syntax - What is type ascription?

Question

Several times I've used the wrong syntax, such as forgetting to use let in this example:

let closure_annotated = |value: i32| -> i32 {
    temp: i32 = fun(5i32);
    temp + value + 1
};

error[E0658]: type ascription is experimental (see issue #23416)
 --> src/main.rs:3:9
  |
3 |         temp: i32 = fun(5i32);
  |         ^^^^^^^^^

I know that this problem is solved by using let, but what is "type ascription" and what is its use?

I found issue #23416 and the feature gate for type ascription, but I could not understand what "type ascription" is or what is its purpose.

Answer 1

Type ascription is the ability to annotate an expression with the type we want it to have. Type ascription in Rust is described in RFC 803.

In some situations, the type of an expression can be ambiguous. For example, this code:

fn main() {
    println!("{:?}", "hello".chars().collect());
}

gives the following error:

error[E0283]: type annotations required: cannot resolve `_: std::iter::FromIterator<char>`
 --> src/main.rs:2:38
  |
2 |     println!("{:?}", "hello".chars().collect());
  |                                      ^^^^^^^

That's because the collect method can return any type that implements the FromIterator trait for the iterator's Item type. With type ascription, one could write:

#![feature(type_ascription)]

fn main() {
    println!("{:?}", "hello".chars().collect(): Vec<char>);
}

Instead of the current (as of Rust 1.33) ways of disambiguating this expression:

fn main() {
    println!("{:?}", "hello".chars().collect::<Vec<char>>());
}

or:

fn main() {
    let vec: Vec<char> = "hello".chars().collect();
    println!("{:?}", vec);
}