What #[derive(Into)] generates
This derive creates the the exact oposite 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.
1 Example usage
// Allow converting into i32 #[derive(Into, PartialEq)] struct MyInt(i32); // Additionally convert refs to the inner type refs #[derive(Into, PartialEq)] #[into(owned, ref, ref_mut)] struct MyInt64(i64); // Specify additional conversions #[derive(Into, PartialEq)] #[into(types(i16, i32))] struct MyInt8(i8); // Even for ref types #[derive(Into, PartialEq)] #[into(owned, ref(types(i64)))] struct MyInt64Wrapped(MyInt64); fn main() { assert!(i32::from(MyInt(2)) == 2i32); assert!(i64::from(MyInt64(6)) == 6i64); assert!(<&i64>::from(&MyInt64(6)) == &6i64); assert!(<&mut i64>::from(&mut MyInt64(6)) == &mut 6i64); assert!(i8::from(MyInt8(7)) == 7i8); assert!(i16::from(MyInt8(7)) == 7i16); assert!(i32::from(MyInt8(7)) == 7i32); assert!(MyInt64::from(MyInt64Wrapped(MyInt64(1))) == MyInt64(1)); assert!(<&MyInt64>::from(&MyInt64Wrapped(MyInt64(1))) == &MyInt64(1)); assert!(<&i64>::from(&MyInt64Wrapped(MyInt64(1))) == &1i64); }
2 Tuple structs
When deriving Into
for a tuple struct with a single field (i.e. a newtype) like this:
#[derive(Into)] struct MyInt(i32);
Code like this will be generated:
impl ::core::convert::From<MyInt> for (i32) { fn from(original: MyInt) -> (i32) { (original.0) } }
The behaviour is a bit different when deriving for a struct with multiple fields, since it returns a tuple. For instance when deriving for a tuple struct with two fields like this:
#[derive(Into)] struct MyInts(i32, i32);
Code like this will be generated:
impl ::core::convert::From<MyInts> for (i32, i32) { fn from(original: MyInts) -> (i32, i32) { (original.0, original.1) } }
3 Regular structs
For regular structs almost the same code is generated as for tuple structs except in the way the field values are assigned to the new struct. When deriving for a regular struct with a single field like this:
#[derive(Into)] struct Point1D { x: i32, }
Code like this will be generated:
impl ::core::convert::From<Point1D> for (i32) { fn from(original: Point1D) -> (i32) { (original.x) } }
The behaviour is again a bit different when deriving for a struct with multiple fields, because this also returns a tuple. For instance when deriving for a tuple struct with two fields like this:
#[derive(Into)] struct Point2D { x: i32, y: i32, }
Code like this will be generated:
impl ::core::convert::From<Point2D> for (i32, i32) { fn from(original: Point2D) -> (i32, i32) { (original.x, original.y) } }
4 Enums
Deriving Into
for enums is not supported as it would not always be successful.
This is what the currently unstable
TryInto
should be
used for, which is currently not supported by this library.