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 Candidate

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

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

impl Debug for StartKind

impl Debug for crossbeam_channel::err::RecvTimeoutError

impl Debug for crossbeam_channel::err::TryRecvError

impl Debug for PrefilterConfig

impl Debug for BernoulliError

impl Debug for rand::distr::uniform::Error

impl Debug for rand::distr::weighted::Error

impl Debug for IndexVec

impl Debug for IndexVecIntoIter

impl Debug for regex::error::Error

impl Debug for StartError

impl Debug for WhichCaptures

impl Debug for State

impl Debug for regex_automata::util::look::Look

impl Debug for regex_automata::util::search::Anchored

impl Debug for regex_automata::util::search::MatchErrorKind

impl Debug for regex_automata::util::search::MatchKind

impl Debug for AssertionKind

impl Debug for Ast

impl Debug for ClassAsciiKind

impl Debug for ClassPerlKind

impl Debug for ClassSet

impl Debug for ClassSetBinaryOpKind

impl Debug for ClassSetItem

impl Debug for ClassUnicodeKind

impl Debug for ClassUnicodeOpKind

impl Debug for regex_syntax::ast::ErrorKind

impl Debug for Flag

impl Debug for FlagsItemKind

impl Debug for GroupKind

impl Debug for HexLiteralKind

impl Debug for LiteralKind

impl Debug for RepetitionKind

impl Debug for RepetitionRange

impl Debug for SpecialLiteralKind

impl Debug for regex_syntax::error::Error

impl Debug for Class

impl Debug for Dot

impl Debug for regex_syntax::hir::ErrorKind

impl Debug for HirKind

impl Debug for regex_syntax::hir::Look

impl Debug for ExtractKind

impl Debug for Utf8Sequence

impl Debug for zerocopy::byteorder::BigEndian

impl Debug for zerocopy::byteorder::LittleEndian

impl Debug for AsciiChar

impl Debug for BacktraceStatus

impl Debug for cs431_homework::test::loom::cmp::Ordering

impl Debug for TryReserveErrorKind

impl Debug for Infallible

impl Debug for VarError

impl Debug for FromBytesWithNulError

impl Debug for c_void

impl Debug for cs431_homework::test::loom::io::ErrorKind

impl Debug for SeekFrom

impl Debug for IpAddr

impl Debug for Ipv6MulticastScope

impl Debug for Shutdown

impl Debug for cs431_homework::test::loom::net::SocketAddr

impl Debug for FpCategory

impl Debug for IntErrorKind

impl Debug for AncillaryError

impl Debug for BacktraceStyle

impl Debug for GetDisjointMutError

impl Debug for SearchStep

impl Debug for cs431_homework::test::loom::sync::atomic::Ordering

Trait Debug