Trait Debug

1.6.0 · 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§

1.6.0 · 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 wasmtime_environ::component::dfg::CoreDef

impl Debug for ComponentItem

impl Debug for wasmtime_environ::component::CoreDef

impl Debug for wasmtime_environ::component::Export

impl Debug for FixedEncoding

impl Debug for GlobalInitializer

impl Debug for InstantiateModule

impl Debug for InterfaceType

impl Debug for StringEncoding

impl Debug for Transcode

impl Debug for TypeDef

impl Debug for CompileError

impl Debug for ConstOp

impl Debug for EngineOrModuleTypeIndex

impl Debug for EntityIndex

impl Debug for wasmtime_environ::EntityType

impl Debug for wasmtime_environ::Initializer

impl Debug for MemoryInitialization

impl Debug for MemoryStyle

impl Debug for wasmtime_environ::RelocationTarget

impl Debug for SettingKind

impl Debug for TableInitialValue

impl Debug for TableSegmentElements

impl Debug for TableStyle

impl Debug for Trap

impl Debug for VMGcKind

impl Debug for WasmCompositeType

impl Debug for WasmError

impl Debug for WasmHeapTopType

impl Debug for WasmHeapType

impl Debug for WasmStorageType

impl Debug for WasmValType

impl Debug for LibCall

impl Debug for wasmtime_environ::wasmparser::AbstractHeapType

impl Debug for wasmtime_environ::wasmparser::BlockType

impl Debug for CanonicalFunction

impl Debug for wasmtime_environ::wasmparser::CanonicalOption

impl Debug for wasmtime_environ::wasmparser::Catch

impl Debug for ComdatSymbolKind

impl Debug for ComponentExternalKind

impl Debug for wasmtime_environ::wasmparser::ComponentOuterAliasKind

impl Debug for wasmtime_environ::wasmparser::ComponentTypeRef

impl Debug for wasmtime_environ::wasmparser::ComponentValType

impl Debug for wasmtime_environ::wasmparser::CompositeType

impl Debug for wasmtime_environ::wasmparser::CoreDumpValue

impl Debug for wasmtime_environ::wasmparser::Encoding

impl Debug for ExternalKind

impl Debug for FrameKind

impl Debug for wasmtime_environ::wasmparser::HeapType

impl Debug for InstantiationArgKind

impl Debug for wasmtime_environ::wasmparser::Ordering

impl Debug for OuterAliasKind

impl Debug for Payload<'_>

impl Debug for wasmtime_environ::wasmparser::PrimitiveValType

impl Debug for RelocAddendKind

impl Debug for RelocationType

impl Debug for wasmtime_environ::wasmparser::StorageType

impl Debug for wasmtime_environ::wasmparser::TagKind

impl Debug for wasmtime_environ::wasmparser::TypeBounds

impl Debug for TypeRef

impl Debug for UnpackedIndex

impl Debug for wasmtime_environ::wasmparser::ValType

impl Debug for AnyTypeId

impl Debug for ComponentAnyTypeId

impl Debug for ComponentCoreTypeId

impl Debug for wasmtime_environ::wasmparser::types::ComponentDefinedType

impl Debug for ComponentEntityType

impl Debug for wasmtime_environ::wasmparser::types::ComponentValType

impl Debug for CoreInstanceTypeKind

impl Debug for wasmtime_environ::wasmparser::types::EntityType

impl Debug for AsciiChar

impl Debug for wasmtime_environ::__core::cmp::Ordering

Trait Debug