rust/library/core/src/error.rs

#![doc = include_str!("error.md")]
#![unstable(feature = "error_in_core", issue = "none")]

#[cfg(test)]
mod tests;

use crate::any::{Demand, Provider, TypeId};
use crate::fmt::{Debug, Display};

/// `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]
pub trait Error: Debug + Display {
    /// 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 [`Demand::provide_value`] and [`Demand::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;
    ///
    /// #[derive(Debug)]
    /// struct MyBacktrace {
    ///     // ...
    /// }
    ///
    /// impl MyBacktrace {
    ///     fn new() -> MyBacktrace {
    ///         // ...
    ///         # MyBacktrace {}
    ///     }
    /// }
    ///
    /// #[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,
    /// }
    ///
    /// 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, demand: &mut Demand<'a>) {
    ///         demand
    ///             .provide_ref::<MyBacktrace>(&self.backtrace)
    ///             .provide_ref::<dyn std::error::Error + 'static>(&self.source);
    ///     }
    /// }
    ///
    /// fn main() {
    ///     let backtrace = MyBacktrace::new();
    ///     let source = SourceError {};
    ///     let error = Error { source, backtrace };
    ///     let dyn_error = &error as &dyn std::error::Error;
    ///     let backtrace_ref = dyn_error.request_ref::<MyBacktrace>().unwrap();
    ///
    ///     assert!(core::ptr::eq(&error.backtrace, backtrace_ref));
    /// }
    /// ```
    #[unstable(feature = "error_generic_member_access", issue = "99301")]
    #[allow(unused_variables)]
    fn provide<'a>(&'a self, demand: &mut Demand<'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.
    #[unstable(feature = "error_type_id", issue = "60784")]
    #[derive(Debug)]
    pub struct Internal;
}

#[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`.
    #[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)
    }

    /// 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 {
    /// 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)
    }

    /// 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 {
    /// 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) }
    }
}

/// 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
    }
}

#[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, demand: &mut Demand<'b>) {
        Error::provide(&**self, demand);
    }
}

#[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 = "char_from_str", since = "1.20.0")]
impl Error for crate::char::ParseCharError {
    #[allow(deprecated)]
    fn description(&self) -> &str {
        self.__description()
    }
}

#[unstable(feature = "duration_checked_float", issue = "83400")]
impl Error for crate::time::FromFloatSecsError {}

#[stable(feature = "frombyteswithnulerror_impls", since = "1.17.0")]
impl Error for crate::ffi::FromBytesWithNulError {
    #[allow(deprecated)]
    fn description(&self) -> &str {
        self.__description()
    }
}

#[unstable(feature = "cstr_from_bytes_until_nul", issue = "95027")]
impl Error for crate::ffi::FromBytesUntilNulError {}