firefox/third_party/rust/derive_more-impl/doc/into.md

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.

# use derive_more::Into;
#
#[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.

# use derive_more::Into;
#
#[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>)].

# use std::borrow::Cow;
#
# use derive_more::Into;
#
#[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(...))].

# use derive_more::Into;
#
#[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)] attribute.

# use std::marker::PhantomData;
#
# use derive_more::Into;
#
# struct Gram;
#
#[derive(Debug, Into, PartialEq)]
#[into(i32, i64, i128)]
struct Mass<Unit> {
    value: i32,
    #[into(skip)]
    _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());
#
# impl<Unit> Mass<Unit> {
#     fn new(value: i32) -> Self {
#         Self {
#             value,
#             _unit: PhantomData,
#         }
#     }
# }

Enums

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