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 NfCtSctpState

impl Debug for NfCtStatus

impl Debug for NfCtTcpFlags

impl Debug for NfCtTcpState

impl Debug for PerfDomainFlags

impl Debug for PerfStateFlags

impl Debug for DpipeActionType

impl Debug for DpipeFieldMappingType

impl Debug for DpipeHeaderId

impl Debug for DpipeMatchType

impl Debug for EswitchEncapMode

impl Debug for EswitchInlineMode

impl Debug for EswitchMode

impl Debug for FlashOverwrite

impl Debug for ParamCmode

impl Debug for PortFlavour

impl Debug for PortFnAttrCap

impl Debug for PortFnOpstate

impl Debug for PortFnState

impl Debug for PortType

impl Debug for RateType

impl Debug for ReloadAction

impl Debug for ResourceScope

impl Debug for ResourceUnit

impl Debug for SbPoolType

impl Debug for SbThresholdType

impl Debug for TrapAction

impl Debug for TrapType

impl Debug for VarAttrType

impl Debug for ClockQualityLevel

impl Debug for FeatureState

impl Debug for LockStatus

impl Debug for LockStatusError

impl Debug for Mode

impl Debug for PinCapabilities

impl Debug for PinDirection

impl Debug for PinOperstate

impl Debug for PinState

impl Debug for PinType

impl Debug for netlink_bindings::dpll::Type

impl Debug for NodeType

impl Debug for C33PseExtState

impl Debug for HeaderFlags

impl Debug for HwtstampSource

impl Debug for InputXfrm

impl Debug for ModuleFwFlashStatus

impl Debug for PhyUpstreamType

impl Debug for PseEvent

impl Debug for RxfhFields

impl Debug for TcpDataSplit

impl Debug for UdpTunnelType

impl Debug for EncapType

impl Debug for Auth

impl Debug for HandlerClass

impl Debug for MsgType

impl Debug for BytecodeOpCode

impl Debug for SockoptFlag

impl Debug for TcpState

impl Debug for EventType

impl Debug for Metric

impl Debug for netlink_bindings::net_shaper::Scope

impl Debug for NapiThreaded

impl Debug for QstatsScope

impl Debug for QueueType

impl Debug for XdpAct

impl Debug for XdpRxMetadata

impl Debug for XskFlags

impl Debug for BitwiseOps

impl Debug for ChainFlags

impl Debug for CmpOps

Trait Debug