Trait core::fmt::Debug1.0.0[−][src]

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

? 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 (libstd, libcore, liballoc, 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.

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

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

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,
)");

Run

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Implementors

`impl Debug for !`[src]

`fn fmt(&self, _: &mut Formatter<'_>) -> Result`[src]

`impl Debug for ()`[src]

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

`impl Debug for core::cmp::Ordering`[src]

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

`impl Debug for Infallible`1.34.0[src]

`fn fmt(&self, _: &mut Formatter<'_>) -> Result`[src]

`impl Debug for c_void`1.16.0[src]

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

`impl Debug for Alignment`1.28.0[src]

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

`impl Debug for FpCategory`[src]

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

`impl Debug for IntErrorKind`[src]

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

`impl Debug for SearchStep`[src]

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

`impl Debug for core::sync::atomic::Ordering`[src]

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

`impl Debug for AllocError`[src]

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

`impl Debug for Layout`1.28.0[src]

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

`impl Debug for LayoutError`1.50.0[src]

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

`impl Debug for TypeId`[src]

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

`impl Debug for float64x1_t`[src]

This is supported on AArch64 only.

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