core::fmt

Trait Debug

1.6.0 · Source

pub trait Debug: PointeeSized {
    // Required method
    fn fmt(&self, f: &mut Formatter<'_>) -> Result;
}

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

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".

Debug

Trait Debug Copy item path

§Stability

§Examples

Required Methods§

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

§Errors

§Examples

Dyn Compatibility§

Implementors§

impl Debug for !

impl Debug for ()

impl Debug for Abi

impl Debug for AddrKind

impl Debug for AddrParseError

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

impl Debug for core::fmt::Alignment

impl Debug for AllocError

impl Debug for Arguments<'_>

impl Debug for Array

impl Debug for AsciiChar

impl Debug for AssertKind

impl Debug for Assume

impl Debug for AtomicBool

impl Debug for AtomicI8

impl Debug for AtomicI16

impl Debug for AtomicI32

impl Debug for AtomicI64

impl Debug for AtomicIsize

impl Debug for AtomicOrdering

impl Debug for AtomicU8

impl Debug for AtomicU16

impl Debug for AtomicU32

impl Debug for AtomicU64

impl Debug for AtomicUsize

impl Debug for BiasedFp

impl Debug for Big8x3

impl Debug for Big32x40

impl Debug for Bool

impl Debug for BorrowError

impl Debug for BorrowMutError

impl Debug for ByteStr

impl Debug for CStr

impl Debug for CaseMappingIter

impl Debug for Char

impl Debug for CharCase

impl Debug for CharErrorKind

impl Debug for CharTryFromError

impl Debug for Chars<'_>

impl Debug for CodePoint

impl Debug for CodePointInner

impl Debug for CodeUnit

impl Debug for Const

impl Debug for Context<'_>

impl Debug for CpuidResult

impl Debug for Custom

impl Debug for CustomOwner

impl Debug for Debug<'_>

impl Debug for DebugAsHex

impl Debug for Decimal

impl Debug for DecimalSeq

impl Debug for DecodeUtf16Error

impl Debug for Decoded

impl Debug for Duration

impl Debug for DynTrait

impl Debug for DynTraitPredicate

impl Debug for core::io::util::Empty

impl Debug for EmptyNeedle

impl Debug for EncodeUtf16<'_>

impl Debug for EncodeWide<'_>

impl Debug for Enum

impl Debug for core::io::error::Error

impl Debug for core::fmt::Error

impl Debug for ErrorKind

impl Debug for core::char::EscapeDebug

impl Debug for core::ascii::EscapeDefault

impl Debug for core::char::EscapeDefault

impl Debug for core::char::EscapeUnicode

impl Debug for Field

impl Debug for FieldId

Trait Debug