Derive Macro derive_more::Debug

#[derive(Debug)]
{
    // Attributes available to this derive:
    #[debug]
}

Available on crate feature debug only.

What #[derive(Debug)] generates

This derive macro is a clever superset of Debug from standard library. Additional features include:

• not imposing redundant trait bounds;
• #[debug(skip)] (or #[debug(ignore)]) attribute to skip formatting struct field or enum variant;
• #[debug("...", args...)] to specify custom formatting either for the whole struct or enum variant, or its particular field;
• #[debug(bounds(...))] to impose additional custom trait bounds.

The format of the format

You supply a format by placing an attribute on a struct or enum variant, or its particular field: #[debug("...", args...)]. The format is exactly like in format!() or any other format_args!()-based macros.

The variables available in the arguments is self and each member of the struct or enum variant, with members of tuple structs being named with a leading underscore and their index, i.e. _0, _1, _2, etc.

Generic data types

When deriving Debug for a generic struct/enum, all generic type arguments used during formatting are bound by respective formatting trait.

E.g., for a structure Foo defined like this:

use derive_more::Debug;

#[derive(Debug)]
struct Foo<'a, T1, T2: Trait, T3, T4> {
    #[debug("{a}")]
    a: T1,
    #[debug("{b}")]
    b: <T2 as Trait>::Type,
    #[debug("{c:?}")]
    c: Vec<T3>,
    #[debug("{d:p}")]
    d: &'a T1,
    #[debug(skip)] // or #[debug(ignore)]
    e: T4,
}

trait Trait { type Type; }

The following where clauses would be generated:

• T1: Display
• <T2 as Trait>::Type: Display
• Vec<T3>: Debug
• &'a T1: Pointer

Custom trait bounds

Sometimes you may want to specify additional trait bounds on your generic type parameters, so that they could be used during formatting. This can be done with a #[debug(bound(...))] attribute.

#[debug(bound(...))] accepts code tokens in a format similar to the format used in angle bracket list (or where clause predicates): T: MyTrait, U: Trait1 + Trait2.

Using #[debug("...", ...)] formatting we’ll try our best to infer trait bounds, but in more advanced cases this isn’t possible. Our aim is to avoid imposing additional bounds, as they can be added with #[debug(bound(...))]. In the example below, we can infer only that V: Display, other bounds have to be supplied by the user:

use std::fmt::Display;
use derive_more::Debug;

#[derive(Debug)]
#[debug(bound(T: MyTrait, U: Display))]
struct MyStruct<T, U, V, F> {
    #[debug("{}", a.my_function())]
    a: T,
    #[debug("{}", b.to_string().len())]
    b: U,
    #[debug("{c}")]
    c: V,
    #[debug(skip)] // or #[debug(ignore)]
    d: F,
}

trait MyTrait { fn my_function(&self) -> i32; }

Example usage

use std::path::PathBuf;
use derive_more::Debug;

#[derive(Debug)]
struct MyInt(i32);

#[derive(Debug)]
struct MyIntHex(#[debug("{_0:x}")] i32);

#[derive(Debug)]
#[debug("{_0} = {_1}")]
struct StructFormat(&'static str, u8);

#[derive(Debug)]
enum E {
    Skipped {
        x: u32,
        #[debug(skip)] // or #[debug(ignore)]
        y: u32,
    },
    Binary {
        #[debug("{i:b}")]
        i: i8,
    },
    Path(#[debug("{}", _0.display())] PathBuf),
    #[debug("{_0}")]
    EnumFormat(bool)
}

assert_eq!(format!("{:?}", MyInt(-2)), "MyInt(-2)");
assert_eq!(format!("{:?}", MyIntHex(-255)), "MyIntHex(ffffff01)");
assert_eq!(format!("{:?}", StructFormat("answer", 42)), "answer = 42");
assert_eq!(format!("{:?}", E::Skipped { x: 10, y: 20 }), "Skipped { x: 10, .. }");
assert_eq!(format!("{:?}", E::Binary { i: -2 }), "Binary { i: 11111110 }");
assert_eq!(format!("{:?}", E::Path("abc".into())), "Path(abc)");
assert_eq!(format!("{:?}", E::EnumFormat(true)), "true");