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rust/library/core/src/error.rs
John Arundel a19472a93e Fix doc nits 
Many tiny changes to stdlib doc comments to make them consistent (for example
"Returns foo", rather than "Return foo", per RFC1574), adding missing periods, paragraph
breaks, backticks for monospace style, and other minor nits.

https://github.com/rust-lang/rfcs/blob/master/text/1574-more-api-documentation-conventions.md#appendix-a-full-conventions-text
2024-07-26 13:26:33 +01:00

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#![doc = include_str!("error.md")]
#![stable(feature = "error_in_core", since = "CURRENT_RUSTC_VERSION")]
#[cfg(test)]
mod tests;
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>`].
///
/// Errors must describe themselves through the [`Display`] and [`Debug`]
/// traits. Error messages are typically concise lowercase sentences without
/// trailing punctuation:
///
/// ```
/// let err = "NaN".parse::<u32>().unwrap_err();
/// assert_eq!(err.to_string(), "invalid digit found in string");
/// ```
///
/// Errors may provide cause information. [`Error::source()`] is generally
/// used when errors cross "abstraction boundaries". If one module must report
/// an error that is caused by an error from a lower-level module, it can allow
/// accessing that error via [`Error::source()`]. This makes it possible for the
/// high-level module to provide its own errors while also revealing some of the
/// implementation for debugging.
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "Error")]
#[rustc_has_incoherent_inherent_impls]
#[allow(multiple_supertrait_upcastable)]
pub trait Error: Debug + Display {
/// Returns the lower-level source of this error, if any.
///
/// # Examples
///
/// ```
/// use std::error::Error;
/// use std::fmt;
///
/// #[derive(Debug)]
/// struct SuperError {
/// source: SuperErrorSideKick,
/// }
///
/// impl fmt::Display for SuperError {
/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
/// write!(f, "SuperError is here!")
/// }
/// }
///
/// impl Error for SuperError {
/// fn source(&self) -> Option<&(dyn Error + 'static)> {
/// Some(&self.source)
/// }
/// }
///
/// #[derive(Debug)]
/// struct SuperErrorSideKick;
///
/// impl fmt::Display for SuperErrorSideKick {
/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
/// write!(f, "SuperErrorSideKick is here!")
/// }
/// }
///
/// impl Error for SuperErrorSideKick {}
///
/// fn get_super_error() -> Result<(), SuperError> {
/// Err(SuperError { source: SuperErrorSideKick })
/// }
///
/// fn main() {
/// match get_super_error() {
/// Err(e) => {
/// println!("Error: {e}");
/// println!("Caused by: {}", e.source().unwrap());
/// }
/// _ => println!("No error"),
/// }
/// }
/// ```
#[stable(feature = "error_source", since = "1.30.0")]
fn source(&self) -> Option<&(dyn Error + 'static)> {
None
}
/// Gets the `TypeId` of `self`.
#[doc(hidden)]
#[unstable(
feature = "error_type_id",
reason = "this is memory-unsafe to override in user code",
issue = "60784"
)]
fn type_id(&self, _: private::Internal) -> TypeId
where
Self: 'static,
{
TypeId::of::<Self>()
}
/// ```
/// if let Err(e) = "xc".parse::<u32>() {
/// // Print `e` itself, no need for description().
/// eprintln!("Error: {e}");
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[deprecated(since = "1.42.0", note = "use the Display impl or to_string()")]
fn description(&self) -> &str {
"description() is deprecated; use Display"
}
#[stable(feature = "rust1", since = "1.0.0")]
#[deprecated(
since = "1.33.0",
note = "replaced by Error::source, which can support downcasting"
)]
#[allow(missing_docs)]
fn cause(&self) -> Option<&dyn Error> {
self.source()
}
/// Provides type-based access to context intended for error reports.
///
/// 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(error_generic_member_access)]
/// use core::fmt;
/// use core::error::{request_ref, Request};
///
/// #[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")]
#[allow(unused_variables)]
fn provide<'a>(&'a self, request: &mut Request<'a>) {}
}
mod private {
// This is a hack to prevent `type_id` from being overridden by `Error`
// implementations, since that can enable unsound downcasting.
#[unstable(feature = "error_type_id", issue = "60784")]
#[derive(Debug)]
pub struct Internal;
}
#[unstable(feature = "never_type", issue = "35121")]
impl Error for ! {}
// Copied from `any.rs`.
impl dyn Error + 'static {
/// Returns `true` if the inner type is the same as `T`.
#[stable(feature = "error_downcast", since = "1.3.0")]
#[inline]
pub fn is<T: Error + 'static>(&self) -> bool {
// Get `TypeId` of the type this function is instantiated with.
let t = TypeId::of::<T>();
// Get `TypeId` of the type in the trait object (`self`).
let concrete = self.type_id(private::Internal);
// Compare both `TypeId`s on equality.
t == concrete
}
/// Returns some reference to the inner value if it is of type `T`, or
/// `None` if it isn't.
#[stable(feature = "error_downcast", since = "1.3.0")]
#[inline]
pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
if self.is::<T>() {
// SAFETY: `is` ensures this type cast is correct
unsafe { Some(&*(self as *const dyn Error as *const T)) }
} else {
None
}
}
/// Returns some mutable reference to the inner value if it is of type `T`, or
/// `None` if it isn't.
#[stable(feature = "error_downcast", since = "1.3.0")]
#[inline]
pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
if self.is::<T>() {
// SAFETY: `is` ensures this type cast is correct
unsafe { Some(&mut *(self as *mut dyn Error as *mut T)) }
} else {
None
}
}
}
impl dyn Error + 'static + Send {
/// Forwards to the method defined on the type `dyn Error`.
#[stable(feature = "error_downcast", since = "1.3.0")]
#[inline]
pub fn is<T: Error + 'static>(&self) -> bool {
<dyn Error + 'static>::is::<T>(self)
}
/// Forwards to the method defined on the type `dyn Error`.
#[stable(feature = "error_downcast", since = "1.3.0")]
#[inline]
pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
<dyn Error + 'static>::downcast_ref::<T>(self)
}
/// Forwards to the method defined on the type `dyn Error`.
#[stable(feature = "error_downcast", since = "1.3.0")]
#[inline]
pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
<dyn Error + 'static>::downcast_mut::<T>(self)
}
}
impl dyn Error + 'static + Send + Sync {
/// Forwards to the method defined on the type `dyn Error`.
#[stable(feature = "error_downcast", since = "1.3.0")]
#[inline]
pub fn is<T: Error + 'static>(&self) -> bool {
<dyn Error + 'static>::is::<T>(self)
}
/// Forwards to the method defined on the type `dyn Error`.
#[stable(feature = "error_downcast", since = "1.3.0")]
#[inline]
pub fn downcast_ref<T: Error + 'static>(&self) -> Option<&T> {
<dyn Error + 'static>::downcast_ref::<T>(self)
}
/// Forwards to the method defined on the type `dyn Error`.
#[stable(feature = "error_downcast", since = "1.3.0")]
#[inline]
pub fn downcast_mut<T: Error + 'static>(&mut self) -> Option<&mut T> {
<dyn Error + 'static>::downcast_mut::<T>(self)
}
}
impl dyn Error {
/// Returns an iterator starting with the current error and continuing with
/// recursively calling [`Error::source`].
///
/// If you want to omit the current error and only use its sources,
/// use `skip(1)`.
///
/// # Examples
///
/// ```
/// #![feature(error_iter)]
/// use std::error::Error;
/// use std::fmt;
///
/// #[derive(Debug)]
/// struct A;
///
/// #[derive(Debug)]
/// struct B(Option<Box<dyn Error + 'static>>);
///
/// impl fmt::Display for A {
/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
/// write!(f, "A")
/// }
/// }
///
/// impl fmt::Display for B {
/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
/// write!(f, "B")
/// }
/// }
///
/// impl Error for A {}
///
/// impl Error for B {
/// fn source(&self) -> Option<&(dyn Error + 'static)> {
/// self.0.as_ref().map(|e| e.as_ref())
/// }
/// }
///
/// let b = B(Some(Box::new(A)));
///
/// // let err : Box<Error> = b.into(); // or
/// let err = &b as &(dyn Error);
///
/// let mut iter = err.sources();
///
/// assert_eq!("B".to_string(), iter.next().unwrap().to_string());
/// assert_eq!("A".to_string(), iter.next().unwrap().to_string());
/// assert!(iter.next().is_none());
/// assert!(iter.next().is_none());
/// ```
#[unstable(feature = "error_iter", issue = "58520")]
#[inline]
pub fn sources(&self) -> Source<'_> {
// You may think this method would be better in the Error trait, and you'd be right.
// Unfortunately that doesn't work, not because of the object safety rules but because we
// save a reference to self in Sources below as a trait object. If this method was
// declared in Error, then self would have the type &T where T is some concrete type which
// implements Error. We would need to coerce self to have type &dyn Error, but that requires
// that Self has a known size (i.e., Self: Sized). We can't put that bound on Error
// since that would forbid Error trait objects, and we can't put that bound on the method
// because that means the method can't be called on trait objects (we'd also need the
// 'static bound, but that isn't allowed because methods with bounds on Self other than
// Sized are not object-safe). Requiring an Unsize bound is not backwards compatible.
Source { current: Some(self) }
}
}
/// Requests 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)
}
/// Requests 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 = Tagged { tag_id: TypeId::of::<I>(), value: TaggedOption::<'a, I>(None) };
err.provide(tagged.as_request());
tagged.value.0
}
///////////////////////////////////////////////////////////////////////////////
// Request and its methods
///////////////////////////////////////////////////////////////////////////////
/// `Request` supports generic, type-driven access to data. Its 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 functions called `request_ref` and
/// `request_value` 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")]
#[repr(transparent)]
pub struct Request<'a>(Tagged<dyn Erased<'a> + 'a>);
impl<'a> Request<'a> {
/// Provides 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)
}
/// Provides 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)
}
/// Provides 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)
}
/// Provides 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)
}
/// Provides 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
}
/// Provides 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
}
/// Checks 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
///
/// Checks if a `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>>()
}
/// Checks 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
///
/// Checks 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 `Tagged::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>> Tagged<TaggedOption<'a, I>> {
pub(crate) fn as_request(&mut self) -> &mut Request<'a> {
let erased = self as &mut Tagged<dyn Erased<'a> + 'a>;
// SAFETY: transmuting `&mut Tagged<dyn Erased<'a> + 'a>` to `&mut Request<'a>` is safe since
// `Request` is repr(transparent).
unsafe { &mut *(erased as *mut Tagged<dyn Erased<'a>> as *mut Request<'a>) }
}
}
/// Represents a type-erased but identifiable object.
///
/// This trait is exclusively implemented by the `TaggedOption` type.
unsafe trait Erased<'a>: 'a {}
unsafe impl<'a, I: tags::Type<'a>> Erased<'a> for TaggedOption<'a, I> {}
struct Tagged<E: ?Sized> {
tag_id: TypeId,
value: E,
}
impl<'a> Tagged<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::<Tagged<TaggedOption<'a, I>>>() }.value)
} 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>() {
Some(
// SAFETY: Just checked whether we're pointing to an I.
&mut unsafe { &mut *(self as *mut Self).cast::<Tagged<TaggedOption<'a, I>>>() }
.value,
)
} else {
None
}
}
}
/// An iterator over an [`Error`] and its sources.
///
/// If you want to omit the initial error and only process
/// its sources, use `skip(1)`.
#[unstable(feature = "error_iter", issue = "58520")]
#[derive(Clone, Debug)]
pub struct Source<'a> {
current: Option<&'a (dyn Error + 'static)>,
}
#[unstable(feature = "error_iter", issue = "58520")]
impl<'a> Iterator for Source<'a> {
type Item = &'a (dyn Error + 'static);
fn next(&mut self) -> Option<Self::Item> {
let current = self.current;
self.current = self.current.and_then(Error::source);
current
}
fn size_hint(&self) -> (usize, Option<usize>) {
if self.current.is_some() { (1, None) } else { (0, Some(0)) }
}
}
#[unstable(feature = "error_iter", issue = "58520")]
impl<'a> crate::iter::FusedIterator for Source<'a> {}
#[stable(feature = "error_by_ref", since = "1.51.0")]
impl<'a, T: Error + ?Sized> Error for &'a T {
#[allow(deprecated, deprecated_in_future)]
fn description(&self) -> &str {
Error::description(&**self)
}
#[allow(deprecated)]
fn cause(&self) -> Option<&dyn Error> {
Error::cause(&**self)
}
fn source(&self) -> Option<&(dyn Error + 'static)> {
Error::source(&**self)
}
fn provide<'b>(&'b self, request: &mut Request<'b>) {
Error::provide(&**self, request);
}
}
#[stable(feature = "fmt_error", since = "1.11.0")]
impl Error for crate::fmt::Error {
#[allow(deprecated)]
fn description(&self) -> &str {
"an error occurred when formatting an argument"
}
}
#[stable(feature = "try_borrow", since = "1.13.0")]
impl Error for crate::cell::BorrowError {
#[allow(deprecated)]
fn description(&self) -> &str {
"already mutably borrowed"
}
}
#[stable(feature = "try_borrow", since = "1.13.0")]
impl Error for crate::cell::BorrowMutError {
#[allow(deprecated)]
fn description(&self) -> &str {
"already borrowed"
}
}
#[stable(feature = "try_from", since = "1.34.0")]
impl Error for crate::char::CharTryFromError {
#[allow(deprecated)]
fn description(&self) -> &str {
"converted integer out of range for `char`"
}
}
#[stable(feature = "duration_checked_float", since = "1.66.0")]
impl Error for crate::time::TryFromFloatSecsError {}
#[stable(feature = "cstr_from_bytes_until_nul", since = "1.69.0")]
impl Error for crate::ffi::FromBytesUntilNulError {}
#[unstable(feature = "get_many_mut", issue = "104642")]
impl<const N: usize> Error for crate::slice::GetManyMutError<N> {}
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