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 PrivateInput

impl Debug for PublicInputError

impl Debug for Dictionary

impl Debug for HashChainError

impl Debug for ProgramHashError

impl Debug for OffsetValue

impl Debug for BuiltinName

impl Debug for MathError

impl Debug for ProgramError

impl Debug for ApUpdate

impl Debug for FpUpdate

impl Debug for Op1Addr

impl Debug for Opcode

impl Debug for PcUpdate

impl Debug for Register

impl Debug for Res

impl Debug for LayoutName

impl Debug for MaybeRelocatable

impl Debug for CairoPieValidationError

impl Debug for CairoRunError

impl Debug for ExecScopeError

impl Debug for HintError

impl Debug for InsufficientAllocatedCellsError

impl Debug for MemoryError

impl Debug for RunnerError

impl Debug for TraceError

impl Debug for VirtualMachineError

impl Debug for BuiltinRunner

impl Debug for BuiltinAdditionalData

impl Debug for CairoArg

impl Debug for RunnerMode

impl Debug for cairo_vm::with_std::cmp::Ordering

impl Debug for Infallible

impl Debug for FpCategory

impl Debug for IntErrorKind

impl Debug for SearchStep

impl Debug for cairo_vm::with_std::sync::atomic::Ordering

impl Debug for RecvTimeoutError

impl Debug for TryRecvError

impl Debug for cairo_vm::with_std::fmt::Alignment

impl Debug for alloc::collections::TryReserveErrorKind

impl Debug for AsciiChar

impl Debug for c_void

impl Debug for IpAddr

impl Debug for Ipv6MulticastScope

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

impl Debug for BacktraceStatus

impl Debug for VarError

impl Debug for SeekFrom

impl Debug for std::io::error::ErrorKind

impl Debug for Shutdown

impl Debug for AncillaryError

impl Debug for BacktraceStyle

impl Debug for _Unwind_Reason_Code

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

impl Debug for LegendreSymbol

impl Debug for SerializationError

impl Debug for ark_std::io::error::ErrorKind

impl Debug for AllowedEnumVariants

impl Debug for bincode::error::DecodeError

impl Debug for bincode::error::EncodeError

impl Debug for IntegerType

impl Debug for bincode::features::serde::DecodeError

impl Debug for bincode::features::serde::EncodeError

impl Debug for BigEndian

impl Debug for LittleEndian

impl Debug for TruncSide

impl Debug for FlushCompress

impl Debug for FlushDecompress

impl Debug for Status

impl Debug for hashbrown::TryReserveError

impl Debug for FromHexError

Trait Debug