pub struct Error { /* private fields */ }
Expand description

The Error type, a wrapper around a dynamic error type.

Error works a lot like Box<dyn std::error::Error>, but with these differences:

  • Error requires that the error is Send, Sync, and 'static.
  • Error guarantees that a backtrace is available, even if the underlying error type does not provide one.
  • Error is represented as a narrow pointer — exactly one word in size instead of two.

Display representations

When you print an error object using “{}” or to_string(), only the outermost underlying error or context is printed, not any of the lower level causes. This is exactly as if you had called the Display impl of the error from which you constructed your anyhow::Error.

Failed to read instrs from ./path/to/instrs.json

To print causes as well using anyhow’s default formatting of causes, use the alternate selector “{:#}”.

Failed to read instrs from ./path/to/instrs.json: No such file or directory (os error 2)

The Debug format “{:?}” includes your backtrace if one was captured. Note that this is the representation you get by default if you return an error from fn main instead of printing it explicitly yourself.

Error: Failed to read instrs from ./path/to/instrs.json

Caused by:
    No such file or directory (os error 2)

and if there is a backtrace available:

Error: Failed to read instrs from ./path/to/instrs.json

Caused by:
    No such file or directory (os error 2)

Stack backtrace:
   0: <E as anyhow::context::ext::StdError>::ext_context
             at /git/anyhow/src/backtrace.rs:26
   1: core::result::Result<T,E>::map_err
             at /git/rustc/src/libcore/result.rs:596
   2: anyhow::context::<impl anyhow::Context<T,E> for core::result::Result<T,E>>::with_context
             at /git/anyhow/src/context.rs:58
   3: testing::main
             at src/main.rs:5
   4: std::rt::lang_start
             at /git/rustc/src/libstd/rt.rs:61
   5: main
   6: __libc_start_main
   7: _start

To see a conventional struct-style Debug representation, use “{:#?}”.

Error {
    context: "Failed to read instrs from ./path/to/instrs.json",
    source: Os {
        code: 2,
        kind: NotFound,
        message: "No such file or directory",
    },
}

If none of the built-in representations are appropriate and you would prefer to render the error and its cause chain yourself, it can be done something like this:

use anyhow::{Context, Result};

fn main() {
    if let Err(err) = try_main() {
        eprintln!("ERROR: {}", err);
        err.chain().skip(1).for_each(|cause| eprintln!("because: {}", cause));
        std::process::exit(1);
    }
}

fn try_main() -> Result<()> {
    ...
}

Implementations§

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impl Error

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pub fn new<E>(error: E) -> Errorwhere E: Error + Send + Sync + 'static,

Create a new error object from any error type.

The error type must be threadsafe and 'static, so that the Error will be as well.

If the error type does not provide a backtrace, a backtrace will be created here to ensure that a backtrace exists.

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pub fn msg<M>(message: M) -> Errorwhere M: Display + Debug + Send + Sync + 'static,

Create a new error object from a printable error message.

If the argument implements std::error::Error, prefer Error::new instead which preserves the underlying error’s cause chain and backtrace. If the argument may or may not implement std::error::Error now or in the future, use anyhow!(err) which handles either way correctly.

Error::msg("...") is equivalent to anyhow!("...") but occasionally convenient in places where a function is preferable over a macro, such as iterator or stream combinators:

use anyhow::{Error, Result};
use futures::stream::{Stream, StreamExt, TryStreamExt};

async fn demo<S>(stream: S) -> Result<Vec<Output>>
where
    S: Stream<Item = Input>,
{
    stream
        .then(ffi::do_some_work) // returns Result<Output, &str>
        .map_err(Error::msg)
        .try_collect()
        .await
}
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pub fn context<C>(self, context: C) -> Errorwhere C: Display + Send + Sync + 'static,

Wrap the error value with additional context.

For attaching context to a Result as it is propagated, the Context extension trait may be more convenient than this function.

The primary reason to use error.context(...) instead of result.context(...) via the Context trait would be if the context needs to depend on some data held by the underlying error:

use anyhow::Result;
use std::fs::File;
use std::path::Path;

struct ParseError {
    line: usize,
    column: usize,
}

fn parse_impl(file: File) -> Result<T, ParseError> {
    ...
}

pub fn parse(path: impl AsRef<Path>) -> Result<T> {
    let file = File::open(&path)?;
    parse_impl(file).map_err(|error| {
        let context = format!(
            "only the first {} lines of {} are valid",
            error.line, path.as_ref().display(),
        );
        anyhow::Error::new(error).context(context)
    })
}
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pub fn backtrace(&self) -> &Backtrace

Get the backtrace for this Error.

In order for the backtrace to be meaningful, one of the two environment variables RUST_LIB_BACKTRACE=1 or RUST_BACKTRACE=1 must be defined and RUST_LIB_BACKTRACE must not be 0. Backtraces are somewhat expensive to capture in Rust, so we don’t necessarily want to be capturing them all over the place all the time.

  • If you want panics and errors to both have backtraces, set RUST_BACKTRACE=1;
  • If you want only errors to have backtraces, set RUST_LIB_BACKTRACE=1;
  • If you want only panics to have backtraces, set RUST_BACKTRACE=1 and RUST_LIB_BACKTRACE=0.
Stability

Standard library backtraces are only available on the nightly channel. Tracking issue: rust-lang/rust#53487.

On stable compilers, this function is only available if the crate’s “backtrace” feature is enabled, and will use the backtrace crate as the underlying backtrace implementation.

[dependencies]
anyhow = { version = "1.0", features = ["backtrace"] }
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pub fn chain(&self) -> Chain<'_>

An iterator of the chain of source errors contained by this Error.

This iterator will visit every error in the cause chain of this error object, beginning with the error that this error object was created from.

Example
use anyhow::Error;
use std::io;

pub fn underlying_io_error_kind(error: &Error) -> Option<io::ErrorKind> {
    for cause in error.chain() {
        if let Some(io_error) = cause.downcast_ref::<io::Error>() {
            return Some(io_error.kind());
        }
    }
    None
}
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pub fn root_cause(&self) -> &(dyn Error + 'static)

The lowest level cause of this error — this error’s cause’s cause’s cause etc.

The root cause is the last error in the iterator produced by chain().

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pub fn is<E>(&self) -> boolwhere E: Display + Debug + Send + Sync + 'static,

Returns true if E is the type held by this error object.

For errors with context, this method returns true if E matches the type of the context C or the type of the error on which the context has been attached. For details about the interaction between context and downcasting, see here.

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

Attempt to downcast the error object to a concrete type.

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pub fn downcast_ref<E>(&self) -> Option<&E>where E: Display + Debug + Send + Sync + 'static,

Downcast this error object by reference.

Example
// If the error was caused by redaction, then return a tombstone instead
// of the content.
match root_cause.downcast_ref::<DataStoreError>() {
    Some(DataStoreError::Censored(_)) => Ok(Poll::Ready(REDACTED_CONTENT)),
    None => Err(error),
}
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pub fn downcast_mut<E>(&mut self) -> Option<&mut E>where E: Display + Debug + Send + Sync + 'static,

Downcast this error object by mutable reference.

Trait Implementations§

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impl AsRef<dyn Error + 'static> for Error

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fn as_ref(&self) -> &(dyn Error + 'static)

Converts this type into a shared reference of the (usually inferred) input type.
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impl AsRef<dyn Error + Send + Sync + 'static> for Error

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fn as_ref(&self) -> &(dyn Error + Send + Sync + 'static)

Converts this type into a shared reference of the (usually inferred) input type.
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impl Debug for Error

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

Formats the value using the given formatter. Read more
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impl Deref for Error

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type Target = dyn Error + Send + Sync + 'static

The resulting type after dereferencing.
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fn deref(&self) -> &<Error as Deref>::Target

Dereferences the value.
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impl DerefMut for Error

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fn deref_mut(&mut self) -> &mut <Error as Deref>::Target

Mutably dereferences the value.
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impl Display for Error

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

Formats the value using the given formatter. Read more
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impl Drop for Error

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fn drop(&mut self)

Executes the destructor for this type. Read more
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impl<E> From<E> for Errorwhere E: Error + Send + Sync + 'static,

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fn from(error: E) -> Error

Converts to this type from the input type.
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impl Provider for Error

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fn provide<'a>(&'a self, demand: &mut Demand<'a>)

🔬This is a nightly-only experimental API. (provide_any)
Data providers should implement this method to provide all values they are able to provide by using demand. Read more

Auto Trait Implementations§

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impl !RefUnwindSafe 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 !UnwindSafe for Error

Blanket Implementations§

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impl<T> Any for Twhere 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 Twhere 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 Twhere 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> Conv for T

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fn conv<T>(self) -> Twhere Self: Into<T>,

Converts self into T using Into<T>. Read more
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impl<T> Conv for T

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fn conv<T>(self) -> Twhere Self: Into<T>,

Converts self into a target type. Read more
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impl<T> FmtForward for T

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fn fmt_binary(self) -> FmtBinary<Self>where Self: Binary,

Causes self to use its Binary implementation when Debug-formatted.
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fn fmt_display(self) -> FmtDisplay<Self>where Self: Display,

Causes self to use its Display implementation when Debug-formatted.
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fn fmt_lower_exp(self) -> FmtLowerExp<Self>where Self: LowerExp,

Causes self to use its LowerExp implementation when Debug-formatted.
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fn fmt_lower_hex(self) -> FmtLowerHex<Self>where Self: LowerHex,

Causes self to use its LowerHex implementation when Debug-formatted.
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fn fmt_octal(self) -> FmtOctal<Self>where Self: Octal,

Causes self to use its Octal implementation when Debug-formatted.
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fn fmt_pointer(self) -> FmtPointer<Self>where Self: Pointer,

Causes self to use its Pointer implementation when Debug-formatted.
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fn fmt_upper_exp(self) -> FmtUpperExp<Self>where Self: UpperExp,

Causes self to use its UpperExp implementation when Debug-formatted.
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fn fmt_upper_hex(self) -> FmtUpperHex<Self>where Self: UpperHex,

Causes self to use its UpperHex implementation when Debug-formatted.
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impl<T> From<!> for T

const: unstable · source§

fn from(t: !) -> T

Converts to this type from the input type.
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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> Instrument for T

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fn instrument(self, span: Span) -> Instrumented<Self>

Instruments this type with the provided Span, returning an Instrumented wrapper. Read more
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fn in_current_span(self) -> Instrumented<Self>

Instruments this type with the current Span, returning an Instrumented wrapper. Read more
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impl<T, U> Into<U> for Twhere U: From<T>,

const: unstable · source§

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> Pipe for Twhere T: ?Sized,

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fn pipe<R>(self, func: impl FnOnce(Self) -> R) -> Rwhere Self: Sized,

Pipes by value. This is generally the method you want to use. Read more
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fn pipe_ref<'a, R>(&'a self, func: impl FnOnce(&'a Self) -> R) -> Rwhere R: 'a,

Borrows self and passes that borrow into the pipe function. Read more
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fn pipe_ref_mut<'a, R>(&'a mut self, func: impl FnOnce(&'a mut Self) -> R) -> Rwhere R: 'a,

Mutably borrows self and passes that borrow into the pipe function. Read more
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fn pipe_borrow<'a, B, R>(&'a self, func: impl FnOnce(&'a B) -> R) -> Rwhere Self: Borrow<B>, B: 'a + ?Sized, R: 'a,

Borrows self, then passes self.borrow() into the pipe function. Read more
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fn pipe_borrow_mut<'a, B, R>( &'a mut self, func: impl FnOnce(&'a mut B) -> R ) -> Rwhere Self: BorrowMut<B>, B: 'a + ?Sized, R: 'a,

Mutably borrows self, then passes self.borrow_mut() into the pipe function. Read more
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fn pipe_as_ref<'a, U, R>(&'a self, func: impl FnOnce(&'a U) -> R) -> Rwhere Self: AsRef<U>, U: 'a + ?Sized, R: 'a,

Borrows self, then passes self.as_ref() into the pipe function.
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fn pipe_as_mut<'a, U, R>(&'a mut self, func: impl FnOnce(&'a mut U) -> R) -> Rwhere Self: AsMut<U>, U: 'a + ?Sized, R: 'a,

Mutably borrows self, then passes self.as_mut() into the pipe function.
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fn pipe_deref<'a, T, R>(&'a self, func: impl FnOnce(&'a T) -> R) -> Rwhere Self: Deref<Target = T>, T: 'a + ?Sized, R: 'a,

Borrows self, then passes self.deref() into the pipe function.
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fn pipe_deref_mut<'a, T, R>(&'a mut self, func: impl FnOnce(&'a mut T) -> R) -> Rwhere Self: DerefMut<Target = T> + Deref, T: 'a + ?Sized, R: 'a,

Mutably borrows self, then passes self.deref_mut() into the pipe function.
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impl<T> Pipe for T

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fn pipe<R>(self, func: impl FnOnce(Self) -> R) -> R

Pipes a value into a function that cannot ordinarily be called in suffix position. Read more
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impl<T> PipeAsRef for T

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fn pipe_as_ref<'a, T, R>(&'a self, func: impl FnOnce(&'a T) -> R) -> Rwhere Self: AsRef<T>, T: 'a, R: 'a,

Pipes a trait borrow into a function that cannot normally be called in suffix position. Read more
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fn pipe_as_mut<'a, T, R>(&'a mut self, func: impl FnOnce(&'a mut T) -> R) -> Rwhere Self: AsMut<T>, T: 'a, R: 'a,

Pipes a trait mutable borrow into a function that cannot normally be called in suffix position. Read more
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impl<T> PipeBorrow for T

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fn pipe_borrow<'a, T, R>(&'a self, func: impl FnOnce(&'a T) -> R) -> Rwhere Self: Borrow<T>, T: 'a, R: 'a,

Pipes a trait borrow into a function that cannot normally be called in suffix position. Read more
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fn pipe_borrow_mut<'a, T, R>( &'a mut self, func: impl FnOnce(&'a mut T) -> R ) -> Rwhere Self: BorrowMut<T>, T: 'a, R: 'a,

Pipes a trait mutable borrow into a function that cannot normally be called in suffix position. Read more
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impl<T> PipeDeref for T

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fn pipe_deref<'a, R>(&'a self, func: impl FnOnce(&'a Self::Target) -> R) -> Rwhere Self: Deref, R: 'a,

Pipes a dereference into a function that cannot normally be called in suffix position. Read more
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fn pipe_deref_mut<'a, R>( &'a mut self, func: impl FnOnce(&'a mut Self::Target) -> R ) -> Rwhere Self: DerefMut, R: 'a,

Pipes a mutable dereference into a function that cannot normally be called in suffix position. Read more
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impl<T> PipeRef for T

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fn pipe_ref<'a, R>(&'a self, func: impl FnOnce(&'a Self) -> R) -> Rwhere R: 'a,

Pipes a reference into a function that cannot ordinarily be called in suffix position. Read more
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fn pipe_mut<'a, R>(&'a mut self, func: impl FnOnce(&'a mut Self) -> R) -> Rwhere R: 'a,

Pipes a mutable reference into a function that cannot ordinarily be called in suffix position. Read more
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impl<T> Pointable for T

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const ALIGN: usize = mem::align_of::<T>()

The alignment of pointer.
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type Init = T

The type for initializers.
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unsafe fn init(init: <T as Pointable>::Init) -> usize

Initializes a with the given initializer. Read more
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unsafe fn deref<'a>(ptr: usize) -> &'a T

Dereferences the given pointer. Read more
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unsafe fn deref_mut<'a>(ptr: usize) -> &'a mut T

Mutably dereferences the given pointer. Read more
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unsafe fn drop(ptr: usize)

Drops the object pointed to by the given pointer. Read more
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impl<T> Same<T> for T

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type Output = T

Should always be Self
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impl<T> Tap for T

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fn tap(self, func: impl FnOnce(&Self)) -> Self

Immutable access to a value. Read more
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fn tap_mut(self, func: impl FnOnce(&mut Self)) -> Self

Mutable access to a value. Read more
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fn tap_borrow<B>(self, func: impl FnOnce(&B)) -> Selfwhere Self: Borrow<B>, B: ?Sized,

Immutable access to the Borrow<B> of a value. Read more
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fn tap_borrow_mut<B>(self, func: impl FnOnce(&mut B)) -> Selfwhere Self: BorrowMut<B>, B: ?Sized,

Mutable access to the BorrowMut<B> of a value. Read more
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fn tap_ref<R>(self, func: impl FnOnce(&R)) -> Selfwhere Self: AsRef<R>, R: ?Sized,

Immutable access to the AsRef<R> view of a value. Read more
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fn tap_ref_mut<R>(self, func: impl FnOnce(&mut R)) -> Selfwhere Self: AsMut<R>, R: ?Sized,

Mutable access to the AsMut<R> view of a value. Read more
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fn tap_deref<T>(self, func: impl FnOnce(&T)) -> Selfwhere Self: Deref<Target = T>, T: ?Sized,

Immutable access to the Deref::Target of a value. Read more
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fn tap_deref_mut<T>(self, func: impl FnOnce(&mut T)) -> Selfwhere Self: DerefMut<Target = T> + Deref, T: ?Sized,

Mutable access to the Deref::Target of a value. Read more
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fn tap_dbg(self, func: impl FnOnce(&Self)) -> Self

Calls .tap() only in debug builds, and is erased in release builds.
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fn tap_mut_dbg(self, func: impl FnOnce(&mut Self)) -> Self

Calls .tap_mut() only in debug builds, and is erased in release builds.
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fn tap_borrow_dbg<B>(self, func: impl FnOnce(&B)) -> Selfwhere Self: Borrow<B>, B: ?Sized,

Calls .tap_borrow() only in debug builds, and is erased in release builds.
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fn tap_borrow_mut_dbg<B>(self, func: impl FnOnce(&mut B)) -> Selfwhere Self: BorrowMut<B>, B: ?Sized,

Calls .tap_borrow_mut() only in debug builds, and is erased in release builds.
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fn tap_ref_dbg<R>(self, func: impl FnOnce(&R)) -> Selfwhere Self: AsRef<R>, R: ?Sized,

Calls .tap_ref() only in debug builds, and is erased in release builds.
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fn tap_ref_mut_dbg<R>(self, func: impl FnOnce(&mut R)) -> Selfwhere Self: AsMut<R>, R: ?Sized,

Calls .tap_ref_mut() only in debug builds, and is erased in release builds.
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fn tap_deref_dbg<T>(self, func: impl FnOnce(&T)) -> Selfwhere Self: Deref<Target = T>, T: ?Sized,

Calls .tap_deref() only in debug builds, and is erased in release builds.
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fn tap_deref_mut_dbg<T>(self, func: impl FnOnce(&mut T)) -> Selfwhere Self: DerefMut<Target = T> + Deref, T: ?Sized,

Calls .tap_deref_mut() only in debug builds, and is erased in release builds.
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impl<T> Tap for T

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fn tap<F, R>(self, func: F) -> Selfwhere F: FnOnce(&Self) -> R,

Provides immutable access for inspection. Read more
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fn tap_dbg<F, R>(self, func: F) -> Selfwhere F: FnOnce(&Self) -> R,

Calls tap in debug builds, and does nothing in release builds.
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fn tap_mut<F, R>(self, func: F) -> Selfwhere F: FnOnce(&mut Self) -> R,

Provides mutable access for modification. Read more
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fn tap_mut_dbg<F, R>(self, func: F) -> Selfwhere F: FnOnce(&mut Self) -> R,

Calls tap_mut in debug builds, and does nothing in release builds.
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impl<T, U> TapAsRef<U> for Twhere U: ?Sized,

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fn tap_ref<F, R>(self, func: F) -> Selfwhere Self: AsRef<T>, F: FnOnce(&T) -> R,

Provides immutable access to the reference for inspection.
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fn tap_ref_dbg<F, R>(self, func: F) -> Selfwhere Self: AsRef<T>, F: FnOnce(&T) -> R,

Calls tap_ref in debug builds, and does nothing in release builds.
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fn tap_ref_mut<F, R>(self, func: F) -> Selfwhere Self: AsMut<T>, F: FnOnce(&mut T) -> R,

Provides mutable access to the reference for modification.
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fn tap_ref_mut_dbg<F, R>(self, func: F) -> Selfwhere Self: AsMut<T>, F: FnOnce(&mut T) -> R,

Calls tap_ref_mut in debug builds, and does nothing in release builds.
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impl<T, U> TapBorrow<U> for Twhere U: ?Sized,

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fn tap_borrow<F, R>(self, func: F) -> Selfwhere Self: Borrow<T>, F: FnOnce(&T) -> R,

Provides immutable access to the borrow for inspection. Read more
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fn tap_borrow_dbg<F, R>(self, func: F) -> Selfwhere Self: Borrow<T>, F: FnOnce(&T) -> R,

Calls tap_borrow in debug builds, and does nothing in release builds.
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fn tap_borrow_mut<F, R>(self, func: F) -> Selfwhere Self: BorrowMut<T>, F: FnOnce(&mut T) -> R,

Provides mutable access to the borrow for modification.
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fn tap_borrow_mut_dbg<F, R>(self, func: F) -> Selfwhere Self: BorrowMut<T>, F: FnOnce(&mut T) -> R,

Calls tap_borrow_mut in debug builds, and does nothing in release builds.
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impl<T> TapDeref for T

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fn tap_deref<F, R>(self, func: F) -> Selfwhere Self: Deref, F: FnOnce(&Self::Target) -> R,

Immutably dereferences self for inspection.
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fn tap_deref_dbg<F, R>(self, func: F) -> Selfwhere Self: Deref, F: FnOnce(&Self::Target) -> R,

Calls tap_deref in debug builds, and does nothing in release builds.
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fn tap_deref_mut<F, R>(self, func: F) -> Selfwhere Self: DerefMut, F: FnOnce(&mut Self::Target) -> R,

Mutably dereferences self for modification.
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fn tap_deref_mut_dbg<F, R>(self, func: F) -> Selfwhere Self: DerefMut, F: FnOnce(&mut Self::Target) -> R,

Calls tap_deref_mut in debug builds, and does nothing in release builds.
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impl<T> ToString for Twhere T: Display + ?Sized,

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default fn to_string(&self) -> String

Converts the given value to a String. Read more
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impl<T> TryConv for T

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fn try_conv<T>(self) -> Result<T, Self::Error>where Self: TryInto<T>,

Attempts to convert self into T using TryInto<T>. Read more
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impl<T> TryConv for T

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fn try_conv<T>(self) -> Result<T, Self::Error>where Self: TryInto<T>,

Attempts to convert self into a target type. Read more
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impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
const: unstable · source§

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 Twhere 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.
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impl<V, T> VZip<V> for Twhere V: MultiLane<T>,

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fn vzip(self) -> V

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impl<T> WithSubscriber for T

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fn with_subscriber<S>(self, subscriber: S) -> WithDispatch<Self>where S: Into<Dispatch>,

Attaches the provided Subscriber to this type, returning a WithDispatch wrapper. Read more
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fn with_current_subscriber(self) -> WithDispatch<Self>

Attaches the current default Subscriber to this type, returning a WithDispatch wrapper. Read more