Trait Debug

1.6.0 · 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".

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 AArch64

impl Debug for ATerm

impl Debug for Aarch64Architecture

impl Debug for Abbreviation

impl Debug for Abbreviations

impl Debug for AbbreviationsCache

impl Debug for AbbreviationsCacheStrategy

impl Debug for wasmtime_environ::__core::mem::type_info::Abi

impl Debug for wasmtime_environ::Abi

impl Debug for AbiTag

impl Debug for AbiTags

impl Debug for AbstractHeapType

impl Debug for AbstractHeapType

impl Debug for AbstractResourceIndex

impl Debug for AccessError

impl Debug for Adapter

impl Debug for AdapterId

impl Debug for AdapterModuleId

impl Debug for AdapterOptions

impl Debug for AddrParseError

impl Debug for Address

impl Debug for AddressSize

impl Debug for AliasableResourceId

impl Debug for wasmtime_environ::__core::mem::Alignment

impl Debug for wasmtime_environ::__core::fmt::Alignment

impl Debug for AllocError

impl Debug for AncillaryError

impl Debug for AnonObjectHeader

impl Debug for AnonObjectHeaderBigobj

impl Debug for AnonObjectHeaderV2

impl Debug for AnyTypeId

impl Debug for ArangeEntry

impl Debug for Architecture

impl Debug for Architecture

impl Debug for Args

impl Debug for ArgsOs

impl Debug for Arguments<'_>

impl Debug for Arm

impl Debug for ArmArchitecture

impl Debug for Array

impl Debug for ArrayType

impl Debug for ArrayType

impl Debug for ArrayType

impl Debug for AsciiChar

impl Debug for Assume

impl Debug for Atom

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 Attribute

impl Debug for AttributeSpecification

impl Debug for AttributeValue

impl Debug for Augmentation

impl Debug for AuxHeader32

impl Debug for AuxHeader64

impl Debug for AuxSymbolSection

impl Debug for AuxSymbolWeak

impl Debug for B0

impl Debug for B1

Trait Debug