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 Body

impl Debug for kvarn::application::Error

impl Debug for H2SendResponse

impl Debug for HttpConnection

impl Debug for PushedResponsePipe

impl Debug for ResponseBodyPipe

impl Debug for ResponsePipe

impl Debug for CachePreferenceError

impl Debug for ClientCachePreference

impl Debug for CompressPreference

impl Debug for PreferredCompression

impl Debug for ServerCachePreference

impl Debug for UriKey

impl Debug for Value

impl Debug for PluginResponseKind

impl Debug for Encryption

impl Debug for kvarn::encryption::Error

impl Debug for BindIpVersion

impl Debug for kvarn::Incoming

impl Debug for SendKind

impl Debug for CacheAction

impl Debug for kvarn::host::CertificateError

impl Debug for Action

impl Debug for kvarn::websocket::Error

impl Debug for kvarn::prelude::IpAddr

impl Debug for kvarn::prelude::SocketAddr

impl Debug for TryReserveErrorKind

impl Debug for BacktraceStatus

impl Debug for VarError

impl Debug for AncillaryError

impl Debug for BacktraceStyle

impl Debug for std::sync::mpsc::RecvTimeoutError

impl Debug for std::sync::mpsc::TryRecvError

impl Debug for _Unwind_Reason_Code

impl Debug for FlushCompress

impl Debug for FlushDecompress

impl Debug for Status

impl Debug for log::Level

impl Debug for log::LevelFilter

impl Debug for petgraph::dot::Config

impl Debug for Directed

impl Debug for Direction

impl Debug for Undirected

impl Debug for Always

impl Debug for socket2::socket::InterfaceIndexOrAddress

impl Debug for Variant

impl Debug for uuid::Version

impl Debug for BernoulliError

impl Debug for WeightedError

impl Debug for IndexVec

impl Debug for IndexVecIntoIter

impl Debug for BytesCow

impl Debug for CacheControlError

impl Debug for kvarn::prelude::utils::parse::Error

impl Debug for RequestParseStage

impl Debug for SanitizeError

impl Debug for AsciiChar

impl Debug for kvarn::prelude::utils::prelude::compact_str::core::cmp::Ordering

impl Debug for Infallible

impl Debug for c_void

impl Debug for FpCategory

impl Debug for IntErrorKind

impl Debug for SearchStep

impl Debug for kvarn::prelude::utils::prelude::compact_str::core::sync::atomic::Ordering

impl Debug for ToCompactStringError

impl Debug for kvarn::prelude::utils::prelude::fmt::Alignment

impl Debug for kvarn::prelude::utils::prelude::io::ErrorKind

impl Debug for SeekFrom

impl Debug for Ipv6MulticastScope

impl Debug for Shutdown

impl Debug for bool

impl Debug for char

Trait Debug