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,
)");

Dyn Compatibility§

This trait is dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors§

1.100.0 · Source§

impl Debug for !

1.0.0 · Source§

Debug

Trait Debug Copy item path

§Stability

§Examples

Required Methods§

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

§Errors

§Examples

Dyn Compatibility§

Implementors§

impl Debug for !

impl Debug for ()

impl Debug for Abi

impl Debug for AccessError

impl Debug for AddrParseError

impl Debug for AggregateError

impl Debug for core::mem::alignment::Alignment

impl Debug for euv_core::fmt::Alignment

impl Debug for AllocError

impl Debug for AncillaryError

impl Debug for AnimationEvent

impl Debug for App

impl Debug for Args

impl Debug for ArgsOs

impl Debug for Arguments<'_>

impl Debug for core::mem::type_info::Array

impl Debug for ArrayBuffer

impl Debug for ArrayBufferOptions

impl Debug for AsciiChar

impl Debug for Assume

impl Debug for Atomic<bool>

impl Debug for Atomic<i8>

impl Debug for Atomic<i16>

impl Debug for Atomic<i32>

impl Debug for Atomic<i64>

impl Debug for Atomic<isize>

impl Debug for Atomic<u8>

impl Debug for Atomic<u16>

impl Debug for Atomic<u32>

impl Debug for Atomic<u64>

impl Debug for Atomic<usize>

impl Debug for AtomicOrdering

impl Debug for AttributeEntry

impl Debug for AttributeValue

impl Debug for AudioBuffer

impl Debug for AudioBufferSourceNode

impl Debug for AudioContext

impl Debug for AudioDestinationNode

impl Debug for AudioListener

impl Debug for AudioNode

impl Debug for AudioParam

impl Debug for AudioScheduledSourceNode

impl Debug for Backtrace

impl Debug for BacktraceFrame

impl Debug for BacktraceStatus

impl Debug for BacktraceStyle

impl Debug for Barrier

impl Debug for BarrierWaitResult

impl Debug for BaseAudioContext

impl Debug for BeforeUnloadEvent

impl Debug for BigInt

impl Debug for BigInt64Array

impl Debug for BigUint64Array

impl Debug for Blob

impl Debug for Boolean

impl Debug for BorrowError

impl Debug for BorrowMutError

impl Debug for BorrowedFd<'_>

impl Debug for Builder

impl Debug for ByteStr

impl Debug for ByteString

impl Debug for CStr

impl Debug for CString

impl Debug for CanvasGradient

impl Debug for CanvasRenderingContext2d

impl Debug for CharCase

impl Debug for CharTryFromError

impl Debug for CharacterData

impl Debug for Chars<'_>

impl Debug for Child

Trait Debug