New keywords
Summary
dynis a strict keyword, in 2015 it is a weak keyword.asyncandawaitare strict keywords.tryis a reserved keyword.
Motivation
dyn Trait for trait objects
The dyn Trait feature is the new syntax for using trait objects. In short:
Box<Trait>becomesBox<dyn Trait>&Traitand&mut Traitbecome&dyn Traitand&mut dyn Trait
And so on. In code:
#![allow(unused)] fn main() { 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.
In the new edition, you should therefore prefer dyn Trait to just Trait
where you need a trait object.
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.
try keyword
The try keyword is reserved for use in try blocks, which have not (as of this writing) been stabilized (tracking issue)