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/// Borrow from self.
///
/// This works similarly to [`Borrow`][::std::borrow::Borrow] but allows
/// borrowing compoundly from `&self` by defining a [generic `Target`]. This
/// means it's not just limited to returning an immediate reference to the
/// borrowed value but can return something which receives lifetime parameters
/// that borrows from self. This is called "compound borrowing" because it lets
/// you return types which contains compound references.
///
/// It is recommended that you use [`borrow`][crate::borrow()] instead of
/// importing this trait.
///
/// <br>
///
/// # What about `std::borrow::Borrow`?
///
/// The [`Borrow`][::std::borrow::Borrow] trait as defined can't perform
/// compound borrows from `&self`. Because the `borrow` method immediately
/// returns *a reference* to the borrowed type.
///
/// ```
/// trait Borrow<Borrowed: ?Sized> {
/// fn borrow(&self) -> &Borrowed;
/// }
/// ```
///
/// This means that there is no way to implement something like
/// `Borrow<Word<'a>>` because it's required that we return a reference which
/// doesn't outlive `'a`, something that can't be satisfied from the call to
/// `&self`.
///
/// ```compile_fail
/// # use std::borrow::Borrow;
/// struct Word<'a>(&'a str);
/// struct OwnedWord(String);
///
/// impl<'a> Borrow<Word<'a>> for OwnedWord {
/// fn borrow(&self) -> &Word<'a> {
/// &Word(self.0.as_str())
/// }
/// }
/// ```
///
/// ```text
/// error[E0515]: cannot return reference to temporary value
/// --> src/borrow.rs:37:9
/// |
/// 9 | &Word(self.0.as_str())
/// | ^---------------------
/// | ||
/// | |temporary value created here
/// | returns a reference to data owned by the current function
/// ```
///
/// The solution implemented in this crate is to use a [generic `Target`], with
/// which we can implement `borrow` like this:
///
/// ```
/// # struct Word<'a>(&'a str);
/// # struct OwnedWord(String);
/// use borrowme::Borrow;
///
/// impl Borrow for OwnedWord {
/// type Target<'a> = Word<'a>;
///
/// fn borrow(&self) -> Self::Target<'_> {
/// Word(self.0.as_str())
/// }
/// }
/// ```
///
/// A catch here is that `Borrow` can only be implemented once for each time,
/// compared to [`Borrow<T>`][::std::borrow::Borrow]. But for our purposes this
/// is fine. This crate is primarily intended to work with two symmetrical types
/// and any deviation from that pattern can be handled by customizing the
/// behavior of the [`#[borrowme]`][crate::borrowme] attribute.
///
/// [generic `Target`]: https://blog.rust-lang.org/2022/10/28/gats-stabilization.html