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 NfCtSctpState

impl Debug for NfCtStatus

impl Debug for NfCtTcpFlags

impl Debug for NfCtTcpState

impl Debug for BitwiseOps

impl Debug for ChainFlags

impl Debug for CmpOps

impl Debug for CtDirection

impl Debug for CtKeys

impl Debug for FibFlags

impl Debug for FibResult

impl Debug for LogFlags

impl Debug for LogLevel

impl Debug for LookupFlags

impl Debug for MetaKeys

impl Debug for NatRangeFlags

impl Debug for NumgenTypes

impl Debug for ObjectType

impl Debug for PayloadBase

impl Debug for QuotaFlags

impl Debug for RangeOps

impl Debug for Registers

impl Debug for RejectTypes

impl Debug for SetFlags

impl Debug for TableFlags

impl Debug for VerdictCode

impl Debug for AttrType

impl Debug for OpFlags

impl Debug for IfaFlags

impl Debug for IfinfoFlags

impl Debug for IflaIcmp6Stats

impl Debug for IflaInet6Stats

impl Debug for IflaVfLinkStateEnum

impl Debug for Ipv4Devconf

impl Debug for Ipv6Devconf

impl Debug for NetkitMode

impl Debug for NetkitPolicy

impl Debug for NetkitScrub

impl Debug for OvpnMode

impl Debug for RtextFilter

impl Debug for VlanFlags

impl Debug for VlanProtocols

impl Debug for NtfExtFlags

impl Debug for NtfFlags

impl Debug for NudState

impl Debug for netlink_bindings::rt_neigh::RtmType

impl Debug for netlink_bindings::rt_route::RtmType

impl Debug for FrAct

impl Debug for ClsFlags

impl Debug for Dualpi2DropEarly

impl Debug for Dualpi2DropOverload

impl Debug for Dualpi2EcnMask

impl Debug for Dualpi2SplitGso

impl Debug for FlowerKeyCtrlFlags

impl Debug for Protocol

impl Debug for WgallowedipFlags

impl Debug for WgdeviceFlags

impl Debug for WgpeerFlags

impl Debug for ErrorReason

impl Debug for IpAddr

impl Debug for netlink_bindings::utils::SocketAddr

impl Debug for TryReserveErrorKind

impl Debug for AsciiChar

impl Debug for core::cmp::Ordering

impl Debug for Infallible

impl Debug for FromBytesWithNulError

impl Debug for c_void

impl Debug for core::fmt::Alignment

impl Debug for DebugAsHex

impl Debug for Sign

impl Debug for AtomicOrdering

Trait Debug