyn Erased<'a> + 'a)) -> &'b mut Request<'a> {
// SAFETY: transmuting `&mut (dyn Erased<'a> + 'a)` to `&mut Request<'a>` is safe since
// `Request` is repr(transparent).
unsafe { &mut *(erased as *mut dyn Erased<'a> as *mut Request<'a>) }
}
/// Provide a value or other type with only static lifetimes.
///
/// # Examples
///
/// Provides an `u8`.
///
/// ```rust
/// #![feature(error_generic_member_access)]
///
/// use core::error::Request;
///
/// #[derive(Debug)]
/// struct SomeConcreteType { field: u8 }
///
/// impl std::fmt::Display for SomeConcreteType {
/// fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
/// write!(f, "{} failed", self.field)
/// }
/// }
///
/// impl std::error::Error for SomeConcreteType {
/// fn provide<'a>(&'a self, request: &mut Request<'a>) {
/// request.provide_value::<u8>(self.field);
/// }
/// }
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
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(error_generic_member_access)]
///
/// use core::error::Request;
///
/// #[derive(Debug)]
/// struct SomeConcreteType { field: String }
///
/// impl std::fmt::Display for SomeConcreteType {
/// fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
/// write!(f, "{} failed", self.field)
/// }
/// }
///
/// impl std::error::Error for SomeConcreteType {
/// fn provide<'a>(&'a self, request: &mut Request<'a>) {
/// request.provide_value_with::<String>(|| self.field.clone());
/// }
/// }
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
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(error_generic_member_access)]
///
/// use core::error::Request;
///
/// #[derive(Debug)]
/// struct SomeConcreteType { field: String }
///
/// impl std::fmt::Display for SomeConcreteType {
/// fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
/// write!(f, "{} failed", self.field)
/// }
/// }
///
/// impl std::error::Error for SomeConcreteType {
/// fn provide<'a>(&'a self, request: &mut Request<'a>) {
/// request.provide_ref::<str>(&self.field);
/// }
/// }
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
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(error_generic_member_access)]
///
/// use core::error::Request;
///
/// #[derive(Debug)]
/// struct SomeConcreteType { business: String, party: String }
/// fn today_is_a_weekday() -> bool { true }
///
/// impl std::fmt::Display for SomeConcreteType {
/// fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
/// write!(f, "{} failed", self.business)
/// }
/// }
///
/// impl std::error::Error for SomeConcreteType {
/// fn provide<'a>(&'a self, request: &mut Request<'a>) {
/// request.provide_ref_with::<str>(|| {
/// if today_is_a_weekday() {
/// &self.business
/// } else {
/// &self.party
/// }
/// });
/// }
/// }
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
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 `Request` 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(error_generic_member_access)]
///
/// use core::error::Request;
/// use core::error::request_value;
///
/// #[derive(Debug)]
/// struct Parent(Option<u8>);
///
/// impl std::fmt::Display for Parent {
/// fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
/// write!(f, "a parent failed")
/// }
/// }
///
/// impl std::error::Error for Parent {
/// fn provide<'a>(&'a self, request: &mut Request<'a>) {
/// if let Some(v) = self.0 {
/// request.provide_value::<u8>(v);
/// }
/// }
/// }
///
/// #[derive(Debug)]
/// 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 std::fmt::Display for Child {
/// fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
/// write!(f, "child failed: \n because of parent: {}", self.parent)
/// }
/// }
///
/// impl std::error::Error for Child {
/// fn provide<'a>(&'a self, request: &mut Request<'a>) {
/// // In general, we don't know if this call will provide
/// // an `u8` value or not...
/// self.parent.provide(request);
///
/// // ...so we check to see if the `u8` is needed before
/// // we run our expensive computation.
/// if request.would_be_satisfied_by_value_of::<u8>() {
/// if let Some(v) = self.an_expensive_computation() {
/// request.provide_value::<u8>(v);
/// }
/// }
///
/// // The request will be satisfied now, regardless of if
/// // the parent provided the value or we did.
/// assert!(!request.would_be_satisfied_by_value_of::<u8>());
/// }
/// }
///
/// let parent = Parent(Some(42));
/// let child = Child { parent };
/// assert_eq!(Some(42), request_value::<u8>(&child));
///
/// let parent = Parent(None);
/// let child = Child { parent };
/// assert_eq!(Some(99), request_value::<u8>(&child));
///
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
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 `Request` 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(error_generic_member_access)]
///
/// use core::error::Request;
/// use core::error::request_ref;
///
/// #[derive(Debug)]
/// struct Parent(Option<String>);
///
/// impl std::fmt::Display for Parent {
/// fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
/// write!(f, "a parent failed")
/// }
/// }
///
/// impl std::error::Error for Parent {
/// fn provide<'a>(&'a self, request: &mut Request<'a>) {
/// if let Some(v) = &self.0 {
/// request.provide_ref::<str>(v);
/// }
/// }
/// }
///
/// #[derive(Debug)]
/// 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 std::fmt::Display for Child {
/// fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
/// write!(f, "{} failed: \n {}", self.name, self.parent)
/// }
/// }
///
/// impl std::error::Error for Child {
/// fn provide<'a>(&'a self, request: &mut Request<'a>) {
/// // In general, we don't know if this call will provide
/// // a `str` reference or not...
/// self.parent.provide(request);
///
/// // ...so we check to see if the `&str` is needed before
/// // we run our expensive computation.
/// if request.would_be_satisfied_by_ref_of::<str>() {
/// if let Some(v) = self.an_expensive_computation() {
/// request.provide_ref::<str>(v);
/// }
/// }
///
/// // The request will be satisfied now, regardless of if
/// // the parent provided the reference or we did.
/// assert!(!request.would_be_satisfied_by_ref_of::<str>());
/// }
/// }
///
/// let parent = Parent(Some("parent".into()));
/// let child = Child { parent, name: "child".into() };
/// assert_eq!(Some("parent"), request_ref::<str>(&child));
///
/// let parent = Parent(None);
/// let child = Child { parent, name: "child".into() };
/// assert_eq!(Some("child"), request_ref::<str>(&child));
/// ```
#[unstable(feature = "error_generic_member_access", issue = "99301")]
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 = "error_generic_member_access", issue = "99301")]
impl<'a> Debug for Request<'a> {
fn fmt(&self, f: &mut Formatter<'_>) -> Result {
f.debug_struct("Request").finish_non_exhaustive()
}
}
///////////////////////////////////////////////////////////////////////////////
// Type tags
///////////////////////////////////////////////////////////////////////////////
pub(crate) 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
//! Request 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(crate) 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(crate) 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(crate) 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(crate) 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(crate) 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)]
pub(crate) struct TaggedOption<'a, I: tags::Type<'a>>(pub Option<I::Reified>);
impl<'a, I: tags::Type<'a>> TaggedOption<'a, I> {
pub(crate) fn as_request(&mut self) -> &mut Request<'a> {
Request::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>()
}
}
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
}
}
}
/// An iterator over an [`Error`] and its sources.
///
/// If you want to omit the initial error and only process
@ -449,8 +1028,8 @@ fn source(&self) -> Option<&(dyn Error + 'static)> {
Error::source(&**self)
}
fn provide<'b>(&'b self, demand: &mut Demand<'b>) {
Error::provide(&**self, demand);
fn provide<'b>(&'b self, request: &mut Request<'b>) {
Error::provide(&**self, request);
}
}

62
library/core/tests/any.rs
View File

@ -147,65 +147,3 @@ trait Foo {
std::any::type_name::<dyn Foo<Bar = i32> + Send + Sync>()
);
}
// Test the `Provider` API.
struct SomeConcreteType {
some_string: String,
}
impl Provider for SomeConcreteType {
fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
demand
.provide_ref::<String>(&self.some_string)
.provide_ref::<str>(&self.some_string)
.provide_value_with::<String>(|| "bye".to_owned());
}
}
// Test the provide and request mechanisms with a by-reference trait object.
#[test]
fn test_provider() {
let obj: &dyn Provider = &SomeConcreteType { some_string: "hello".to_owned() };
assert_eq!(&**request_ref::<String>(obj).unwrap(), "hello");
assert_eq!(&*request_value::<String>(obj).unwrap(), "bye");
assert_eq!(request_value::<u8>(obj), None);
}
// Test the provide and request mechanisms with a boxed trait object.
#[test]
fn test_provider_boxed() {
let obj: Box<dyn Provider> = Box::new(SomeConcreteType { some_string: "hello".to_owned() });
assert_eq!(&**request_ref::<String>(&*obj).unwrap(), "hello");
assert_eq!(&*request_value::<String>(&*obj).unwrap(), "bye");
assert_eq!(request_value::<u8>(&*obj), None);
}
// Test the provide and request mechanisms with a concrete object.
#[test]
fn test_provider_concrete() {
let obj = SomeConcreteType { some_string: "hello".to_owned() };
assert_eq!(&**request_ref::<String>(&obj).unwrap(), "hello");
assert_eq!(&*request_value::<String>(&obj).unwrap(), "bye");
assert_eq!(request_value::<u8>(&obj), None);
}
trait OtherTrait: Provider {}
impl OtherTrait for SomeConcreteType {}
impl dyn OtherTrait {
fn get_ref<T: 'static + ?Sized>(&self) -> Option<&T> {
request_ref::<T>(self)
}
}
// Test the provide and request mechanisms via an intermediate trait.
#[test]
fn test_provider_intermediate() {
let obj: &dyn OtherTrait = &SomeConcreteType { some_string: "hello".to_owned() };
assert_eq!(obj.get_ref::<str>().unwrap(), "hello");
}

66
library/core/tests/error.rs Normal file
View File

@ -0,0 +1,66 @@
use core::error::{request_value, request_ref, Request};
// Test the `Request` API.
#[derive(Debug)]
struct SomeConcreteType {
some_string: String,
}
impl std::fmt::Display for SomeConcreteType {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, "A")
}
}
impl std::error::Error for SomeConcreteType {
fn provide<'a>(&'a self, request: &mut Request<'a>) {
request
.provide_ref::<String>(&self.some_string)
.provide_ref::<str>(&self.some_string)
.provide_value_with::<String>(|| "bye".to_owned());
}
}
// Test the Error.provide and request mechanisms with a by-reference trait object.
#[test]
fn test_error_generic_member_access() {
let obj = &SomeConcreteType { some_string: "hello".to_owned() };
assert_eq!(request_ref::<String>(&*obj).unwrap(), "hello");
assert_eq!(request_value::<String>(&*obj).unwrap(), "bye");
assert_eq!(request_value::<u8>(&obj), None);
}
// Test the Error.provide and request mechanisms with a by-reference trait object.
#[test]
fn test_request_constructor() {
let obj: &dyn std::error::Error = &SomeConcreteType { some_string: "hello".to_owned() };
assert_eq!(request_ref::<String>(&*obj).unwrap(), "hello");
assert_eq!(request_value::<String>(&*obj).unwrap(), "bye");
assert_eq!(request_value::<u8>(&obj), None);
}
// Test the Error.provide and request mechanisms with a boxed trait object.
#[test]
fn test_error_generic_member_access_boxed() {
let obj: Box<dyn std::error::Error> =
Box::new(SomeConcreteType { some_string: "hello".to_owned() });
assert_eq!(request_ref::<String>(&*obj).unwrap(), "hello");
assert_eq!(request_value::<String>(&*obj).unwrap(), "bye");
// NOTE: Box<E> only implements Error when E: Error + Sized, which means we can't pass a
// Box<dyn Error> to request_value.
//assert_eq!(request_value::<String>(&obj).unwrap(), "bye");
}
// Test the Error.provide and request mechanisms with a concrete object.
#[test]
fn test_error_generic_member_access_concrete() {
let obj = SomeConcreteType { some_string: "hello".to_owned() };
assert_eq!(request_ref::<String>(&obj).unwrap(), "hello");
assert_eq!(request_value::<String>(&obj).unwrap(), "bye");
assert_eq!(request_value::<u8>(&obj), None);
}

4
library/core/tests/lib.rs
View File

@ -105,7 +105,9 @@
#![feature(const_slice_from_ref)]
#![feature(waker_getters)]
#![feature(slice_flatten)]
#![feature(provide_any)]
#![feature(error_generic_member_access)]
#![feature(error_in_core)]
#![feature(trait_upcasting)]
#![feature(utf8_chunks)]
#![feature(is_ascii_octdigit)]
#![feature(get_many_mut)]

13
library/std/src/error.rs
View File

@ -9,6 +9,8 @@
#[stable(feature = "rust1", since = "1.0.0")]
pub use core::error::Error;
#[unstable(feature = "error_generic_member_access", issue = "99301")]
pub use core::error::{request_ref, Request};
mod private {
// This is a hack to prevent `type_id` from being overridden by `Error`
@ -371,11 +373,10 @@ pub fn pretty(mut self, pretty: bool) -> Self {
///
/// ```rust
/// #![feature(error_reporter)]
/// #![feature(provide_any)]
/// #![feature(error_generic_member_access)]
/// # use std::error::Error;
/// # use std::fmt;
/// use std::any::Demand;
/// use std::error::Request;
/// use std::error::Report;
/// use std::backtrace::Backtrace;
///
@ -405,8 +406,8 @@ pub fn pretty(mut self, pretty: bool) -> Self {
/// }
///
/// impl Error for SuperErrorSideKick {
/// fn provide<'a>(&'a self, demand: &mut Demand<'a>) {
/// demand.provide_ref::<Backtrace>(&self.backtrace);
/// fn provide<'a>(&'a self, request: &mut Request<'a>) {
/// request.provide_ref::<Backtrace>(&self.backtrace);
/// }
/// }
///
@ -459,11 +460,11 @@ impl<E> Report<E>
fn backtrace(&self) -> Option<&Backtrace> {
// have to grab the backtrace on the first error directly since that error may not be
// 'static
let backtrace = (&self.error as &dyn Error).request_ref();
let backtrace = request_ref(&self.error);
let backtrace = backtrace.or_else(|| {
self.error
.source()
.map(|source| source.sources().find_map(|source| source.request_ref()))
.map(|source| source.sources().find_map(|source| request_ref(source)))
.flatten()
});
backtrace

4
library/std/src/error/tests.rs
View File

@ -1,6 +1,6 @@
use super::Error;
use crate::fmt;
use core::any::Demand;
use core::error::Request;
#[derive(Debug, PartialEq)]
struct A;
@ -199,7 +199,7 @@ fn source(&self) -> Option<&(dyn Error + 'static)> {
self.source.as_deref()
}
fn provide<'a>(&'a self, req: &mut Demand<'a>) {
fn provide<'a>(&'a self, req: &mut Request<'a>) {
self.backtrace.as_ref().map(|bt| req.provide_ref::<Backtrace>(bt));
}
}

1
library/std/src/lib.rs
View File

@ -306,7 +306,6 @@
#![feature(pointer_is_aligned)]
#![feature(portable_simd)]
#![feature(prelude_2024)]
#![feature(provide_any)]
#![feature(ptr_as_uninit)]
#![feature(raw_os_nonzero)]
#![feature(round_ties_even)]