? formatting.

Debug should format the output in a programmer-facing, debugging context.

Generally speaking, you should justderive aDebug implementation.

When used with the alternate format specifier#?, the output is pretty-printed.

For more information on formatters, seethe module-level documentation.

This trait can be used with#[derive] if all fields implementDebug. Whenderived for structs, it will use the name of thestruct, then{, then acomma-separated list of each field’s name andDebug value, then}. Forenums, it will use the name of the variant and, if applicable,(, then theDebug values of the fields, then).

§Stability

DerivedDebug formats are not stable, and so may change with future Rustversions. Additionally,Debug implementations of types provided by thestandard library (std,core,alloc, etc.) are not stable, andmay also change with future Rust versions.

§Examples

Deriving an implementation:

#[derive(Debug)]structPoint {    x: i32,    y: i32,}letorigin = Point { x:0, y:0};assert_eq!(format!("The origin is: {origin:?}"),"The origin is: Point { x: 0, y: 0 }",);

Manually implementing:

usestd::fmt;structPoint {    x: i32,    y: i32,}implfmt::DebugforPoint {fnfmt(&self, f:&mutfmt::Formatter<'_>) -> fmt::Result {        f.debug_struct("Point")         .field("x",&self.x)         .field("y",&self.y)         .finish()    }}letorigin = 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 theFormatter struct to help you with manualimplementations, such asdebug_struct.

Types that do not wish to use the standard suite of debug representationsprovided by theFormatter trait (debug_struct,debug_tuple,debug_list,debug_set,debug_map) can do something totally custom bymanually writing an arbitrary representation to theFormatter.

implfmt::DebugforPoint {fnfmt(&self, f:&mutfmt::Formatter<'_>) -> fmt::Result {write!(f,"Point [{} {}]",self.x,self.y)    }}

Debug implementations using eitherderive or the debug builder APIonFormatter support pretty-printing using the alternate flag:{:#?}.

Pretty-printing with#?:

#[derive(Debug)]structPoint {    x: i32,    y: i32,}letorigin = Point { x:0, y:0};letexpected ="The origin is: Point {    x: 0,    y: 0,}";assert_eq!(format!("The origin is: {origin:#?}"), expected);