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,
)");

Debug

Trait Debug Copy item path

§Stability

§Examples

Required Methods§

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

§Errors

§Examples

Implementors§

impl Debug for Adt

impl Debug for ArithOp

impl Debug for AsmOperand

impl Debug for AsmPiece

impl Debug for AssocItem

impl Debug for AttrKind

impl Debug for BinaryOp

impl Debug for CallableExpr

impl Debug for CmpOp

impl Debug for CommentPlacement

impl Debug for CommentShape

impl Debug for ElseBranch

impl Debug for Expr

impl Debug for ExternItem

impl Debug for FieldKind

impl Debug for FieldList

impl Debug for GenericArg

impl Debug for GenericParam

impl Debug for capability_3p::ast::Item

impl Debug for capability_3p::ast::LiteralKind

impl Debug for LogicOp

impl Debug for Macro

impl Debug for NameLike

impl Debug for NameOrNameRef

impl Debug for capability_3p::ast::Ordering

impl Debug for Pat

impl Debug for PathSegmentKind

impl Debug for Radix

impl Debug for RangeOp

impl Debug for SelfParamKind

impl Debug for Stmt

impl Debug for StructKind

impl Debug for TraitOrAlias

impl Debug for capability_3p::ast::Type

impl Debug for TypeBoundKind

impl Debug for TypeOrConstParam

impl Debug for UnaryOp

impl Debug for capability_3p::colored::Color

impl Debug for Styles

impl Debug for AgentCoordinateBuilderError

impl Debug for BatchChoiceBuilderError

impl Debug for BatchDownloadError

impl Debug for BatchErrorDataBuilderError

impl Debug for BatchErrorDetailsBuilderError

impl Debug for BatchErrorFileProcessingOperation

impl Debug for BatchErrorProcessingError

impl Debug for BatchErrorResponseBodyBuilderError

impl Debug for BatchExecutionResultBuilderError

impl Debug for BatchFileState

impl Debug for BatchFileTripleBuilderError

impl Debug for BatchFileTripleReconciliationOperation

impl Debug for BatchIndex

impl Debug for BatchInputCreationError

impl Debug for BatchInputDataBuilderError

impl Debug for BatchMessageBuilderError

impl Debug for BatchMessageContentBuilderError

impl Debug for BatchMetadataBuilderError

impl Debug for BatchMetadataError

impl Debug for BatchOutputDataBuilderError

impl Debug for BatchOutputProcessingError

impl Debug for BatchProcessingError

impl Debug for BatchReconciliationError

impl Debug for BatchRequestRecordBuilderError

impl Debug for BatchResponseBody

impl Debug for BatchResponseContentBuilderError

impl Debug for BatchResponseRecordBuilderError

impl Debug for BatchSuccessResponseBodyBuilderError

impl Debug for BatchSuccessResponseHandlingError

impl Debug for BatchTokenDetailsBuilderError

impl Debug for BatchUsageBuilderError

impl Debug for BatchValidationError

Trait Debug