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 &dyn TargetIsa

impl Debug for cumulus_primitives_core::AggregateMessageOrigin

impl Debug for CoreInfoExistsAtMaxOnce

impl Debug for CumulusDigestItem

impl Debug for MessageSendError

impl Debug for RelayBlockIdentifier

impl Debug for ServiceQuality

impl Debug for RelayChainError

impl Debug for FilterAction

impl Debug for AnalysisChoice

impl Debug for BenchmarkError

impl Debug for BenchmarkParameter

impl Debug for frame_election_provider_support::onchain::Error

impl Debug for ExecutiveError

impl Debug for frame_remote_externalities::Transport

impl Debug for StorageAccessPayload

impl Debug for DispatchClass

impl Debug for Pays

impl Debug for Never

impl Debug for InitilizationType

impl Debug for FailedMigrationHandling

impl Debug for SteppedMigrationError

impl Debug for ExecuteOverweightError

impl Debug for ProcessMessageError

impl Debug for frame_support::traits::reality::Judgement

impl Debug for frame_support::traits::reality::Statement

impl Debug for Truth

impl Debug for Social

impl Debug for frame_support::traits::schedule::LookupError

impl Debug for BalanceStatus

impl Debug for DepositConsequence

impl Debug for ExistenceRequirement

impl Debug for Fortitude

impl Debug for Precision

impl Debug for Preservation

impl Debug for Provenance

impl Debug for Restriction

impl Debug for PaymentStatus

impl Debug for frame_support::traits::try_runtime::Select

impl Debug for UpgradeCheckSelect

impl Debug for ViewFunctionDispatchError

impl Debug for DecRefStatus

impl Debug for IncRefStatus

impl Debug for frame_system::Phase

impl Debug for RefStatus

impl Debug for ListError

impl Debug for pallet_bags_list::mock::OriginCaller

impl Debug for pallet_bags_list::mock::RuntimeCall

impl Debug for pallet_bags_list::mock::RuntimeError

impl Debug for pallet_bags_list::mock::RuntimeEvent

impl Debug for pallet_bags_list::mock::RuntimeFreezeReason

impl Debug for pallet_bags_list::mock::RuntimeHoldReason

impl Debug for pallet_bags_list::mock::RuntimeLockId

impl Debug for pallet_bags_list::mock::RuntimeSlashReason

impl Debug for pallet_bags_list::mock::RuntimeTask

impl Debug for pallet_bags_list::mock::RuntimeViewFunction

impl Debug for pallet_balances::pallet::UnexpectedKind

impl Debug for AdjustmentDirection

impl Debug for Reasons

impl Debug for CoreAssignment

impl Debug for CompletionStatus

impl Debug for pallet_broker::types::Finality

impl Debug for pallet_conviction_voting::conviction::Conviction

impl Debug for pallet_conviction_voting::traits::Status

impl Debug for pallet_delegated_staking::pallet::HoldReason

impl Debug for pallet_democracy::conviction::Conviction

impl Debug for MetadataOwner

impl Debug for VoteThreshold

impl Debug for ElectionCompute

impl Debug for FeasibilityError

impl Debug for MinerError

impl Debug for Renouncing

Trait Debug