Rust那些事之ToOwned trait
默认的Clone trait有两个问题：
只支持固定大小的类型
转换也只能从&T到T，不能够从&T到U的转换。
pub trait Clone: Sized那么如何实现呢？于是便有了ToOwned trait。
ToOwned内部有一个关联类型Owned，实现ToOwned要求其实现Borrow trait。
pub trait ToOwned {
type Owned: Borrow<Self>;
fn to_owned(&self) -> Self::Owned;
fn clone_into(&self, target: &mut Self::Owned) {
*target = self.to_owned();
}
}例如str的ToOwned实现如下，可以从&str变为String类型。
impl ToOwned for str {
type Owned = String;
#[inline]
fn to_owned(&self) -> String {
unsafe { String::from_utf8_unchecked(self.as_bytes().to_owned()) }
}
fn clone_into(&self, target: &mut String) {
let mut b = mem::take(target).into_bytes();
self.as_bytes().clone_into(&mut b);
*target = unsafe { String::from_utf8_unchecked(b) }
}
}ToOwned也有默认的实现，对于任意类型T必须要求其实现Clone trait，然后调用Clone的接口。即：默认实现等价于Clone的实现。
impl<T> ToOwned for T
where
T: Clone,
{
type Owned = T;
fn to_owned(&self) -> T {
self.clone()
}
fn clone_into(&self, target: &mut T) {
target.clone_from(self);
}
}往期回顾：
