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rust/library/core/src/error.rs
Marijn Schouten 1a1b52acbf clippy fix: indentation
2025-07-04 11:52:17 +00:00

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#![doc = include_str!("error.md")]
#![stable(feature = "error_in_core", since = "1.81.0")]
use crate::any::TypeId;
use crate::fmt::{self, Debug, Display, Formatter};
/// `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.
///
/// # Example
///
/// Implementing the `Error` trait only requires that `Debug` and `Display` are implemented too.
///
/// ```
/// use std::error::Error;
/// use std::fmt;
/// use std::path::PathBuf;
///
/// #[derive(Debug)]
/// struct ReadConfigError {
/// path: PathBuf
/// }
///
/// impl fmt::Display for ReadConfigError {
/// fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
/// let path = self.path.display();
/// write!(f, "unable to read configuration at {path}")
/// }
/// }
///
/// impl Error for ReadConfigError {}
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[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 dyn-incompatibility rules but
// because we save a reference to `self` in `Source`s 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
// dyn-incompatible). 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<'_>) -> fmt::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 {}
#[stable(feature = "get_many_mut", since = "1.86.0")]
impl Error for crate::slice::GetDisjointMutError {}
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