Trait Debug

1.0.0 · Source

pub trait Debug {
    // Required method
    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>;
}

Expand description

? formatting.

Debug should format the output in a programmer-facing, debugging context.

Generally speaking, you should just derive a Debug implementation.

When used with the alternate format specifier #?, the output is pretty-printed.

For more information on formatters, see the module-level documentation.

This trait can be used with #[derive] if all fields implement Debug. When derived for structs, it will use the name of the struct, then {, then a comma-separated list of each field’s name and Debug value, then }. For enums, it will use the name of the variant and, if applicable, (, then the Debug values of the fields, then ).

§Stability

Derived Debug formats are not stable, and so may change with future Rust versions. Additionally, Debug implementations of types provided by the standard library (std, core, alloc, etc.) are not stable, and may also change with future Rust versions.

§Examples

Deriving an implementation:

#[derive(Debug)]
struct Point {
    x: i32,
    y: i32,
}

let origin = Point { x: 0, y: 0 };

assert_eq!(
    format!("The origin is: {origin:?}"),
    "The origin is: Point { x: 0, y: 0 }",
);

Manually implementing:

use std::fmt;

struct Point {
    x: i32,
    y: i32,
}

impl fmt::Debug for Point {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Point")
         .field("x", &self.x)
         .field("y", &self.y)
         .finish()
    }
}

let origin = Point { x: 0, y: 0 };

assert_eq!(
    format!("The origin is: {origin:?}"),
    "The origin is: Point { x: 0, y: 0 }",
);

There are a number of helper methods on the Formatter struct to help you with manual implementations, such as debug_struct.

Types that do not wish to use the standard suite of debug representations provided by the Formatter trait (debug_struct, debug_tuple, debug_list, debug_set, debug_map) can do something totally custom by manually writing an arbitrary representation to the Formatter.

impl fmt::Debug for Point {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Point [{} {}]", self.x, self.y)
    }
}

Debug implementations using either derive or the debug builder API on Formatter support pretty-printing using the alternate flag: {:#?}.

Pretty-printing with #?:

#[derive(Debug)]
struct Point {
    x: i32,
    y: i32,
}

let origin = Point { x: 0, y: 0 };

let expected = "The origin is: Point {
    x: 0,
    y: 0,
}";
assert_eq!(format!("The origin is: {origin:#?}"), expected);

Required Methods§

1.0.0 · Source

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

Formats the value using the given formatter.

§Errors

This function should return Err if, and only if, the provided Formatter returns Err. String formatting is considered an infallible operation; this function only returns a Result because writing to the underlying stream might fail and it must provide a way to propagate the fact that an error has occurred back up the stack.

§Examples

use std::fmt;

struct Position {
    longitude: f32,
    latitude: f32,
}

impl fmt::Debug for Position {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("")
         .field(&self.longitude)
         .field(&self.latitude)
         .finish()
    }
}

let position = Position { longitude: 1.987, latitude: 2.983 };
assert_eq!(format!("{position:?}"), "(1.987, 2.983)");

assert_eq!(format!("{position:#?}"), "(
    1.987,
    2.983,
)");

impl Debug for proc_macro::Spacing

1.29.0 · Source§

impl Debug for proc_macro::TokenTree

impl Debug for proc_macro2::TokenTree

impl Debug for proc_macro::Punct

1.29.0 · Source§

impl Debug for proc_macro::Span

Prints a span in a form convenient for debugging.

1.15.0 · Source§

impl Debug for proc_macro::TokenStream

impl Debug for proc_macro2::Span

Prints a span in a form convenient for debugging.

Source §

impl Debug for proc_macro2::TokenStream

impl Debug for Event

When the alternate flag is enabled this will print platform specific details, for example the fields of the kevent structure on platforms that use kqueue(2). Note however that the output of this implementation is not consider a part of the stable API.

Styles have a special Debug implementation that only shows the fields that are set. Fields that haven’t been touched aren’t included in the output.

This behaviour gets bypassed when using the alternate formatting mode format!("{:#?}").

use nu_ansi_term::Color::{Red, Blue};
assert_eq!("Style { fg(Red), on(Blue), bold, italic }",
           format!("{:?}", Red.on(Blue).bold().italic()));

§

impl<T, O> Debug for BitVec<T, O>
where T: BitStore, O: BitOrder,

§

impl<T, O> Debug for Drain<'_, T, O>
where T: BitStore, O: BitOrder,

§

impl<T, O> Debug for IntoIter<T, O>
where T: BitStore, O: BitOrder,

Original

§

impl<T, O> Debug for Iter<'_, T, O>
where T: BitStore, O: BitOrder,

Original

§

impl<T, O> Debug for IterMut<'_, T, O>
where T: BitStore, O: BitOrder,

impl<const T: u32> Debug for ConstU32<T>

§

impl<const T: u64> Debug for ConstU64<T>

§

impl<const T: u128> Debug for ConstU128<T>

§

Trait DebugCopy item path

§Stability

§Examples

Required Methods§

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

§Errors

§Examples

Implementors§

impl Debug for &dyn TargetIsa

impl Debug for ArithmeticError

impl Debug for Rounding

impl Debug for SignedRounding

impl Debug for AnalysisChoice

impl Debug for BenchmarkError

impl Debug for BenchmarkParameter

impl Debug for DecRefStatus

impl Debug for IncRefStatus

impl Debug for polkadot_sdk_frame::prelude::frame_system::Phase

impl Debug for RefStatus

impl Debug for ApiError

impl Debug for CallContext

impl Debug for CheckInherentsError

impl Debug for SignatureResult

impl Debug for ExecutiveError

impl Debug for ExtrinsicInclusionMode

impl Debug for polkadot_sdk_frame::runtime::prelude::Sr25519Keyring

impl Debug for polkadot_sdk_frame::runtime::testing_prelude::DispatchError

impl Debug for StateVersion

impl Debug for DepositConsequence

impl Debug for Fortitude

impl Debug for Precision

impl Debug for Preservation

impl Debug for Provenance

impl Debug for InvalidTransaction

impl Debug for BalanceStatus

impl Debug for ExecuteOverweightError

impl Debug for ExistenceRequirement

impl Debug for ProcessMessageError

impl Debug for polkadot_sdk_frame::traits::TrieError

impl Debug for polkadot_sdk_frame::traits::TryStateSelect

impl Debug for UpgradeCheckSelect

impl Debug for alloc::collections::TryReserveErrorKind

impl Debug for AsciiChar

impl Debug for core::cmp::Ordering

impl Debug for Infallible

impl Debug for FromBytesWithNulError

impl Debug for c_void

impl Debug for core::fmt::Alignment

impl Debug for DebugAsHex

impl Debug for core::fmt::Sign

impl Debug for IpAddr

impl Debug for Ipv6MulticastScope

impl Debug for core::net::socket_addr::SocketAddr

impl Debug for FpCategory

impl Debug for IntErrorKind

impl Debug for core::slice::GetDisjointMutError

impl Debug for SearchStep

impl Debug for core::sync::atomic::Ordering

impl Debug for proc_macro::diagnostic::Level

impl Debug for ConversionErrorKind

impl Debug for proc_macro::Delimiter

impl Debug for proc_macro::Spacing

impl Debug for proc_macro::TokenTree

impl Debug for BacktraceStatus

impl Debug for VarError

impl Debug for std::fs::TryLockError

impl Debug for std::io::SeekFrom

impl Debug for std::io::error::ErrorKind

impl Debug for Shutdown

impl Debug for AncillaryError

impl Debug for BacktraceStyle

impl Debug for RecvTimeoutError

impl Debug for std::sync::mpsc::TryRecvError

impl Debug for hex::error::FromHexError

impl Debug for itertools::with_position::Position

impl Debug for log::Level

impl Debug for log::LevelFilter

impl Debug for TAffine

impl Debug for TGeneral

impl Debug for TProjective

Trait Debug