What #[derive(From)] generates

The point of deriving this type is that it makes it easy to create a new instance of the type by using the .into() method on the value(s) that it should contain. This is done by implementing the From trait for the type that is passed to the derive.

Structs

For structs with a single field you can call .into() on the desired content itself after deriving From.

# use derive_more::From;
#
#[derive(Debug, From, PartialEq)]
struct Int(i32);

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

For structs that have multiple fields .into() needs to be called on a tuple containing the desired content for each field.

# use derive_more::From;
#
#[derive(Debug, From, PartialEq)]
struct Point(i32, i32);

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

To specify concrete types to derive convert from use #[from(<types>)].

# use std::borrow::Cow;
#
# use derive_more::From;
#
#[derive(Debug, From, PartialEq)]
#[from(Cow<'static, str>, String, &'static str)]
struct Str(Cow<'static, str>);

assert_eq!(Str("&str".into()), "&str".into());
assert_eq!(Str("String".into()), "String".to_owned().into());
assert_eq!(Str("Cow".into()), Cow::Borrowed("Cow").to_owned().into());

#[derive(Debug, From, PartialEq)]
#[from((i16, i16), (i32, i32))]
struct Point {
    x: i32,
    y: i32,
}

assert_eq!(Point { x: 1_i32, y: 2_i32 }, (1_i16, 2_i16).into());
assert_eq!(Point { x: 3_i32, y: 4_i32 }, (3_i32, 4_i32).into());

Also, you can forward implementation to the inner type, which means deriving From for any type, that derives From inner type.

# use std::borrow::Cow;
#
# use derive_more::From;
#
#[derive(Debug, From, PartialEq)]
#[from(forward)]
struct Str {
    inner: Cow<'static, str>,
}

assert_eq!(Str { inner: "&str".into() }, "&str".into());
assert_eq!(Str { inner: "String".into() }, "String".to_owned().into());
assert_eq!(Str { inner: "Cow".into() }, Cow::Borrowed("Cow").to_owned().into());

Enums

For enums .into() works for each variant as if they were structs. This includes specifying concrete types via #[from(<types>)] or forwarding implementation with #[from(forward)].

# use derive_more::From;
#
#[derive(Debug, From, PartialEq)]
enum IntOrPoint {
    Int(i32),
    Point {
        x: i32,
        y: i32,
    },
}

assert_eq!(IntOrPoint::Int(1), 1.into());
assert_eq!(IntOrPoint::Point { x: 1, y: 2 }, (1, 2).into());

By default, From is generated for every enum variant, but you can skip some variants via #[from(skip)] (or #[from(ignore)]) or only concrete fields via #[from].

# mod from {
# use derive_more::From;
#[derive(Debug, From, PartialEq)]
enum Int {
    #[from]
    Derived(i32),
    NotDerived(i32),
}
# }

// Is equivalent to:

# mod skip {
# use derive_more::From;
#[derive(Debug, From, PartialEq)]
enum Int {
    Derived(i32),
    #[from(skip)] // or #[from(ignore)]
    NotDerived(i32),
}
# }

Example usage

# use derive_more::From;
#
// Allow converting from i32
#[derive(From, PartialEq)]
struct MyInt(i32);

// Forward from call to the field, so allow converting
// from anything that can be converted into an i64 (so most integers)
#[derive(From, PartialEq)]
#[from(forward)]
struct MyInt64(i64);

// You can ignore a variant
#[derive(From, PartialEq)]
enum MyEnum {
    SmallInt(i32),
    NamedBigInt { int: i64 },
    #[from(ignore)]
    NoFromImpl(i64),
}

// Or explicitly annotate the ones you need
#[derive(From, PartialEq)]
enum MyEnum2 {
    #[from]
    SmallInt(i32),
    #[from]
    NamedBigInt { int: i64 },
    NoFromImpl(i64),
}

// And even specify additional conversions for them
#[derive(From, PartialEq)]
enum MyEnum3 {
    #[from(i8, i32)]
    SmallInt(i32),
    #[from(i16, i64)]
    NamedBigInt { int: i64 },
    NoFromImpl(i64),
}

assert!(MyInt(2) == 2.into());
assert!(MyInt64(6) == 6u8.into());
assert!(MyEnum::SmallInt(123) == 123i32.into());
assert!(MyEnum::SmallInt(123) != 123i64.into());
assert!(MyEnum::NamedBigInt{int: 123} == 123i64.into());
assert!(MyEnum3::SmallInt(123) == 123i8.into());
assert!(MyEnum3::NamedBigInt{int: 123} == 123i16.into());