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 &dyn TargetIsa

impl Debug for AhoCorasickKind

impl Debug for aho_corasick::packed::api::MatchKind

impl Debug for aho_corasick::util::error::MatchErrorKind

impl Debug for Candidate

impl Debug for aho_corasick::util::search::Anchored

impl Debug for aho_corasick::util::search::MatchKind

impl Debug for aho_corasick::util::search::StartKind

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

impl Debug for arbitrary::error::Error

impl Debug for base16ct::error::Error

impl Debug for ParseAlphabetError

impl Debug for base64::decode::DecodeError

impl Debug for DecodeSliceError

impl Debug for EncodeSliceError

impl Debug for DecodePaddingMode

impl Debug for base64ct::errors::Error

impl Debug for LineEnding

impl Debug for bip39::Error

impl Debug for Language

impl Debug for bs58::alphabet::Error

impl Debug for bs58::decode::Error

impl Debug for bs58::encode::Error

impl Debug for byte_slice_cast::Error

impl Debug for byteorder::BigEndian

impl Debug for byteorder::LittleEndian

impl Debug for cobs::dec::DecodeError

impl Debug for DecoderState

impl Debug for const_format::__ascii_case_conv::Case

impl Debug for const_oid::error::Error

impl Debug for cpp_demangle::ast::ArrayType

impl Debug for BaseUnresolvedName

impl Debug for BuiltinType

impl Debug for CallOffset

impl Debug for ClassEnumType

impl Debug for CtorDtorName

impl Debug for Decltype

impl Debug for DestructorName

impl Debug for cpp_demangle::ast::Encoding

impl Debug for ExceptionSpec

impl Debug for ExplicitObjectParameter

impl Debug for ExprPrimary

impl Debug for cpp_demangle::ast::Expression

impl Debug for GlobalCtorDtor

impl Debug for LocalName

impl Debug for MangledName

impl Debug for cpp_demangle::ast::Name

impl Debug for NestedName

impl Debug for OperatorName

impl Debug for ParametricBuiltinType

impl Debug for cpp_demangle::ast::Prefix

impl Debug for PrefixHandle

impl Debug for RefQualifier

impl Debug for SimpleOperatorName

impl Debug for SpecialName

impl Debug for StandardBuiltinType

impl Debug for Substitution

impl Debug for TemplateArg

impl Debug for TemplateTemplateParamHandle

impl Debug for cpp_demangle::ast::Type

impl Debug for TypeHandle

impl Debug for UnqualifiedName

impl Debug for UnresolvedName

impl Debug for UnresolvedType

impl Debug for UnresolvedTypeHandle

impl Debug for UnscopedName

impl Debug for UnscopedTemplateNameHandle

impl Debug for VectorType

impl Debug for WellKnownComponent

impl Debug for DemangleNodeType

impl Debug for cpp_demangle::error::Error

Trait Debug