# borrowme
[
](https://github.com/udoprog/borrowme)
[
](https://crates.io/crates/borrowme)
[
](https://docs.rs/borrowme)
[
](https://github.com/udoprog/borrowme/actions?query=branch%3Amain)
The missing compound borrowing for Rust.
Rust comes with two sibling traits which that can convert from owned to
borrowed: [`ToOwned`][std-to-owned], [`Borrow`][std-borrow] and
[`BorrowMut`][std-borrow-mut].
These can convert most simple types such as `&str` to and from `String`. But
lets think of this in a broader perspective. How to we convert a type that
*has lifetimes*, to one which *does not*? This crate defines its own
[`ToOwned`], [`Borrow`] and [`BorrowMut`] traits which serve a similar
purpose to the ones in `std` but are implemented so that they can do this
not only for simple references but also for *compound types* which receives
lifetimes.
To help us implement these traits the [`#[borrowme]`][borrowme] attribute
macro is provided ([see this section][borrowme-derive] for why it's not a
derive).
```rust
#[borrowme]
#[derive(Clone)]
#[borrowed_attr(derive(Copy))]
struct Word<'a> {
text: &'a str,
}
```
From this we get the following types and implementations:
```rust
#[derive(Clone, Copy)]
struct Word<'a> {
text: &'a str,
}
#[derive(Clone)]
struct OwnedWord {
text: String,
}
impl borrowme::ToOwned for Word<'_> {
type Owned = OwnedWord;
fn to_owned(&self) -> OwnedWord {
/* .. */
}
}
impl borrowme::Borrow for OwnedWord {
type Target<'a> = Word<'a>;
fn borrow(&self) -> Word<'_> {
/* .. */
}
}
```
By itself this isn't much, but here's the big trick. Types using this crate
can be composed and converted into their borrowed or owned counterparts as
needed:
```rust
use std::collections::HashMap;
#[borrowme]
struct Word<'a> {
text: &'a str,
}
#[borrowme]
struct Dictionary<'a> {
words: HashMap<&'a str, Word<'a>>,
}
let dictionary = Dictionary {
/* .. */
};
let owned_dictionary: OwnedDictionary = borrowme::to_owned(&dictionary);
let dictionary2: Dictionary<'_> = borrowme::borrow(&owned_dictionary);
```
[`Borrow`]: https://docs.rs/borrowme/latest/borrowme/trait.Borrow.html
[`BorrowMut`]: https://docs.rs/borrowme/latest/borrowme/trait.BorrowMut.html
[`ToOwned`]: https://docs.rs/borrowme/latest/borrowme/trait.ToOwned.html
[borrowme-derive]: https://docs.rs/borrowme/latest/borrowme/attr.borrowme.html#why-isnt-this-a-derive
[borrowme]: https://docs.rs/borrowme/latest/borrowme/attr.borrowme.html
[generic associated types]: https://blog.rust-lang.org/2022/10/28/gats-stabilization.html
[std-borrow-mut]: https://doc.rust-lang.org/std/borrow/trait.BorrowMut.html
[std-borrow]: https://doc.rust-lang.org/std/borrow/trait.Borrow.html
[std-to-owned]: https://doc.rust-lang.org/std/borrow/trait.ToOwned.html