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 sails_rs::errors::Error

impl Debug for RtlError

impl Debug for AsciiChar

impl Debug for sails_rs::prelude::cmp::Ordering

impl Debug for sails_rs::prelude::collections::TryReserveErrorKind

impl Debug for sails_rs::prelude::collections::hashbrown::TryReserveError

impl Debug for Infallible

impl Debug for ErrorReplyReason

impl Debug for ReplyCode

impl Debug for SignalCode

impl Debug for SimpleExecutionError

impl Debug for SimpleUnavailableActorError

impl Debug for SuccessReplyReason

impl Debug for FromBytesWithNulError

impl Debug for c_void

impl Debug for FpCategory

impl Debug for IntErrorKind

impl Debug for GetDisjointMutError

impl Debug for SearchStep

impl Debug for sails_rs::prelude::fmt::Alignment

impl Debug for DebugAsHex

impl Debug for Sign

impl Debug for AtomicOrdering

impl Debug for IpAddr

impl Debug for Ipv6MulticastScope

impl Debug for core::net::socket_addr::SocketAddr

impl Debug for core::sync::atomic::Ordering

impl Debug for BacktraceStatus

impl Debug for VarError

impl Debug for std::fs::TryLockError

impl Debug for SeekFrom

impl Debug for ErrorKind

impl Debug for Shutdown

impl Debug for AncillaryError

impl Debug for BacktraceStyle

impl Debug for RecvTimeoutError

impl Debug for TryRecvError

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

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 BigEndian

impl Debug for LittleEndian

impl Debug for Case

impl Debug for BinaryError

impl Debug for TruncSide

impl Debug for PollNext

impl Debug for gcore::errors::Error

impl Debug for ExecutionError

impl Debug for ExtError

impl Debug for MemoryError

impl Debug for MessageError

impl Debug for ReservationError

impl Debug for gear_ss58::Error

impl Debug for ConversionError

impl Debug for gstd::common::errors::Error

impl Debug for UsageError

impl Debug for hashbrown::TryReserveError

impl Debug for hex::error::FromHexError

impl Debug for primitive_types::Error

impl Debug for BernoulliError

impl Debug for WeightedError

impl Debug for IndexVec

impl Debug for IndexVecIntoIter

impl Debug for rustc_hex::FromHexError

impl Debug for MetaForm

impl Debug for PortableForm

impl Debug for TypeDefPrimitive

impl Debug for PathError

impl Debug for FromDecStrErr

Trait Debug