Trait std::convert::Into

1.0.0 · source · [−]
pub trait Into<T> {
    const fn into(self) -> T;
}
Expand description

A value-to-value conversion that consumes the input value. The opposite of From.

One should avoid implementing Into and implement From instead. Implementing From automatically provides one with an implementation of Into thanks to the blanket implementation in the standard library.

Prefer using Into over From when specifying trait bounds on a generic function to ensure that types that only implement Into can be used as well.

Note: This trait must not fail. If the conversion can fail, use TryInto.

Generic Implementations

  • From<T> for U implies Into<U> for T
  • Into is reflexive, which means that Into<T> for T is implemented

Implementing Into for conversions to external types in old versions of Rust

Prior to Rust 1.41, if the destination type was not part of the current crate then you couldn’t implement From directly. For example, take this code:

struct Wrapper<T>(Vec<T>);
impl<T> From<Wrapper<T>> for Vec<T> {
    fn from(w: Wrapper<T>) -> Vec<T> {
        w.0
    }
}
Run

This will fail to compile in older versions of the language because Rust’s orphaning rules used to be a little bit more strict. To bypass this, you could implement Into directly:

struct Wrapper<T>(Vec<T>);
impl<T> Into<Vec<T>> for Wrapper<T> {
    fn into(self) -> Vec<T> {
        self.0
    }
}
Run

It is important to understand that Into does not provide a From implementation (as From does with Into). Therefore, you should always try to implement From and then fall back to Into if From can’t be implemented.

Examples

String implements Into<Vec<u8>>:

In order to express that we want a generic function to take all arguments that can be converted to a specified type T, we can use a trait bound of Into<T>. For example: The function is_hello takes all arguments that can be converted into a Vec<u8>.

fn is_hello<T: Into<Vec<u8>>>(s: T) {
   let bytes = b"hello".to_vec();
   assert_eq!(bytes, s.into());
}

let s = "hello".to_string();
is_hello(s);
Run

Required Methods

Converts this type into the (usually inferred) input type.

Implementors