Trait Debug

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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,
)");

Trait DebugCopy item path

§Stability

§Examples

Required Methods§

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

§Errors

§Examples

Implementors§

impl Debug for allocator_api2::stable::raw_vec::TryReserveErrorKind

impl Debug for DecoderState

impl Debug for DwarfFileType

impl Debug for Format

impl Debug for gimli::common::SectionId

impl Debug for gimli::common::Vendor

impl Debug for RunTimeEndian

impl Debug for AbbreviationsCacheStrategy

impl Debug for Pointer

impl Debug for gimli::read::Error

impl Debug for IndexSectionId

impl Debug for ColumnType

impl Debug for Value

impl Debug for ValueType

impl Debug for gimli::write::cfi::CallFrameInstruction

impl Debug for ConvertError

impl Debug for Address

impl Debug for gimli::write::Error

impl Debug for Reference

impl Debug for LineString

impl Debug for gimli::write::loc::Location

impl Debug for gimli::write::range::Range

impl Debug for gimli::write::relocate::RelocationTarget

impl Debug for gimli::write::unit::AttributeValue

impl Debug for hashbrown::TryReserveError

impl Debug for GetDisjointMutError

impl Debug for Level

impl Debug for LevelFilter

impl Debug for PrefilterConfig

impl Debug for AddressSize

impl Debug for object::common::Architecture

impl Debug for object::common::BinaryFormat

impl Debug for ComdatKind

impl Debug for FileFlags

impl Debug for RelocationEncoding

impl Debug for RelocationFlags

impl Debug for RelocationKind

impl Debug for SectionFlags

impl Debug for SectionKind

impl Debug for object::common::SegmentFlags

impl Debug for SubArchitecture

impl Debug for SymbolKind

impl Debug for SymbolScope

impl Debug for object::endian::Endianness

impl Debug for ImportType

impl Debug for CompressionFormat

impl Debug for FileKind

impl Debug for ObjectKind

impl Debug for object::read::RelocationTarget

impl Debug for object::read::SymbolSection

impl Debug for ResourceNameOrId

impl Debug for CoffExportStyle

impl Debug for Name

impl Debug for Mangling

impl Debug for StandardSection

impl Debug for StandardSegment

impl Debug for object::write::SymbolSection

impl Debug for postcard::error::Error

impl Debug for Op

impl Debug for CollectionAllocErr

impl Debug for CDataModel

impl Debug for Size

impl Debug for target_lexicon::parse_error::ParseError

impl Debug for Aarch64Architecture

impl Debug for target_lexicon::targets::Architecture

impl Debug for ArmArchitecture

impl Debug for target_lexicon::targets::BinaryFormat

impl Debug for CleverArchitecture

impl Debug for CustomVendor

impl Debug for Environment

impl Debug for Mips32Architecture

impl Debug for Mips64Architecture

Trait Debug