core::fmt

Trait Debug

1.6.0 · source

pub trait Debug {
    // 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,
)");

Trait DebugCopy item path

§Stability

§Examples

Required Methods§

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

§Errors

§Examples

Implementors§

impl Debug for AsciiChar

impl Debug for core::cmp::Ordering

impl Debug for Infallible

impl Debug for c_void

impl Debug for IpAddr

impl Debug for Ipv6MulticastScope

impl Debug for SocketAddr

impl Debug for FpCategory

impl Debug for IntErrorKind

impl Debug for SearchStep

impl Debug for core::sync::atomic::Ordering

impl Debug for core::fmt::Alignment

impl Debug for bool

impl Debug for char

impl Debug for f16

impl Debug for f32

impl Debug for f64

impl Debug for f128

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 AllocError

impl Debug for Layout

impl Debug for LayoutError

impl Debug for TypeId

impl Debug for float64x1_t

impl Debug for float64x1x2_t

impl Debug for float64x1x3_t

impl Debug for float64x1x4_t

impl Debug for float64x2_t

impl Debug for float64x2x2_t

impl Debug for float64x2x3_t

impl Debug for float64x2x4_t

impl Debug for float32x2_t

impl Debug for float32x2x2_t

impl Debug for float32x2x3_t

impl Debug for float32x2x4_t

impl Debug for float32x4_t

impl Debug for float32x4x2_t

impl Debug for float32x4x3_t

impl Debug for float32x4x4_t

impl Debug for int8x4_t

impl Debug for int8x8_t

impl Debug for int8x8x2_t

impl Debug for int8x8x3_t

impl Debug for int8x8x4_t

impl Debug for int8x16_t

impl Debug for int8x16x2_t

impl Debug for int8x16x3_t

impl Debug for int8x16x4_t

impl Debug for int16x2_t

impl Debug for int16x4_t

impl Debug for int16x4x2_t

impl Debug for int16x4x3_t

impl Debug for int16x4x4_t

impl Debug for int16x8_t

impl Debug for int16x8x2_t

impl Debug for int16x8x3_t

impl Debug for int16x8x4_t

impl Debug for int32x2_t

Trait Debug