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`From` and `Into`

src/std-traits/from-and-into.md

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<!-- Copyright 2023 Google LLC SPDX-License-Identifier: CC-BY-4.0 -->

From and Into

Types implement From and Into to facilitate type conversions. Unlike as, these traits correspond to lossless, infallible conversions.

rust,editable
# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn main() {
    let s = String::from("hello");
    let addr = std::net::Ipv4Addr::from([127, 0, 0, 1]);
    let one = i16::from(true);
    let bigger = i32::from(123_i16);
    println!("{s}, {addr}, {one}, {bigger}");
}

Into is automatically implemented when From is implemented:

rust,editable
# // Copyright 2023 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
fn main() {
    let s: String = "hello".into();
    let addr: std::net::Ipv4Addr = [127, 0, 0, 1].into();
    let one: i16 = true.into();
    let bigger: i32 = 123_i16.into();
    println!("{s}, {addr}, {one}, {bigger}");
}
<details>
  • That's why it is common to only implement From, as your type will get Into implementation too.
  • When declaring a function argument input type like "anything that can be converted into a String", the rule is opposite, you should use Into. Your function will accept types that implement From and those that only implement Into.
</details>