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 TruncSide

impl Debug for hashbrown::TryReserveError

impl Debug for indexmap::GetDisjointMutError

impl Debug for proc_macro2::Spacing

impl Debug for proc_macro_crate::Error

impl Debug for FoundCrate

impl Debug for AttrStyle

impl Debug for Meta

impl Debug for Fields

impl Debug for Data

impl Debug for Expr

impl Debug for Member

impl Debug for PointerMutability

impl Debug for RangeLimits

impl Debug for CapturedParam

impl Debug for GenericParam

impl Debug for TraitBoundModifier

impl Debug for TypeParamBound

impl Debug for WherePredicate

impl Debug for FnArg

impl Debug for ForeignItem

impl Debug for ImplItem

impl Debug for ImplRestriction

impl Debug for syn::item::Item

impl Debug for StaticMutability

impl Debug for TraitItem

impl Debug for UseTree

impl Debug for Lit

impl Debug for MacroDelimiter

impl Debug for BinOp

impl Debug for UnOp

impl Debug for Pat

impl Debug for GenericArgument

impl Debug for PathArguments

impl Debug for FieldMutability

impl Debug for Visibility

impl Debug for Stmt

impl Debug for ReturnType

impl Debug for syn::ty::Type

impl Debug for Offset

impl Debug for toml_edit::item::Item

impl Debug for Value

impl Debug for Encoding

impl Debug for IntegerRadix

impl Debug for ScalarKind

impl Debug for Expected

impl Debug for TokenKind

impl Debug for EventKind

impl Debug for Endianness

impl Debug for Needed

impl Debug for StrContext

impl Debug for StrContextValue

impl Debug for CompareResult

impl Debug for Equality

impl Debug for Target

impl Debug for DefineMacroInput

impl Debug for EngineInput

impl Debug for WithinLoop

impl Debug for FoundNearbyNodeKind

impl Debug for PreprocessedValue

impl Debug for derive_deftly_macros::meta::Scope

impl Debug for SuppliedScope

impl Debug for UsageMode

impl Debug for ExpectedDriverKind

impl Debug for OpContext

impl Debug for OptionDetails

impl Debug for ChangeCase

impl Debug for derive_deftly_macros::paste::Item

impl Debug for RepeatOver

impl Debug for DefinitionBody

impl Debug for OrigDollarHandled

Trait Debug