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,
)");

Trait DebugCopy item path

§Stability

§Examples

Required Methods§

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

§Errors

§Examples

Implementors§

impl Debug for NamedColor

impl Debug for DisplayModeBar

impl Debug for DoubleClick

impl Debug for ImageButtonFormats

impl Debug for ModeBarButtonName

impl Debug for PlotGLPixelRatio

impl Debug for smo::Alignment

impl Debug for Anchor

impl Debug for AxisSide

impl Debug for Calendar

impl Debug for CellErrorType

impl Debug for ColorScale

impl Debug for ColorScalePalette

impl Debug for ConstrainText

impl Debug for DashType

impl Debug for DataType

impl Debug for DeError

impl Debug for DebugAsHex

impl Debug for Direction

impl Debug for ErrorType

impl Debug for Ex

impl Debug for ExponentFormat

impl Debug for smo::Fill

impl Debug for GradientType

impl Debug for HoverInfo

impl Debug for smo::HoverOn

impl Debug for ImageFormat

impl Debug for Infallible

impl Debug for LineShape

impl Debug for MarkerSymbol

impl Debug for Mode

impl Debug for Month

impl Debug for OdsError

impl Debug for Orientation

impl Debug for PlotType

impl Debug for smo::Position

impl Debug for smo::Reference

impl Debug for RoundingError

impl Debug for SecondsFormat

impl Debug for SheetType

impl Debug for SheetVisible

impl Debug for Show

impl Debug for Side

impl Debug for Sign

impl Debug for SizeMode

impl Debug for TextAnchor

impl Debug for TextPosition

impl Debug for ThicknessMode

impl Debug for TickMode

impl Debug for Ticks

impl Debug for TryReserveErrorKind

impl Debug for Value

impl Debug for Visible

impl Debug for Weekday

impl Debug for XlsError

impl Debug for XlsbError

impl Debug for XlsxError

impl Debug for Category

impl Debug for Colons

impl Debug for Fixed

impl Debug for Numeric

impl Debug for OffsetPrecision

impl Debug for smo::format::Pad

impl Debug for ParseErrorKind

impl Debug for AsciiChar

impl Debug for core::cmp::Ordering

impl Debug for FromBytesWithNulError

impl Debug for c_void

impl Debug for IpAddr

impl Debug for Ipv6MulticastScope

impl Debug for core::net::socket_addr::SocketAddr

impl Debug for FpCategory

Trait Debug