Derive Macro derive_more_impl::Display

source ·

#[derive(Display)]
{
    // Attributes available to this derive:
    #[display]
}

Available on crate feature display only.

Expand description

§What `#[derive(Display)]` generates

Deriving Display will generate a Display implementation, with a fmt method that matches self and each of its variants. In the case of a struct or union, only a single variant is available, and it is thus equivalent to a simple let statement. In the case of an enum, each of its variants is matched.

For each matched variant, a write! expression will be generated with the supplied format, or an automatically inferred one.

You specify the format on each variant by writing e.g. #[display("my val: {}", some_val * 2)]. For enums, you can either specify it on each variant, or on the enum as a whole.

For variants that don’t have a format specified, it will simply defer to the format of the inner variable. If there is no such variable, or there is more than 1, an error is generated.

§The format of the format

You supply a format by attaching an attribute of the syntax: #[display("...", args...)]. The format supplied is passed verbatim to write!.

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. Due to ownership/lifetime limitations the member variables are all references to the fields, except when used directly in the format string. For most purposes this detail doesn’t matter, but it is quite important when using Pointer formatting. If you don’t use the {field:p} syntax, you have to dereference once to get the address of the field itself, instead of the address of the reference to the field:

#[derive(Display)]
#[display("{field:p} {:p}", *field)]
struct RefInt<'a> {
    field: &'a i32,
}

let a = &123;
assert_eq!(format!("{}", RefInt{field: &a}), format!("{a:p} {:p}", a));

For enums you can also specify a shared format on the enum itself instead of the variant. This format is used for each of the variants, and can be customized per variant by including the special {_variant} placeholder in this shared format, which is then replaced by the format string that’s provided on the variant.

§Other formatting traits

The syntax does not change, but the name of the attribute is the snake case version of the trait. E.g. Octal -> octal, Pointer -> pointer, UpperHex -> upper_hex.

Note, that Debug has a slightly different API and semantics, described in its docs, and so, requires a separate debug feature.

§Generic data types

When deriving Display (or other formatting trait) for a generic struct/enum, all generic type arguments used during formatting are bound by respective formatting trait. Bounds can only be inferred this way if a field is used directly in the interpolation.

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

#[derive(Display)]
#[display("{a} {b} {c:?} {d:p}")]
struct Foo<'a, T1, T2: Trait, T3> {
    a: T1,
    b: <T2 as Trait>::Type,
    c: Vec<T3>,
    d: &'a T1,
}

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 #[display(bound(...))] attribute.

#[display(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.

#[display("fmt", ...)] arguments are parsed as an arbitrary Rust expression and passed to generated write! as-is, it’s impossible to meaningfully infer any kind of trait bounds for generic type parameters used this way. That means that you’ll have to explicitly specify all the required trait bounds of the expression. Either in the struct/enum definition, or via #[display(bound(...))] attribute.

Explicitly specified bounds are added to the inferred ones. Note how no V: Display bound is necessary, because it’s inferred already.

#[derive(Display)]
#[display(bound(T: MyTrait, U: Display))]
#[display("{} {} {}", a.my_function(), b.to_string().len(), c)]
struct MyStruct<T, U, V> {
    a: T,
    b: U,
    c: V,
}

§Transparency

If the #[display("...", args...)] attribute is omitted, the implementation transparently delegates to the format of the inner type, so all the additional formatting parameters do work as expected:

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

assert_eq!(format!("{:03}", MyInt(7)), "007");

If the #[display("...", args...)] attribute is specified and can be trivially substituted with a transparent delegation call to the inner type, then additional formatting parameters will work too:

#[derive(Display)]
#[display("{_0:o}")] // the same as calling `Octal::fmt()`
struct MyOctalInt(i32);

// so, additional formatting parameters do work transparently
assert_eq!(format!("{:03}", MyOctalInt(9)), "011");

#[derive(Display)]
#[display("{_0:02b}")]   // cannot be trivially substituted with `Binary::fmt()`,
struct MyBinaryInt(i32); // because of specified formatting parameters

// so, additional formatting parameters have no effect
assert_eq!(format!("{:07}", MyBinaryInt(2)), "10");

If, for some reason, transparency in trivial cases is not desired, it may be suppressed explicitly either with the format_args!() macro usage:

#[derive(Display)]
#[display("{}", format_args!("{_0:o}"))] // `format_args!()` obscures the inner type
struct MyOctalInt(i32);

// so, additional formatting parameters have no effect
assert_eq!(format!("{:07}", MyOctalInt(9)), "11");

Or by adding formatting parameters which cause no visual effects:

#[derive(Display)]
#[display("{_0:^o}")] // `^` is centering, but in absence of additional width has no effect
struct MyOctalInt(i32);

// and so, additional formatting parameters have no effect
assert_eq!(format!("{:07}", MyOctalInt(9)), "11");

§Example usage

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

#[derive(Display)]
#[display("({x}, {y})")]
struct Point2D {
    x: i32,
    y: i32,
}

#[derive(Display)]
#[display("Enum E: {_variant}")]
enum E {
    Uint(u32),
    #[display("I am B {:b}", i)]
    Binary {
        i: i8,
    },
    #[display("I am C {}", _0.display())]
    Path(PathBuf),
}

#[derive(Display)]
#[display("Enum E2: {_0:?}")]
enum E2 {
    Uint(u32),
    String(&'static str, &'static str),
}

#[derive(Display)]
#[display("Hello there!")]
union U {
    i: u32,
}

#[derive(Octal)]
#[octal("7")]
struct S;

#[derive(UpperHex)]
#[upper_hex("UpperHex")]
struct UH;

#[derive(Display)]
struct Unit;

#[derive(Display)]
struct UnitStruct {}

#[derive(Display)]
#[display("{}", self.sign())]
struct PositiveOrNegative {
    x: i32,
}

impl PositiveOrNegative {
    fn sign(&self) -> &str {
        if self.x >= 0 {
            "Positive"
        } else {
            "Negative"
        }
    }
}

assert_eq!(MyInt(-2).to_string(), "-2");
assert_eq!(Point2D { x: 3, y: 4 }.to_string(), "(3, 4)");
assert_eq!(E::Uint(2).to_string(), "Enum E: 2");
assert_eq!(E::Binary { i: -2 }.to_string(), "Enum E: I am B 11111110");
assert_eq!(E::Path("abc".into()).to_string(), "Enum E: I am C abc");
assert_eq!(E2::Uint(2).to_string(), "Enum E2: 2");
assert_eq!(E2::String("shown", "ignored").to_string(), "Enum E2: \"shown\"");
assert_eq!(U { i: 2 }.to_string(), "Hello there!");
assert_eq!(format!("{:o}", S), "7");
assert_eq!(format!("{:X}", UH), "UpperHex");
assert_eq!(Unit.to_string(), "Unit");
assert_eq!(UnitStruct {}.to_string(), "UnitStruct");
assert_eq!(PositiveOrNegative { x: 1 }.to_string(), "Positive");
assert_eq!(PositiveOrNegative { x: -1 }.to_string(), "Negative");

Derive Macro derive_more_impl::DisplayCopy item path

§What #[derive(Display)] generates