rustmax::std::fmt

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 anstyle_parse::state::definitions::Action

impl Debug for anstyle_parse::state::definitions::State

impl Debug for cexpr::ErrorKind

impl Debug for EvalResult

impl Debug for CChar

impl Debug for cexpr::token::Kind

impl Debug for Tz

impl Debug for clang_sys::Version

impl Debug for LabelStyle

impl Debug for Severity

impl Debug for codespan_reporting::files::Error

impl Debug for DisplayStyle

impl Debug for colorchoice::ColorChoice

impl Debug for CoderResult

impl Debug for DecoderResult

impl Debug for EncoderResult

impl Debug for Latin1Bidi

impl Debug for DwarfFileType

impl Debug for gimli::common::Format

impl Debug for SectionId

impl Debug for Vendor

impl Debug for RunTimeEndian

impl Debug for AbbreviationsCacheStrategy

impl Debug for Pointer

impl Debug for gimli::read::Error

impl Debug for IndexSectionId

impl Debug for ColumnType

impl Debug for gimli::read::value::Value

impl Debug for ValueType

impl Debug for globset::ErrorKind

impl Debug for hashbrown::TryReserveError

impl Debug for AdditionError

impl Debug for CreationError

impl Debug for RecordError

impl Debug for SubtractionError

impl Debug for httparse::Error

impl Debug for BaseUnit

impl Debug for FixedAt

impl Debug for Kilo

impl Debug for humantime::date::Error

impl Debug for humantime::duration::Error

impl Debug for GetTimezoneError

impl Debug for TrieResult

impl Debug for CodePointInversionListError

impl Debug for CodePointInversionListAndStringListError

impl Debug for TrieType

impl Debug for icu_collections::codepointtrie::error::Error

impl Debug for ExtensionType

impl Debug for ParserError

impl Debug for icu_locid_transform::directionality::Direction

impl Debug for TransformResult

impl Debug for LocaleTransformError

impl Debug for NormalizerError

impl Debug for Decomposed

impl Debug for BidiPairingProperties

impl Debug for PropertiesError

impl Debug for GeneralCategory

impl Debug for CheckedBidiPairedBracketType

impl Debug for BufferFormat

impl Debug for DataErrorKind

impl Debug for LocaleFallbackPriority

impl Debug for LocaleFallbackSupplement

impl Debug for ProcessingError

impl Debug for ProcessingSuccess

impl Debug for ignore::Error

Trait Debug