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Restructure negative trait bounds section.

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LukeMathWalker 2024-05-24 12:43:34 +02:00
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book/src/04_traits/09_from.md
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@ -69,7 +69,20 @@ pub struct Foo<T: Sized>
} }
``` ```
You can opt out of this behavior by using a **negative trait bound**: In the case of `From<T>`, the trait definition is equivalent to:
```rust
pub trait From<T: Sized>: Sized {
fn from(value: T) -> Self;
}
```
In other words, _both_ `T` and the type implementing `From<T>` must be `Sized`, even
though the former bound is implicit.
### Negative trait bounds
You can opt out of the implicit `Sized` bound with a **negative trait bound**:
```rust ```rust
pub struct Foo<T: ?Sized> { pub struct Foo<T: ?Sized> {
@ -82,8 +95,6 @@ pub struct Foo<T: ?Sized> {
This syntax reads as "`T` may or may not be `Sized`", and it allows you to This syntax reads as "`T` may or may not be `Sized`", and it allows you to
bind `T` to a DST (e.g. `Foo<str>`). It is a special case, though: negative trait bounds are exclusive to `Sized`, bind `T` to a DST (e.g. `Foo<str>`). It is a special case, though: negative trait bounds are exclusive to `Sized`,
you can't use them with other traits. you can't use them with other traits.
In the case of `From<T>`, we want _both_ `T` and the type implementing `From<T>` to be `Sized`, even
though the former bound is implicit.
## `&str` to `String` ## `&str` to `String`