Error in alloc::io - Rust
Description: The error type for I/O operations of the `Read`, `Write`, `Seek`, and associated traits.

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## Error

# Struct Error

Source

    pub struct Error { /* private fields */ }

🔬This is a nightly-only experimental API. (`alloc_io` #154046)
Expand description

The error type for I/O operations of the `Read`, `Write`, `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`.

## Implementations§

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### impl Error

1.0.0 · Source

#### pub fn new<E>(kind: ErrorKind, error: E) -> Error
where E: Into<Box<dyn Error + Send + Sync>>,

Creates a new I/O error from a known kind of error as well as an arbitrary error payload.

This function is used to generically create I/O errors which do not originate from the OS itself. The `error` argument is an arbitrary payload which will be contained in this `Error`.

Note that this function allocates memory on the heap. If no extra payload is required, use the `From` conversion from `ErrorKind`.

##### §Examples

    use std::io::{Error, ErrorKind};

    // errors can be created from strings
    let custom_error = Error::new(ErrorKind::Other, "oh no!");

    // errors can also be created from other errors
    let custom_error2 = Error::new(ErrorKind::Interrupted, custom_error);

    // creating an error without payload (and without memory allocation)
    let eof_error = Error::from(ErrorKind::UnexpectedEof);

1.74.0 · Source

#### pub fn other<E>(error: E) -> Error
where E: Into<Box<dyn Error + Send + Sync>>,

Creates a new I/O error from an arbitrary error payload.

This function is used to generically create I/O errors which do not originate from the OS itself. It is a shortcut for `Error::new` with `ErrorKind::Other`.

##### §Examples

    use std::io::Error;

    // errors can be created from strings
    let custom_error = Error::other("oh no!");

    // errors can also be created from other errors
    let custom_error2 = Error::other(custom_error);

1.3.0 · Source

#### pub fn into_inner(self) -> Option<Box<dyn Error + Send + Sync>>

Consumes the `Error`, returning its inner error (if any).

If this `Error` was constructed via `new` or `other`, then this function will return `Some`, otherwise it will return `None`.

##### §Examples

    use std::io::{Error, ErrorKind};

    fn print_error(err: Error) {
        if let Some(inner_err) = err.into_inner() {
            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!"));
    }

1.79.0 · Source

#### pub fn downcast<E>(self) -> Result<E, Self>
where E: Error + Send + Sync + 'static,

Attempts to downcast the custom boxed error to `E`.

If this `Error` contains a custom boxed error, then it would attempt downcasting on the boxed error, otherwise it will return `Err`.

If the custom boxed error has the same type as `E`, it will return `Ok`, otherwise it will also return `Err`.

This method is meant to be a convenience routine for calling `Box<dyn Error + Sync + Send>::downcast` on the custom boxed error, returned by `Error::into_inner`.

##### §Examples

    use std::fmt;
    use std::io;
    use std::error::Error;

    #[derive(Debug)]
    enum E {
        Io(io::Error),
        SomeOtherVariant,
    }

    impl fmt::Display for E {
       // ...
    }
    impl Error for E {}

    impl From<io::Error> for E {
        fn from(err: io::Error) -> E {
            err.downcast::<E>()
                .unwrap_or_else(E::Io)
        }
    }

    impl From<E> for io::Error {
        fn from(err: E) -> io::Error {
            match err {
                E::Io(io_error) => io_error,
                e => io::Error::new(io::ErrorKind::Other, e),
            }
        }
    }

    let e = E::SomeOtherVariant;
    // Convert it to an io::Error
    let io_error = io::Error::from(e);
    // Cast it back to the original variant
    let e = E::from(io_error);
    assert!(matches!(e, E::SomeOtherVariant));

    let io_error = io::Error::from(io::ErrorKind::AlreadyExists);
    // Convert it to E
    let e = E::from(io_error);
    // Cast it back to the original variant
    let io_error = io::Error::from(e);
    assert_eq!(io_error.kind(), io::ErrorKind::AlreadyExists);
    assert!(io_error.get_ref().is_none());
    assert!(io_error.raw_os_error().is_none());

Source§

### impl Error

1.0.0 · Source

#### pub fn raw_os_error(&self) -> Option<i32>

Returns the OS error that this error represents (if any).

If this `Error` was constructed via `last_os_error` or `from_raw_os_error`, then this function will return `Some`, otherwise it will return `None`.

##### §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!"));
    }

1.3.0 · Source

#### pub fn get_ref(&self) -> Option<&(dyn Error + Send + Sync + 'static)>

Returns a reference to the inner error wrapped by this error (if any).

If this `Error` was constructed via `new` then this function will return `Some`, otherwise it will return `None`.

##### §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!"));
    }

1.3.0 · Source

#### pub fn get_mut(&mut self) -> Option<&mut (dyn Error + Send + Sync + 'static)>

Returns a mutable reference to the inner error wrapped by this error (if any).

If this `Error` was constructed via `new` then this function will return `Some`, otherwise it will return `None`.

##### §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())));
    }

1.0.0 · Source

#### pub fn kind(&self) -> ErrorKind

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` for more details.

##### §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!"));
    }

## Trait Implementations§

1.0.0 · Source§

### impl Debug for Error

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter. Read more
1.0.0 · Source§

### impl Display for Error

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#### fn fmt(&self, fmt: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter. Read more
1.0.0 · Source§

### impl Error for Error

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#### fn cause(&self) -> Option<&dyn Error>

👎Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

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#### fn source(&self) -> Option<&(dyn Error + 'static)>

Returns the lower-level source of this error, if any. Read more
1.0.0 · Source§

#### fn description(&self) -> &str

👎Deprecated since 1.42.0:

use the Display impl or to_string()

Read more
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#### fn provide<'a>(&'a self, request: &mut Request<'a>)

🔬This is a nightly-only experimental API. (`error_generic_member_access` #99301)
Provides type-based access to context intended for error reports. Read more
1.14.0 · Source§

### impl From<ErrorKind> for Error

Intended for use for errors not exposed to the user, where allocating onto the heap (for normal construction via Error::new) is too costly.

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#### fn from(kind: ErrorKind) -> 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}"));

1.0.0 · Source§

### impl From<NulError> for Error

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#### fn from(_: NulError) -> Error

Converts a `crate::ffi::NulError` into a `Error`.

1.78.0 · Source§

### impl From<TryReserveError> for Error

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#### fn from(_: TryReserveError) -> Error

Converts `TryReserveError` to an error with `ErrorKind::OutOfMemory`.

`TryReserveError` won’t be available as the error `source()`, but this may change in the future.

## Auto Trait Implementations§

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### impl !RefUnwindSafe for Error

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### impl !UnwindSafe for Error

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### impl Freeze for Error

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### impl Send for Error

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### impl Sync for Error

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### impl Unpin for Error

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### impl UnsafeUnpin for Error

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> ToString for T
where T: Display + ?Sized,

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#### fn to_string(&self) -> String

Converts the given value to a `String`. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.