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,
)");

Trait DebugCopy item path

§Stability

§Examples

Required Methods§

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

§Errors

§Examples

Implementors§

impl Debug for AhoCorasickKind

impl Debug for aho_corasick::packed::api::MatchKind

impl Debug for aho_corasick::util::error::MatchErrorKind

impl Debug for aho_corasick::util::prefilter::Candidate

impl Debug for aho_corasick::util::search::Anchored

impl Debug for aho_corasick::util::search::MatchKind

impl Debug for StartKind

impl Debug for async_compression::Level

impl Debug for ParseAlphabetError

impl Debug for base64::decode::DecodeError

impl Debug for DecodeSliceError

impl Debug for EncodeSliceError

impl Debug for DecodePaddingMode

impl Debug for base64ct::errors::Error

impl Debug for LineEnding

impl Debug for PadType

impl Debug for bs58::alphabet::Error

impl Debug for bs58::decode::Error

impl Debug for bs58::encode::Error

impl Debug for BigEndian

impl Debug for LittleEndian

impl Debug for Colons

impl Debug for Fixed

impl Debug for Numeric

impl Debug for chrono::format::OffsetPrecision

impl Debug for Pad

impl Debug for ParseErrorKind

impl Debug for SecondsFormat

impl Debug for chrono::month::Month

impl Debug for RoundingError

impl Debug for chrono::weekday::Weekday

impl Debug for Tz

impl Debug for FmtKind

impl Debug for NumberFmt

impl Debug for deadpool::managed::builder::BuildError

impl Debug for QueueMode

impl Debug for TimeoutType

impl Debug for deadpool_runtime::Runtime

impl Debug for SpawnBlockingError

impl Debug for InteractError

impl Debug for decancer::Error

impl Debug for Translation

impl Debug for TruncSide

impl Debug for FlushCompress

impl Debug for FlushDecompress

impl Debug for flate2::mem::Status

impl Debug for Meaning

impl Debug for PollNext

impl Debug for hashbrown::TryReserveError

impl Debug for hashbrown::TryReserveError

impl Debug for TagKind

impl Debug for html5ever::tokenizer::interface::Token

impl Debug for AttrValueKind

impl Debug for DoctypeIdKind

impl Debug for RawKind

impl Debug for ScriptEscapeKind

impl Debug for html5ever::tokenizer::states::State

impl Debug for httparse::Error

impl Debug for GetTimezoneError

impl Debug for CalendarComponent

impl Debug for Related

impl Debug for Trigger

impl Debug for CalendarDateTime

impl Debug for DatePerhapsTime

impl Debug for icalendar::properties::Class

impl Debug for EventStatus

impl Debug for TodoStatus

impl Debug for ValueType

impl Debug for TrieResult

impl Debug for CodePointInversionListError

impl Debug for CodePointInversionListAndStringListError

impl Debug for TrieType

Trait Debug