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 AArch64

impl Debug for Abbreviation

impl Debug for Abbreviations

impl Debug for AbbreviationsCache

impl Debug for AbbreviationsCacheStrategy

impl Debug for Abi

impl Debug for AccessError

impl Debug for AcqRel

impl Debug for AcquiredSegment

impl Debug for AddrParseError

impl Debug for AddressSize

impl Debug for Adler32

impl Debug for AixFileHeader

impl Debug for AixHeader

impl Debug for AixMemberOffset

impl Debug for moirai_core::platform::self::Alignment

impl Debug for moirai_core::platform::self::Alignment

impl Debug for AllocError

impl Debug for AllocationDelta

impl Debug for AncillaryError

impl Debug for AnonObjectHeader

impl Debug for AnonObjectHeaderBigobj

impl Debug for AnonObjectHeaderV2

impl Debug for ArangeEntry

impl Debug for Architecture

impl Debug for ArchiveKind

impl Debug for ArchiveOffset

impl Debug for ArenaMemoryStats

impl Debug for Args

impl Debug for ArgsOs

impl Debug for Arguments<'_>

impl Debug for Arm

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 AtomicCounter

impl Debug for AtomicOrdering

impl Debug for AttributeSpecification

impl Debug for Augmentation

impl Debug for AuxHeader32

impl Debug for AuxHeader64

impl Debug for BackendMemoryStats

impl Debug for std::backtrace::Backtrace

impl Debug for backtrace::capture::Backtrace

impl Debug for std::backtrace::BacktraceFrame

impl Debug for backtrace::capture::BacktraceFrame

impl Debug for BacktraceStatus

impl Debug for BacktraceStyle

impl Debug for BacktraceSymbol

impl Debug for Barrier

impl Debug for BarrierWaitResult

impl Debug for BaseAddresses

impl Debug for gimli::endianity::BigEndian

impl Debug for object::endian::BigEndian

impl Debug for BinSnapshot

impl Debug for BinaryFormat

impl Debug for BindError

impl Debug for BlockAux32

Trait Debug