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# New keywords
## Summary
- `dyn` is a [strict keyword][strict], in 2015 it is a [weak keyword].
- `async` and `await` are [strict keywords][strict].
- `try` is a [reserved keyword].
[strict]: https://doc.rust-lang.org/reference/keywords.html#strict-keywords
[weak keyword]: https://doc.rust-lang.org/reference/keywords.html#weak-keywords
[reserved keyword]: https://doc.rust-lang.org/reference/keywords.html#reserved-keywords
## Motivation
### `dyn Trait` for trait objects
The `dyn Trait` feature is the new syntax for using trait objects. In short:
* `Box<Trait>` becomes `Box<dyn Trait>`
* `&Trait` and `&mut Trait` become `&dyn Trait` and `&mut dyn Trait`
And so on. In code:
```rust
trait Trait {}
impl Trait for i32 {}
// old
fn function1() -> Box<Trait> {
# unimplemented!()
}
// new
fn function2() -> Box<dyn Trait> {
# unimplemented!()
}
```
That's it!
#### Why?
Using just the trait name for trait objects turned out to be a bad decision.
The current syntax is often ambiguous and confusing, even to veterans,
and favors a feature that is not more frequently used than its alternatives,
is sometimes slower, and often cannot be used at all when its alternatives can.
Furthermore, with `impl Trait` arriving, "`impl Trait` vs `dyn Trait`" is much
more symmetric, and therefore a bit nicer, than "`impl Trait` vs `Trait`".
`impl Trait` is explained [here][impl-trait].
In the new edition, you should therefore prefer `dyn Trait` to just `Trait`
where you need a trait object.
[impl-trait]: ../../rust-by-example/trait/impl_trait.html
### `async` and `await`
These keywords are reserved to implement the async-await feature of Rust, which was ultimately [released to stable in 1.39.0](https://blog.rust-lang.org/2019/11/07/Async-await-stable.html).
### `try` keyword
The `try` keyword is reserved for use in `try` blocks, which have not (as of this writing) been stabilized ([tracking issue](https://github.com/rust-lang/rust/issues/31436))
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