Derive Macro derive_more::Into

#[derive(Into)]
{
    // Attributes available to this derive:
    #[into]
}

Available on crate feature into only.

What #[derive(Into)] generates

This derive creates the exact opposite of #[derive(From)]. Instead of allowing you to create a new instance of the struct from the values it should contain, it allows you to extract the values from the struct. One thing to note is that this derive doesn’t actually generate an implementation for the Into trait. Instead, it derives From for the values contained in the struct and thus has an indirect implementation of Into as recommended by the docs.

Structs

For structs with a single field you can call .into() to extract the inner type.

#[derive(Debug, Into, PartialEq)]
struct Int(i32);

assert_eq!(2, Int(2).into());

For structs having multiple fields, .into() extracts a tuple containing the desired content for each field.

#[derive(Debug, Into, PartialEq)]
struct Point(i32, i32);

assert_eq!((1, 2), Point(1, 2).into());

To specify concrete types for deriving conversions into, use #[into(<types>)].

#[derive(Debug, Into, PartialEq)]
#[into(Cow<'static, str>, String)]
struct Str(Cow<'static, str>);

assert_eq!("String".to_owned(), String::from(Str("String".into())));
assert_eq!(Cow::Borrowed("Cow"), <Cow<_>>::from(Str("Cow".into())));

#[derive(Debug, Into, PartialEq)]
#[into((i64, i64), (i32, i32))]
struct Point {
    x: i32,
    y: i32,
}

assert_eq!((1_i64, 2_i64), Point { x: 1_i32, y: 2_i32 }.into());
assert_eq!((3_i32, 4_i32), Point { x: 3_i32, y: 4_i32 }.into());

In addition to converting to owned types, this macro supports deriving into reference (mutable or not) via #[into(ref(...))]/#[into(ref_mut(...))].

#[derive(Debug, Into, PartialEq)]
#[into(owned, ref(i32), ref_mut)]
struct Int(i32);

assert_eq!(2, Int(2).into());
assert_eq!(&2, <&i32>::from(&Int(2)));
assert_eq!(&mut 2, <&mut i32>::from(&mut Int(2)));

In case there are fields, that shouldn’t be included in the conversion, use the #[into(skip)] (or #[into(ignore)]) attribute.

#[derive(Debug, Into, PartialEq)]
#[into(i32, i64, i128)]
struct Mass<Unit> {
    value: i32,
    #[into(skip)] // or #[into(ignore)]
    _unit: PhantomData<Unit>,
}

assert_eq!(5, Mass::<Gram>::new(5).into());
assert_eq!(5_i64, Mass::<Gram>::new(5).into());
assert_eq!(5_i128, Mass::<Gram>::new(5).into());

Fields

The #[into] attribute can also be applied to specific fields of a struct.

#[derive(Into)]
struct Data {
    id: i32,
    #[into]
    raw: f64
}

assert_eq!(42.0, Data { id: 1, raw: 42.0 }.into());

In such cases, no conversion into a tuple of all fields is generated, unless an explicit struct attribute is present.

#[derive(Into)]
#[into]
struct Data {
    id: i32,
    #[into]
    raw: f64
}

assert_eq!(42.0, Data { id: 1, raw: 42.0 }.into());
assert_eq!((1, 42.0), Data { id: 1, raw: 42.0 }.into());

The #[into(<types>)] syntax can be used on fields as well.

#[derive(Into, Clone)]
#[into(owned, ref((u8, str)), ref_mut)]
struct Foo {
   #[into(owned(u64), ref)]
   a: u8,
   b: String,
   #[into(skip)]
   _c: PhantomData<Whatever>,
}

let mut foo = Foo { a: 1, b: "string".to_owned(), _c: PhantomData };

assert_eq!((1_u8, "string".to_owned()), foo.clone().into());
assert_eq!((&1_u8, "string"), <(&u8, &str)>::from(&foo));
assert_eq!((&mut 1_u8, &mut "string".to_owned()), <(&mut u8, &mut String)>::from(&mut foo));
assert_eq!(1_u64, foo.clone().into());
assert_eq!(&1_u8, <&u8>::from(&foo));

Fields, having specific conversions into them, can also be skipped for top-level tuple conversions.

#[derive(Into)]
#[into(ref((str, f64)))]
struct Foo {
    #[into(ref)]
    #[into(skip)]
    a: u8,
    b: String,
    c: f64,
}

let foo = Foo { a: 1, b: "string".to_owned(), c: 3.0 };

assert_eq!(("string", &3.0), (&foo).into());
assert_eq!(&1_u8, <&u8>::from(&foo));

Enums

Deriving Into for enums is not supported as it would not always be successful, so TryInto should be used instead.