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".

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 CharacterSet

impl Debug for otter_nodejs_tests::base64::DecodeError

impl Debug for otter_nodejs_tests::bundles::Kind

impl Debug for LoadError

impl Debug for otter_nodejs_tests::bundles::State

impl Debug for otter_nodejs_tests::chrono::Month

impl Debug for RoundingError

impl Debug for SecondsFormat

impl Debug for otter_nodejs_tests::chrono::Weekday

impl Debug for Colons

impl Debug for Fixed

impl Debug for Numeric

impl Debug for OffsetPrecision

impl Debug for Pad

impl Debug for ParseErrorKind

impl Debug for otter_nodejs_tests::chrono_tz::Tz

impl Debug for TruncSide

impl Debug for AccountScope

impl Debug for AccountsSaveError

impl Debug for ApiPieceOpError

impl Debug for AssetUrlKey

impl Debug for otter_nodejs_tests::BigEndian

impl Debug for otter_nodejs_tests::ErrorKind

impl Debug for Explode

impl Debug for Fatal

impl Debug for IOccultIlk

impl Debug for Inapplicable

impl Debug for Infallible

impl Debug for InternalError

impl Debug for InvalidScopedName

impl Debug for otter_nodejs_tests::IpAddr

impl Debug for LibraryLoadMFIncompat

impl Debug for LOccultIlk

impl Debug for LibraryLoadError

impl Debug for LinkKind

impl Debug for otter_nodejs_tests::LittleEndian

impl Debug for VersionError

impl Debug for MgmtCommand

impl Debug for MgmtError

impl Debug for MgmtGameInstruction

impl Debug for MgmtGameResponse

impl Debug for MgmtGameUpdateMode

impl Debug for MgmtResponse

impl Debug for OccDisplacement

impl Debug for OccultationKindAlwaysOk

impl Debug for OldNewIndex

impl Debug for otter_nodejs_tests::Ordering

impl Debug for Outline

impl Debug for PacketFrameReadError

impl Debug for PacketFrameWriteError

impl Debug for PathResolveContext

impl Debug for PathResolveMethod

impl Debug for PieceAngle

impl Debug for PieceAngleSpec

impl Debug for PieceMoveable

impl Debug for PieceOpErrorPartiallyProcessed

impl Debug for PieceUpdateOps

impl Debug for PreparedUpdateEntry

impl Debug for PresentationLayout

impl Debug for ProgressUpdateMode

impl Debug for SVGProcessingError

impl Debug for SVGSizeError

impl Debug for SVGWidthOrHeight

impl Debug for Shape

impl Debug for ShapelibConfig1

impl Debug for otter_nodejs_tests::SocketAddr

impl Debug for SpecError

impl Debug for SpecialClientRendering

impl Debug for TablePermission

impl Debug for TablePlayerSpec

Trait Debug