library/core/src/io/error.rs

#![unstable(feature = "core_io", issue = "154046")]

// On 64-bit platforms, `io::Error` may use a bit-packed representation to
// reduce size. However, this representation assumes that error codes are
// always 32-bit wide.
//
// This assumption is invalid on 64-bit UEFI, where error codes are 64-bit.
// Therefore, the packed representation is explicitly disabled for UEFI
// targets, and the unpacked representation must be used instead.
#[cfg_attr(
    all(target_pointer_width = "64", not(target_os = "uefi")),
    path = "error/repr_bitpacked.rs"
)]
#[cfg_attr(
    not(all(target_pointer_width = "64", not(target_os = "uefi"))),
    path = "error/repr_unpacked.rs"
)]
mod repr;

#[cfg_attr(
    all(target_has_atomic_load_store = "ptr", not(no_io_statics)),
    path = "error/os_functions_atomic.rs"
)]
#[cfg_attr(
    not(all(target_has_atomic_load_store = "ptr", not(no_io_statics))),
    path = "error/os_functions.rs"
)]
mod os_functions;

use self::os_functions::{decode_error_kind, format_os_error, is_interrupted, set_functions};
use self::repr::Repr;
use crate::{error, fmt, result};

/// A specialized [`Result`] type for I/O operations.
///
/// This type is broadly used across [`std::io`] for any operation which may
/// produce an error.
///
/// This type alias is generally used to avoid writing out [`io::Error`] directly and
/// is otherwise a direct mapping to [`Result`].
///
/// While usual Rust style is to import types directly, aliases of [`Result`]
/// often are not, to make it easier to distinguish between them. [`Result`] is
/// generally assumed to be [`core::result::Result`][`Result`], and so users of this alias
/// will generally use `io::Result` instead of shadowing the [prelude]'s import
/// of [`core::result::Result`][`Result`].
///
/// [`std::io`]: ../../std/io/index.html
/// [`io::Error`]: Error
/// [`Result`]: crate::result::Result
/// [prelude]: crate::prelude
///
/// # Examples
///
/// A convenience function that bubbles an `io::Result` to its caller:
///
/// ```
/// use std::io;
///
/// fn get_string() -> io::Result<String> {
///     let mut buffer = String::new();
///
///     io::stdin().read_line(&mut buffer)?;
///
///     Ok(buffer)
/// }
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[doc(search_unbox)]
pub type Result<T> = result::Result<T, Error>;

/// The error type for I/O operations of the [`Read`][Read], [`Write`][Write], [`Seek`][Seek], and
/// associated traits.
///
/// Errors mostly originate from the underlying OS, but custom instances of
/// `Error` can be created with crafted error messages and a particular value of
/// [`ErrorKind`].
///
/// [Read]: ../../std/io/trait.Read.html
/// [Write]: ../../std/io/trait.Write.html
/// [Seek]: ../../std/io/trait.Seek.html
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_has_incoherent_inherent_impls]
pub struct Error {
    repr: Repr,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Debug for Error {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Debug::fmt(&self.repr, f)
    }
}

/// Common errors constants for use in std
#[doc(hidden)]
impl Error {
    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    pub const INVALID_UTF8: Self =
        const_error!(ErrorKind::InvalidData, "stream did not contain valid UTF-8");

    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    pub const READ_EXACT_EOF: Self =
        const_error!(ErrorKind::UnexpectedEof, "failed to fill whole buffer");

    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    pub const UNKNOWN_THREAD_COUNT: Self = const_error!(
        ErrorKind::NotFound,
        "the number of hardware threads is not known for the target platform",
    );

    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    pub const UNSUPPORTED_PLATFORM: Self =
        const_error!(ErrorKind::Unsupported, "operation not supported on this platform");

    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    pub const WRITE_ALL_EOF: Self =
        const_error!(ErrorKind::WriteZero, "failed to write whole buffer");

    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    pub const ZERO_TIMEOUT: Self =
        const_error!(ErrorKind::InvalidInput, "cannot set a 0 duration timeout");

    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    pub const NO_ADDRESSES: Self =
        const_error!(ErrorKind::InvalidInput, "could not resolve to any addresses");
}

// Only derive debug in tests, to make sure it
// doesn't accidentally get printed.
#[cfg_attr(test, derive(Debug))]
enum ErrorData<C> {
    Os(RawOsError),
    Simple(ErrorKind),
    SimpleMessage(&'static SimpleMessage),
    Custom(C),
}

// `#[repr(align(4))]` is probably redundant, it should have that value or
// higher already. We include it just because repr_bitpacked.rs's encoding
// requires an alignment >= 4 (note that `#[repr(align)]` will not reduce the
// alignment required by the struct, only increase it).
//
// If we add more variants to ErrorData, this can be increased to 8, but it
// should probably be behind `#[cfg_attr(target_pointer_width = "64", ...)]` or
// whatever cfg we're using to enable the `repr_bitpacked` code, since only the
// that version needs the alignment, and 8 is higher than the alignment we'll
// have on 32 bit platforms.
//
// (For the sake of being explicit: the alignment requirement here only matters
// if `error/repr_bitpacked.rs` is in use — for the unpacked repr it doesn't
// matter at all)
#[doc(hidden)]
#[unstable(feature = "io_const_error_internals", issue = "none")]
#[repr(align(4))]
#[derive(Debug)]
pub struct SimpleMessage {
    pub kind: ErrorKind,
    pub message: &'static str,
}

/// Creates a new I/O error from a known kind of error and a string literal.
///
/// Contrary to [`Error::new`][new], this macro does not allocate and can be used in
/// `const` contexts.
///
/// [new]: ../../alloc/io/struct.Error.html#method.new
///
/// # Example
/// ```
/// #![feature(io_const_error)]
/// use std::io::{const_error, Error, ErrorKind};
///
/// const FAIL: Error = const_error!(ErrorKind::Unsupported, "tried something that never works");
///
/// fn not_here() -> Result<(), Error> {
///     Err(FAIL)
/// }
/// ```
#[rustc_macro_transparency = "semiopaque"]
#[unstable(feature = "io_const_error", issue = "133448")]
#[allow_internal_unstable(core_io, hint_must_use, io_const_error_internals)]
pub macro const_error($kind:expr, $message:expr $(,)?) {
    $crate::hint::must_use($crate::io::Error::from_static_message(
        const { &$crate::io::SimpleMessage { kind: $kind, message: $message } },
    ))
}

/// Intended for use for errors not exposed to the user, where allocating onto
/// the heap (for normal construction via Error::new) is too costly.
#[stable(feature = "io_error_from_errorkind", since = "1.14.0")]
impl From<ErrorKind> for Error {
    /// Converts an [`ErrorKind`] into an [`Error`].
    ///
    /// This conversion creates a new error with a simple representation of error kind.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::{Error, ErrorKind};
    ///
    /// let not_found = ErrorKind::NotFound;
    /// let error = Error::from(not_found);
    /// assert_eq!("entity not found", format!("{error}"));
    /// ```
    #[inline]
    fn from(kind: ErrorKind) -> Error {
        Error { repr: Repr::new_simple(kind) }
    }
}

impl Error {
    /// # Safety
    ///
    /// The provided `CustomOwner` must have been constructed from a `Box` from the `alloc` crate.
    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    #[must_use]
    #[inline]
    pub unsafe fn from_custom_owner(custom: CustomOwner) -> Error {
        Error { repr: Repr::new_custom(custom) }
    }

    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    #[must_use]
    #[inline]
    pub fn into_custom_owner(self) -> result::Result<CustomOwner, Self> {
        if matches!(self.repr.data(), ErrorData::Custom(..)) {
            let ErrorData::Custom(c) = self.repr.into_data() else {
                // SAFETY: Checked above using `matches!`.
                unsafe { crate::hint::unreachable_unchecked() }
            };
            Ok(c)
        } else {
            Err(self)
        }
    }

    /// Creates a new I/O error from a known kind of error as well as a constant
    /// message.
    ///
    /// This function does not allocate.
    ///
    /// You should not use this directly, and instead use the `const_error!`
    /// macro: `io::const_error!(ErrorKind::Something, "some_message")`.
    ///
    /// This function should maybe change to `from_static_message<const MSG: &'static
    /// str>(kind: ErrorKind)` in the future, when const generics allow that.
    #[inline]
    #[doc(hidden)]
    #[unstable(feature = "io_const_error_internals", issue = "none")]
    pub const fn from_static_message(msg: &'static SimpleMessage) -> Error {
        Self { repr: Repr::new_simple_message(msg) }
    }

    /// # Safety
    ///
    /// `functions` must point to data that is entirely constant; it must
    /// not be created during runtime.
    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    #[must_use]
    #[inline]
    pub unsafe fn from_raw_os_error_with_functions(
        code: RawOsError,
        functions: &'static OsFunctions,
    ) -> Error {
        // SAFETY: Caller ensures `functions` is a constant not created at runtime.
        unsafe {
            set_functions(functions);
        }
        Error { repr: Repr::new_os(code) }
    }

    /// Returns the OS error that this error represents (if any).
    ///
    /// If this [`Error`] was constructed via [`last_os_error`][last_os_error] or
    /// [`from_raw_os_error`][from_raw_os_error], then this function will return [`Some`], otherwise
    /// it will return [`None`].
    ///
    /// [last_os_error]: ../../std/io/struct.Error.html#method.last_os_error
    /// [from_raw_os_error]: ../../std/io/struct.Error.html#method.from_raw_os_error
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::{Error, ErrorKind};
    ///
    /// fn print_os_error(err: &Error) {
    ///     if let Some(raw_os_err) = err.raw_os_error() {
    ///         println!("raw OS error: {raw_os_err:?}");
    ///     } else {
    ///         println!("Not an OS error");
    ///     }
    /// }
    ///
    /// fn main() {
    ///     // Will print "raw OS error: ...".
    ///     print_os_error(&Error::last_os_error());
    ///     // Will print "Not an OS error".
    ///     print_os_error(&Error::new(ErrorKind::Other, "oh no!"));
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    #[inline]
    pub fn raw_os_error(&self) -> Option<RawOsError> {
        match self.repr.data() {
            ErrorData::Os(i) => Some(i),
            ErrorData::Custom(..) => None,
            ErrorData::Simple(..) => None,
            ErrorData::SimpleMessage(..) => None,
        }
    }

    /// Returns a reference to the inner error wrapped by this error (if any).
    ///
    /// If this [`Error`] was constructed via [`new`][new] then this function will
    /// return [`Some`], otherwise it will return [`None`].
    ///
    /// [new]: ../../alloc/io/struct.Error.html#method.new
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::{Error, ErrorKind};
    ///
    /// fn print_error(err: &Error) {
    ///     if let Some(inner_err) = err.get_ref() {
    ///         println!("Inner error: {inner_err:?}");
    ///     } else {
    ///         println!("No inner error");
    ///     }
    /// }
    ///
    /// fn main() {
    ///     // Will print "No inner error".
    ///     print_error(&Error::last_os_error());
    ///     // Will print "Inner error: ...".
    ///     print_error(&Error::new(ErrorKind::Other, "oh no!"));
    /// }
    /// ```
    #[stable(feature = "io_error_inner", since = "1.3.0")]
    #[must_use]
    #[inline]
    pub fn get_ref(&self) -> Option<&(dyn error::Error + Send + Sync + 'static)> {
        match self.repr.data() {
            ErrorData::Os(..) => None,
            ErrorData::Simple(..) => None,
            ErrorData::SimpleMessage(..) => None,
            ErrorData::Custom(c) => Some(c.error_ref()),
        }
    }

    /// Returns a mutable reference to the inner error wrapped by this error
    /// (if any).
    ///
    /// If this [`Error`] was constructed via [`new`][new] then this function will
    /// return [`Some`], otherwise it will return [`None`].
    ///
    /// [new]: ../../alloc/io/struct.Error.html#method.new
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::{Error, ErrorKind};
    /// use std::{error, fmt};
    /// use std::fmt::Display;
    ///
    /// #[derive(Debug)]
    /// struct MyError {
    ///     v: String,
    /// }
    ///
    /// impl MyError {
    ///     fn new() -> MyError {
    ///         MyError {
    ///             v: "oh no!".to_string()
    ///         }
    ///     }
    ///
    ///     fn change_message(&mut self, new_message: &str) {
    ///         self.v = new_message.to_string();
    ///     }
    /// }
    ///
    /// impl error::Error for MyError {}
    ///
    /// impl Display for MyError {
    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
    ///         write!(f, "MyError: {}", self.v)
    ///     }
    /// }
    ///
    /// fn change_error(mut err: Error) -> Error {
    ///     if let Some(inner_err) = err.get_mut() {
    ///         inner_err.downcast_mut::<MyError>().unwrap().change_message("I've been changed!");
    ///     }
    ///     err
    /// }
    ///
    /// fn print_error(err: &Error) {
    ///     if let Some(inner_err) = err.get_ref() {
    ///         println!("Inner error: {inner_err}");
    ///     } else {
    ///         println!("No inner error");
    ///     }
    /// }
    ///
    /// fn main() {
    ///     // Will print "No inner error".
    ///     print_error(&change_error(Error::last_os_error()));
    ///     // Will print "Inner error: ...".
    ///     print_error(&change_error(Error::new(ErrorKind::Other, MyError::new())));
    /// }
    /// ```
    #[stable(feature = "io_error_inner", since = "1.3.0")]
    #[must_use]
    #[inline]
    pub fn get_mut(&mut self) -> Option<&mut (dyn error::Error + Send + Sync + 'static)> {
        match self.repr.data_mut() {
            ErrorData::Os(..) => None,
            ErrorData::Simple(..) => None,
            ErrorData::SimpleMessage(..) => None,
            ErrorData::Custom(c) => Some(c.error_mut()),
        }
    }

    /// Returns the corresponding [`ErrorKind`] for this error.
    ///
    /// This may be a value set by Rust code constructing custom `io::Error`s,
    /// or if this `io::Error` was sourced from the operating system,
    /// it will be a value inferred from the system's error encoding.
    /// See [`last_os_error`][last_os_error] for more details.
    ///
    /// [last_os_error]: ../../std/io/struct.Error.html#method.last_os_error
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::{Error, ErrorKind};
    ///
    /// fn print_error(err: Error) {
    ///     println!("{:?}", err.kind());
    /// }
    ///
    /// fn main() {
    ///     // As no error has (visibly) occurred, this may print anything!
    ///     // It likely prints a placeholder for unidentified (non-)errors.
    ///     print_error(Error::last_os_error());
    ///     // Will print "AddrInUse".
    ///     print_error(Error::new(ErrorKind::AddrInUse, "oh no!"));
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[must_use]
    #[inline]
    pub fn kind(&self) -> ErrorKind {
        match self.repr.data() {
            ErrorData::Os(code) => decode_error_kind(code),
            ErrorData::Custom(c) => c.kind,
            ErrorData::Simple(kind) => kind,
            ErrorData::SimpleMessage(m) => m.kind,
        }
    }

    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    #[doc(hidden)]
    #[inline]
    pub fn is_interrupted(&self) -> bool {
        match self.repr.data() {
            ErrorData::Os(code) => is_interrupted(code),
            ErrorData::Custom(c) => c.kind == ErrorKind::Interrupted,
            ErrorData::Simple(kind) => kind == ErrorKind::Interrupted,
            ErrorData::SimpleMessage(m) => m.kind == ErrorKind::Interrupted,
        }
    }
}

impl fmt::Debug for Repr {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self.data() {
            ErrorData::Os(code) => fmt
                .debug_struct("Os")
                .field("code", &code)
                .field("kind", &decode_error_kind(code))
                .field(
                    "message",
                    &fmt::from_fn(|fmt| {
                        write!(fmt, "\"{}\"", fmt::from_fn(|fmt| format_os_error(code, fmt)))
                    }),
                )
                .finish(),
            ErrorData::Custom(c) => fmt::Debug::fmt(&c, fmt),
            ErrorData::Simple(kind) => fmt.debug_tuple("Kind").field(&kind).finish(),
            ErrorData::SimpleMessage(msg) => fmt
                .debug_struct("Error")
                .field("kind", &msg.kind)
                .field("message", &msg.message)
                .finish(),
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl fmt::Display for Error {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self.repr.data() {
            ErrorData::Os(code) => {
                let detail = fmt::from_fn(|fmt| format_os_error(code, fmt));
                write!(fmt, "{detail} (os error {code})")
            }
            ErrorData::Custom(c) => fmt::Display::fmt(c.error_ref(), fmt),
            ErrorData::Simple(kind) => kind.fmt(fmt),
            ErrorData::SimpleMessage(msg) => msg.message.fmt(fmt),
        }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl error::Error for Error {
    #[allow(deprecated)]
    fn cause(&self) -> Option<&dyn error::Error> {
        match self.repr.data() {
            ErrorData::Os(..) => None,
            ErrorData::Simple(..) => None,
            ErrorData::SimpleMessage(..) => None,
            ErrorData::Custom(c) => c.error_ref().cause(),
        }
    }

    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
        match self.repr.data() {
            ErrorData::Os(..) => None,
            ErrorData::Simple(..) => None,
            ErrorData::SimpleMessage(..) => None,
            ErrorData::Custom(c) => c.error_ref().source(),
        }
    }
}

fn _assert_error_is_sync_send() {
    fn _is_sync_send<T: Sync + Send>() {}
    _is_sync_send::<Error>();
}

#[doc(hidden)]
#[derive(Debug)]
#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
pub struct OsFunctions {
    pub format_os_error: fn(_: RawOsError, _: &mut fmt::Formatter<'_>) -> fmt::Result,
    pub decode_error_kind: fn(_: RawOsError) -> ErrorKind,
    pub is_interrupted: fn(_: RawOsError) -> bool,
}

impl OsFunctions {
    const DEFAULT: &'static OsFunctions = &OsFunctions {
        format_os_error: |_, _| Ok(()),
        decode_error_kind: |_| ErrorKind::Uncategorized,
        is_interrupted: |_| false,
    };
}

// As with `SimpleMessage`: `#[repr(align(4))]` here is just because
// repr_bitpacked's encoding requires it. In practice it almost certainly be
// already be this high or higher.
#[doc(hidden)]
#[repr(align(4))]
#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
pub struct Custom {
    kind: ErrorKind,
    error: crate::ptr::NonNull<dyn error::Error + Send + Sync>,
    error_drop: unsafe fn(*mut (dyn error::Error + Send + Sync)),
    outer_drop: unsafe fn(*mut Self),
}

// SAFETY: All members of `Custom` are `Send`
#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
unsafe impl Send for Custom {}

// SAFETY: All members of `Custom` are `Sync`
#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
unsafe impl Sync for Custom {}

#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
impl fmt::Debug for Custom {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Custom").field("kind", &self.kind).field("error", self.error_ref()).finish()
    }
}

#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
impl Drop for Custom {
    fn drop(&mut self) {
        // SAFETY: `Custom::from_raw` ensures this call is safe.
        unsafe {
            (self.error_drop)(self.error.as_ptr());
        }
    }
}

impl Custom {
    /// # Safety
    ///
    /// * `error` must be valid for up to a static lifetime, and own its pointee.
    /// * `error_drop` must be safe to call for the pointer `error` exactly once.
    /// * `outer_drop` must be safe to call on a pointer to this instance of `Custom`
    ///   if it were stored within a [`CustomOwner`].
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    pub unsafe fn from_raw(
        kind: ErrorKind,
        error: crate::ptr::NonNull<dyn error::Error + Send + Sync>,
        error_drop: unsafe fn(*mut (dyn error::Error + Send + Sync)),
        outer_drop: unsafe fn(*mut Self),
    ) -> Custom {
        Custom { kind, error, error_drop, outer_drop }
    }

    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    pub fn into_raw(self) -> crate::ptr::NonNull<dyn error::Error + Send + Sync> {
        let ptr = self.error;
        core::mem::forget(self);
        ptr
    }

    fn error_ref(&self) -> &(dyn error::Error + Send + Sync + 'static) {
        // SAFETY:
        // `from_raw` ensures `error` is a valid pointer up to a static lifetime
        // and is owned by `self`
        unsafe { self.error.as_ref() }
    }

    fn error_mut(&mut self) -> &mut (dyn error::Error + Send + Sync + 'static) {
        // SAFETY:
        // `from_raw` ensures `error` is a valid pointer up to a static lifetime
        // and is owned by `self`
        unsafe { self.error.as_mut() }
    }
}

#[derive(Debug)]
#[repr(transparent)]
#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
#[doc(hidden)]
pub struct CustomOwner(crate::ptr::NonNull<Custom>);

// SAFETY: Custom is `Send`
#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
unsafe impl Send for CustomOwner {}

// SAFETY: Custom is `Sync`
#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
unsafe impl Sync for CustomOwner {}

#[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
impl Drop for CustomOwner {
    fn drop(&mut self) {
        // SAFETY: `CustomOwner::from_raw` ensures this call is safe.
        unsafe {
            (self.0.as_ref().outer_drop)(self.0.as_ptr());
        }
    }
}

impl CustomOwner {
    /// # Safety
    ///
    /// * The `outer_drop` of the provided `custom` must be safe to call exactly once.
    #[doc(hidden)]
    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    pub unsafe fn from_raw(custom: crate::ptr::NonNull<Custom>) -> CustomOwner {
        CustomOwner(custom)
    }

    #[unstable(feature = "core_io_internals", reason = "exposed only for libstd", issue = "none")]
    pub fn into_raw(self) -> crate::ptr::NonNull<Custom> {
        let ptr = self.0;
        core::mem::forget(self);
        ptr
    }

    #[allow(dead_code, reason = "only used for unpacked representation")]
    fn custom_ref(&self) -> &Custom {
        // SAFETY:
        // `from_raw` ensures `0` is a valid pointer up to a static lifetime
        // and is owned by `self`
        unsafe { self.0.as_ref() }
    }

    #[allow(dead_code, reason = "only used for unpacked representation")]
    fn custom_mut(&mut self) -> &mut Custom {
        // SAFETY:
        // `from_raw` ensures `0` is a valid pointer up to a static lifetime
        // and is owned by `self`
        unsafe { self.0.as_mut() }
    }
}

/// The type of raw OS error codes.
///
/// This is an [`i32`] on all currently supported platforms, but platforms
/// added in the future (such as UEFI) may use a different primitive type like
/// [`usize`]. Use `as` or [`into`] conversions where applicable to ensure maximum
/// portability.
///
/// [`into`]: Into::into
#[unstable(feature = "raw_os_error_ty", issue = "107792")]
pub type RawOsError = cfg_select! {
    target_os = "uefi" => usize,
    _ => i32,
};

/// A list specifying general categories of I/O error.
///
/// This list is intended to grow over time and it is not recommended to
/// exhaustively match against it.
///
/// It is used with the [`io::Error`][error] type.
///
/// [error]: Error
///
/// # Handling errors and matching on `ErrorKind`
///
/// In application code, use `match` for the `ErrorKind` values you are
/// expecting; use `_` to match "all other errors".
///
/// In comprehensive and thorough tests that want to verify that a test doesn't
/// return any known incorrect error kind, you may want to cut-and-paste the
/// current full list of errors from here into your test code, and then match
/// `_` as the correct case. This seems counterintuitive, but it will make your
/// tests more robust. In particular, if you want to verify that your code does
/// produce an unrecognized error kind, the robust solution is to check for all
/// the recognized error kinds and fail in those cases.
#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
#[stable(feature = "rust1", since = "1.0.0")]
#[cfg_attr(not(test), rustc_diagnostic_item = "io_errorkind")]
#[allow(deprecated)]
#[non_exhaustive]
pub enum ErrorKind {
    /// An entity was not found, often a file.
    #[stable(feature = "rust1", since = "1.0.0")]
    NotFound,
    /// The operation lacked the necessary privileges to complete.
    #[stable(feature = "rust1", since = "1.0.0")]
    PermissionDenied,
    /// The connection was refused by the remote server.
    #[stable(feature = "rust1", since = "1.0.0")]
    ConnectionRefused,
    /// The connection was reset by the remote server.
    #[stable(feature = "rust1", since = "1.0.0")]
    ConnectionReset,
    /// The remote host is not reachable.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    HostUnreachable,
    /// The network containing the remote host is not reachable.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    NetworkUnreachable,
    /// The connection was aborted (terminated) by the remote server.
    #[stable(feature = "rust1", since = "1.0.0")]
    ConnectionAborted,
    /// The network operation failed because it was not connected yet.
    #[stable(feature = "rust1", since = "1.0.0")]
    NotConnected,
    /// A socket address could not be bound because the address is already in
    /// use elsewhere.
    #[stable(feature = "rust1", since = "1.0.0")]
    AddrInUse,
    /// A nonexistent interface was requested or the requested address was not
    /// local.
    #[stable(feature = "rust1", since = "1.0.0")]
    AddrNotAvailable,
    /// The system's networking is down.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    NetworkDown,
    /// The operation failed because a pipe was closed.
    #[stable(feature = "rust1", since = "1.0.0")]
    BrokenPipe,
    /// An entity already exists, often a file.
    #[stable(feature = "rust1", since = "1.0.0")]
    AlreadyExists,
    /// The operation needs to block to complete, but the blocking operation was
    /// requested to not occur.
    #[stable(feature = "rust1", since = "1.0.0")]
    WouldBlock,
    /// A filesystem object is, unexpectedly, not a directory.
    ///
    /// For example, a filesystem path was specified where one of the intermediate directory
    /// components was, in fact, a plain file.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    NotADirectory,
    /// The filesystem object is, unexpectedly, a directory.
    ///
    /// A directory was specified when a non-directory was expected.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    IsADirectory,
    /// A non-empty directory was specified where an empty directory was expected.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    DirectoryNotEmpty,
    /// The filesystem or storage medium is read-only, but a write operation was attempted.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    ReadOnlyFilesystem,
    /// Loop in the filesystem or IO subsystem; often, too many levels of symbolic links.
    ///
    /// There was a loop (or excessively long chain) resolving a filesystem object
    /// or file IO object.
    ///
    /// On Unix this is usually the result of a symbolic link loop; or, of exceeding the
    /// system-specific limit on the depth of symlink traversal.
    #[unstable(feature = "io_error_more", issue = "86442")]
    FilesystemLoop,
    /// Stale network file handle.
    ///
    /// With some network filesystems, notably NFS, an open file (or directory) can be invalidated
    /// by problems with the network or server.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    StaleNetworkFileHandle,
    /// A parameter was incorrect.
    #[stable(feature = "rust1", since = "1.0.0")]
    InvalidInput,
    /// Data not valid for the operation were encountered.
    ///
    /// Unlike [`InvalidInput`], this typically means that the operation
    /// parameters were valid, however the error was caused by malformed
    /// input data.
    ///
    /// For example, a function that reads a file into a string will error with
    /// `InvalidData` if the file's contents are not valid UTF-8.
    ///
    /// [`InvalidInput`]: ErrorKind::InvalidInput
    #[stable(feature = "io_invalid_data", since = "1.2.0")]
    InvalidData,
    /// The I/O operation's timeout expired, causing it to be canceled.
    #[stable(feature = "rust1", since = "1.0.0")]
    TimedOut,
    /// An error returned when an operation could not be completed because a
    /// call to [`write`][write] returned [`Ok(0)`].
    ///
    /// This typically means that an operation could only succeed if it wrote a
    /// particular number of bytes but only a smaller number of bytes could be
    /// written.
    ///
    /// [write]: ../../std/io/trait.Write.html#tymethod.write
    /// [`Ok(0)`]: Ok
    #[stable(feature = "rust1", since = "1.0.0")]
    WriteZero,
    /// The underlying storage (typically, a filesystem) is full.
    ///
    /// This does not include out of quota errors.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    StorageFull,
    /// Seek on unseekable file.
    ///
    /// Seeking was attempted on an open file handle which is not suitable for seeking - for
    /// example, on Unix, a named pipe opened with `File::open`.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    NotSeekable,
    /// Filesystem quota or some other kind of quota was exceeded.
    #[stable(feature = "io_error_quota_exceeded", since = "1.85.0")]
    QuotaExceeded,
    /// File larger than allowed or supported.
    ///
    /// This might arise from a hard limit of the underlying filesystem or file access API, or from
    /// an administratively imposed resource limitation.  Simple disk full, and out of quota, have
    /// their own errors.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    FileTooLarge,
    /// Resource is busy.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    ResourceBusy,
    /// Executable file is busy.
    ///
    /// An attempt was made to write to a file which is also in use as a running program.  (Not all
    /// operating systems detect this situation.)
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    ExecutableFileBusy,
    /// Deadlock (avoided).
    ///
    /// A file locking operation would result in deadlock.  This situation is typically detected, if
    /// at all, on a best-effort basis.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    Deadlock,
    /// Cross-device or cross-filesystem (hard) link or rename.
    #[stable(feature = "io_error_crosses_devices", since = "1.85.0")]
    CrossesDevices,
    /// Too many (hard) links to the same filesystem object.
    ///
    /// The filesystem does not support making so many hardlinks to the same file.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    TooManyLinks,
    /// A filename was invalid.
    ///
    /// This error can also occur if a length limit for a name was exceeded.
    #[stable(feature = "io_error_invalid_filename", since = "1.87.0")]
    InvalidFilename,
    /// Program argument list too long.
    ///
    /// When trying to run an external program, a system or process limit on the size of the
    /// arguments would have been exceeded.
    #[stable(feature = "io_error_a_bit_more", since = "1.83.0")]
    ArgumentListTooLong,
    /// This operation was interrupted.
    ///
    /// Interrupted operations can typically be retried.
    #[stable(feature = "rust1", since = "1.0.0")]
    Interrupted,

    /// This operation is unsupported on this platform.
    ///
    /// This means that the operation can never succeed.
    #[stable(feature = "unsupported_error", since = "1.53.0")]
    Unsupported,

    // ErrorKinds which are primarily categorisations for OS error
    // codes should be added above.
    //
    /// An error returned when an operation could not be completed because an
    /// "end of file" was reached prematurely.
    ///
    /// This typically means that an operation could only succeed if it read a
    /// particular number of bytes but only a smaller number of bytes could be
    /// read.
    #[stable(feature = "read_exact", since = "1.6.0")]
    UnexpectedEof,

    /// An operation could not be completed, because it failed
    /// to allocate enough memory.
    #[stable(feature = "out_of_memory_error", since = "1.54.0")]
    OutOfMemory,

    /// The operation was partially successful and needs to be checked
    /// later on due to not blocking.
    #[unstable(feature = "io_error_inprogress", issue = "130840")]
    InProgress,

    /// The process or the whole system has reached its limit on the number of
    /// open files or sockets.
    #[unstable(feature = "io_error_too_many_open_files", issue = "158319")]
    TooManyOpenFiles,

    // "Unusual" error kinds which do not correspond simply to (sets
    // of) OS error codes, should be added just above this comment.
    // `Other` and `Uncategorized` should remain at the end:
    //
    /// A custom error that does not fall under any other I/O error kind.
    ///
    /// This can be used to construct your own [`Error`][error]s that do not match any
    /// [`ErrorKind`].
    ///
    /// This [`ErrorKind`] is not used by the standard library.
    ///
    /// Errors from the standard library that do not fall under any of the I/O
    /// error kinds cannot be `match`ed on, and will only match a wildcard (`_`) pattern.
    /// New [`ErrorKind`]s might be added in the future for some of those.
    ///
    /// [error]: Error
    #[stable(feature = "rust1", since = "1.0.0")]
    Other,

    /// Any I/O error from the standard library that's not part of this list.
    ///
    /// Errors that are `Uncategorized` now may move to a different or a new
    /// [`ErrorKind`] variant in the future. It is not recommended to match
    /// an error against `Uncategorized`; use a wildcard match (`_`) instead.
    #[unstable(feature = "io_error_uncategorized", issue = "none")]
    #[doc(hidden)]
    Uncategorized,
}

impl ErrorKind {
    const fn as_str(&self) -> &'static str {
        use ErrorKind::*;
        match *self {
            // tidy-alphabetical-start
            AddrInUse => "address in use",
            AddrNotAvailable => "address not available",
            AlreadyExists => "entity already exists",
            ArgumentListTooLong => "argument list too long",
            BrokenPipe => "broken pipe",
            ConnectionAborted => "connection aborted",
            ConnectionRefused => "connection refused",
            ConnectionReset => "connection reset",
            CrossesDevices => "cross-device link or rename",
            Deadlock => "deadlock",
            DirectoryNotEmpty => "directory not empty",
            ExecutableFileBusy => "executable file busy",
            FileTooLarge => "file too large",
            FilesystemLoop => "filesystem loop or indirection limit (e.g. symlink loop)",
            HostUnreachable => "host unreachable",
            InProgress => "in progress",
            Interrupted => "operation interrupted",
            InvalidData => "invalid data",
            InvalidFilename => "invalid filename",
            InvalidInput => "invalid input parameter",
            IsADirectory => "is a directory",
            NetworkDown => "network down",
            NetworkUnreachable => "network unreachable",
            NotADirectory => "not a directory",
            NotConnected => "not connected",
            NotFound => "entity not found",
            NotSeekable => "seek on unseekable file",
            Other => "other error",
            OutOfMemory => "out of memory",
            PermissionDenied => "permission denied",
            QuotaExceeded => "quota exceeded",
            ReadOnlyFilesystem => "read-only filesystem or storage medium",
            ResourceBusy => "resource busy",
            StaleNetworkFileHandle => "stale network file handle",
            StorageFull => "no storage space",
            TimedOut => "timed out",
            TooManyLinks => "too many links",
            TooManyOpenFiles => "too many open files",
            Uncategorized => "uncategorized error",
            UnexpectedEof => "unexpected end of file",
            Unsupported => "unsupported",
            WouldBlock => "operation would block",
            WriteZero => "write zero",
            // tidy-alphabetical-end
        }
    }

    // This compiles to the same code as the check+transmute, but doesn't require
    // unsafe, or to hard-code max ErrorKind or its size in a way the compiler
    // couldn't verify.
    #[inline]
    #[allow(dead_code, reason = "only used for packed representation")]
    const fn from_prim(ek: u32) -> Option<Self> {
        macro_rules! from_prim {
            ($prim:expr => $Enum:ident { $($Variant:ident),* $(,)? }) => {{
                // Force a compile error if the list gets out of date.
                const _: fn(e: $Enum) = |e: $Enum| match e {
                    $($Enum::$Variant => (),)*
                };
                match $prim {
                    $(v if v == ($Enum::$Variant as _) => Some($Enum::$Variant),)*
                    _ => None,
                }
            }}
        }
        from_prim!(ek => ErrorKind {
            NotFound,
            PermissionDenied,
            ConnectionRefused,
            ConnectionReset,
            HostUnreachable,
            NetworkUnreachable,
            ConnectionAborted,
            NotConnected,
            AddrInUse,
            AddrNotAvailable,
            NetworkDown,
            BrokenPipe,
            AlreadyExists,
            WouldBlock,
            NotADirectory,
            IsADirectory,
            DirectoryNotEmpty,
            ReadOnlyFilesystem,
            FilesystemLoop,
            StaleNetworkFileHandle,
            InvalidInput,
            InvalidData,
            TimedOut,
            WriteZero,
            StorageFull,
            NotSeekable,
            QuotaExceeded,
            FileTooLarge,
            ResourceBusy,
            ExecutableFileBusy,
            Deadlock,
            CrossesDevices,
            TooManyLinks,
            InvalidFilename,
            ArgumentListTooLong,
            Interrupted,
            Other,
            UnexpectedEof,
            Unsupported,
            OutOfMemory,
            InProgress,
            TooManyOpenFiles,
            Uncategorized,
        })
    }
}

#[stable(feature = "io_errorkind_display", since = "1.60.0")]
impl fmt::Display for ErrorKind {
    /// Shows a human-readable description of the [`ErrorKind`].
    ///
    /// This is similar to `impl Display for Error`, but doesn't require first converting to Error.
    ///
    /// # Examples
    /// ```
    /// use core::io::ErrorKind;
    /// assert_eq!("entity not found", ErrorKind::NotFound.to_string());
    /// ```
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt.write_str(self.as_str())
    }
}