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# 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`.
```rust
# 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.
```rust
# 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>)]`.
```rust
# 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.
```rust
# 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)]`.
```rust
# 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]`.
```rust
# 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
```rust
# 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());
```
|
