Rust by Example

println!("Yes. It's definitely linux!");

} else {

println!("Yes. It's definitely *not* linux!");

}

}

הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

See also:

the reference, cfg!, and macros.

Custom

Some conditionals like target_os are implicitly provided by rustc, but custom conditionals must be passed to rustc using the --cfg flag.

#[cfg(some_condition)]

fn conditional_function() {

println!("condition met!");

}

fn main() {

conditional_function();

}

הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Try to run this to see what happens without the custom cfg flag.

With the custom cfg flag:

$ rustc --cfg some_condition custom.rs && ./custom

condition met!

Generics

Generics is the topic of generalizing types and functionalities to broader cases. This is extremely useful for reducing code duplication in many ways, but can call for rather involving syntax. Namely, being generic requires taking great care to specify over which types a generic type is actually considered valid. The simplest and most common use of generics is for type parameters.

A type parameter is specified as generic by the use of angle brackets and upper camel case: . "Generic type parameters" are typically represented as . In Rust, "generic" also describes anything that accepts one or more generic type parameters . Any type specified as a generic type parameter is generic, and everything else is concrete (non-generic).

For example, defining a generic function named foo that takes an argument T of any type:

fn foo(arg: T) { ... }

Because T has been specified as a generic type parameter using , it is considered generic when used here as (arg: T). This is the case even if T has previously been defined as a struct.

This example shows some of the syntax in action:

// A concrete type `A`.

struct A;

// In defining the type `Single`, the first use of `A` is not preceded by ``.

// Therefore, `Single` is a concrete type, and `A` is defined as above.

struct Single(A);

// ^ Here is `Single`s first use of the type `A`.