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,
)");

Debug

Trait Debug Copy 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 aho_corasick::util::search::StartKind

impl Debug for DivError

impl Debug for PositDecodeError

impl Debug for amplify_num::hex::Error

impl Debug for arbitrary::error::Error

impl Debug for ascii::ascii_char::AsciiChar

impl Debug for OidParseError

impl Debug for asn1_rs::asn1_types::real::Real

impl Debug for Explicit

impl Debug for Implicit

impl Debug for asn1_rs::class::Class

impl Debug for ASN1TimeZone

impl Debug for DerConstraint

impl Debug for asn1_rs::error::Error

impl Debug for asn1_rs::error::SerializeError

impl Debug for asn1_rs::length::Length

impl Debug for async_channel::TryRecvError

impl Debug for async_channel::TryRecvError

impl Debug for TokioTpErr

impl Debug for async_native_tls::acceptor::Error

impl Debug for async_signal::Signal

impl Debug for JsonCodecError

impl Debug for TlsProtocolId

impl Debug for ParsedPublicKeyFormat

impl Debug for aws_lc_rs::cipher::AlgorithmId

impl Debug for DecryptionContext

impl Debug for EncryptionContext

impl Debug for OperatingMode

impl Debug for KbkdfCtrHmacAlgorithmId

impl Debug for SskdfDigestAlgorithmId

impl Debug for SskdfHmacAlgorithmId

impl Debug for aws_lc_rs::kem::AlgorithmId

impl Debug for BlockCipherId

impl Debug for EncryptionAlgorithmId

impl Debug for KeySize

impl Debug for point_conversion_form_t

impl Debug for base16ct::error::Error

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 base64ct::line_ending::LineEnding

impl Debug for bincode::error::ErrorKind

impl Debug for byteorder::BigEndian

impl Debug for byteorder::LittleEndian

impl Debug for LzmaDecoderParams

impl Debug for LzmaEncoderParams

impl Debug for LzmaFilter

impl Debug for Xz2FileFormat

impl Debug for compression_core::level::Level

impl Debug for PopError

impl Debug for const_oid::error::Error

impl Debug for GenError

impl Debug for BitOrder

impl Debug for DecodeKind

impl Debug for der::error::ErrorKind

impl Debug for der::tag::class::Class

impl Debug for der::tag::Tag

impl Debug for TagMode

impl Debug for PrettyPrinterFlag

impl Debug for BinaryError

impl Debug for TruncSide

impl Debug for RV

Trait Debug