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 !

impl Debug for ()

impl Debug for capability_3p::mem::type_info::Abi

impl Debug for capability_3p::ast::Abi

impl Debug for futures_util::abortable::AbortHandle

impl Debug for capability_3p::task::AbortHandle

impl Debug for AbortRegistration

impl Debug for Aborted

impl Debug for Accept

impl Debug for rustls::server::server_conn::Accepted

impl Debug for AcceptedAlert

impl Debug for rustix::backend::fs::types::Access

impl Debug for rustix::backend::fs::types::Access

impl Debug for AccessControl

impl Debug for AccessError

impl Debug for AccessKind

impl Debug for AccessMode

impl Debug for AcquireError

impl Debug for reqwest::redirect::Action

impl Debug for tower_http::follow_redirect::policy::Action

impl Debug for anstyle_parse::state::definitions::Action

impl Debug for ActivateSlotError

impl Debug for ActivationState

impl Debug for Actual

impl Debug for AdaptiveSyncState

impl Debug for AddGlyphsRequest<'_>

impl Debug for AddTrapsRequest<'_>

impl Debug for async_openai::types::upload::AddUploadPartRequest

impl Debug for async_openai::types::upload::AddUploadPartRequest

impl Debug for AddrIncoming

impl Debug for core::net::parser::AddrParseError

impl Debug for ipnet::parser::AddrParseError

impl Debug for rustls_pki_types::server_name::AddrParseError

impl Debug for AddrStream

impl Debug for AddressFamily

impl Debug for Adt

impl Debug for rustix::backend::fs::types::Advice

impl Debug for rustix::backend::fs::types::Advice

impl Debug for memmap2::advice::Advice

impl Debug for AgentCoordinate

impl Debug for AgentCoordinateBuilderError

impl Debug for AhoCorasick

impl Debug for AhoCorasickBuilder

impl Debug for AhoCorasickKind

impl Debug for AlertDescription

impl Debug for ring::aead::algorithm::Algorithm

impl Debug for ring::aead::quic::Algorithm

impl Debug for ring::agreement::Algorithm

impl Debug for ring::digest::Algorithm

impl Debug for ring::hkdf::Algorithm

impl Debug for ring::hmac::Algorithm

impl Debug for AlgorithmIdentifier

impl Debug for capability_3p::mem::Alignment

impl Debug for capability_3p::fmt::Alignment

impl Debug for AllCounts

impl Debug for AllocColorCellsReply

impl Debug for AllocColorCellsRequest

impl Debug for AllocColorPlanesReply

impl Debug for AllocColorPlanesRequest

impl Debug for AllocColorReply

impl Debug for AllocColorRequest

impl Debug for core::alloc::AllocError

impl Debug for zerocopy::error::AllocError

impl Debug for AllocNamedColorReply

impl Debug for AllocNamedColorRequest<'_>

impl Debug for rocket::shield::policy::Allow

impl Debug for x11rb_protocol::protocol::xproto::Allow

impl Debug for AllowEventsRequest

impl Debug for AllowNull

impl Debug for AllowedTools

Trait Debug