Trait std::fmt::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 }");

Run

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 }");

Run

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)
    }
}

Run

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 };

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

Run

Required Methods§

source

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

Formats the value using the given formatter.

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 std::fmt::Debug

Stability

Examples

Required Methods§

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

Examples

Implementors§

impl Debug for hashbrown::TryReserveError

impl Debug for AsciiChar

impl Debug for BacktraceStatus

impl Debug for BytesOrWide

impl Debug for RawFrame

impl Debug for std::cmp::Ordering

impl Debug for TryReserveErrorKind

impl Debug for Infallible

impl Debug for VarError

impl Debug for c_void

impl Debug for SeekFrom

impl Debug for ErrorKind

impl Debug for IpAddr

impl Debug for Ipv6MulticastScope

impl Debug for Shutdown

impl Debug for std::net::SocketAddr

impl Debug for FpCategory

impl Debug for IntErrorKind

impl Debug for AncillaryError

impl Debug for BacktraceStyle

impl Debug for State

impl Debug for MustAbort

impl Debug for Which

impl Debug for SearchStep

impl Debug for std::sync::atomic::Ordering

impl Debug for Selected

impl Debug for RecvTimeoutError

impl Debug for TryRecvError

impl Debug for ProgramKind

impl Debug for std::sys::unix::process::process_common::Stdio

impl Debug for std::fmt::Alignment

impl Debug for bool

impl Debug for char

impl Debug for f32

impl Debug for f64

impl Debug for i8

impl Debug for i16

impl Debug for i32

impl Debug for i64

impl Debug for i128

impl Debug for isize

impl Debug for !

impl Debug for str

impl Debug for u8

impl Debug for u16

impl Debug for u32

impl Debug for u64

impl Debug for u128

impl Debug for ()

impl Debug for usize

impl Debug for CpuidResult

impl Debug for __m128

impl Debug for __m128bh

impl Debug for __m128d

impl Debug for __m128i

impl Debug for __m256

impl Debug for __m256bh

impl Debug for __m256d

impl Debug for __m256i

impl Debug for __m512

impl Debug for __m512bh

impl Debug for __m512d

impl Debug for __m512i

impl Debug for FromBytesUntilNulError

impl Debug for TimSortRun

impl Debug for TryDemangleError

impl Debug for AllocError

impl Debug for Global

impl Debug for Layout

impl Debug for LayoutError

impl Debug for System

impl Debug for TypeId

impl Debug for TryFromSliceError