What is raw identifier in rust?

April 21, 2022 · 2 min read

Maintainer of inspektor

Every programming language has a set of keywords that are only used for certain things. In rust, for example, the keyword for is used to represent looping.

Because keywords have meaning in programming languages, they cannot be used to name a function or variable. for example, the keywords for or in cannot be used as variable names.

Although keywords are not intended to be used to name variables, you can do so in rust by using a raw identifier.

The program below will not compile in rust because in is a reserved keyword.


#[derive(Debug)]
struct Test{
    in: String
}

fn main() {
    let a = Test{
        in: "sadf".to_string()
    };
    println!("{:?}", a);
}

output:

error: expected identifier, found keyword `in`
 --> src/main.rs:4:5
  |
4 |     in: String
  |     ^^ expected identifier, found keyword
  |
help: you can escape reserved keywords to use them as identifiers
  |
4 |     r#in: String
  |     ~~~~

error: expected identifier, found keyword `in`
 --> src/main.rs:9:9
  |
9 |         in: "sadf".to_string()
  |         ^^ expected identifier, found keyword
  |
help: you can escape reserved keywords to use them as identifiers
  |
9 |         r#in: "sadf".to_string()
  |         ~~~~

However, we can make the program work by prefixing the keyword with r#.

r# tells the compiler that the incoming token is an identifier rather than a keyword.


#[derive(Debug)]
struct Test{
    r#in: String
}

fn main() {
    let a = Test{
        r#in: "sadf".to_string()
    };
    println!("{:?}", a);
}

output:

Test { in: "sadf" }

It's very useful for rust because it allows rust to introduce new keywords.

Assume we have a crate built with the 2015 rust edition that exposes the identifier try. Later, try was reserved for a feature in the 2018 edition. As a result, we must use a raw identifier to call try

reference