use Basics

Goal of This Episode

Learn to shorten long paths with use, and understand why we could use Option, Vec, and friends without any use before.

Concept

Rust ships with a great many built-in functions, types, and traits. To organize them, the standard library sorts things into modules. Every type has a full path describing which module it lives in, with segments separated by :: — like std::string::String (String lives in std’s string module), std::vec::Vec, std::fmt::Display. Normally, using a type means writing out its full path.

But strangely, we’ve been using Vec, String, Option, and Result all along without ever writing full paths like std::vec::Vec. Why?

Because Rust has a mechanism called the prelude — Rust imports the most commonly used functions, types, and traits into every file by default. Vec, String, Option, Result, Some, None, Ok, Err, plus common traits like Clone and Copy, are all in the prelude, so no full paths are needed.

But not everything is in the prelude. The trait std::fmt::Display, for instance, isn’t. To use it, you either write the full path — or bring it in with use.

The use Syntax

use std::fmt::Display;

fn main() {}

This line means: “Bring std::fmt::Display into the current scope; from now on, just write Display.”

use brings an existing name into the current scope, which lets you write a shorter path. Without use, you write std::fmt::Display; with it, just Display.

Example Code

use std::cmp::max;

fn main() {
    // Without use, the full path is needed:
    println!("The smaller is: {}", std::cmp::min(3, 7));

    // With use, just max will do:
    println!("The larger is: {}", max(3, 7));
    println!("The larger is: {}", max(10, -2));
}

std::cmp::max and std::cmp::min are standard-library functions returning the larger or smaller of two values. They’re not in the prelude, so it’s either the full path or a use.

Recap

• use std::fmt::Display; shortens the long path; afterward, just write Display.
• use brings an existing name into the current scope, which lets you write a shorter path.
• Rust’s compiler imports the prelude’s common types and traits by default (Vec, String, Option, Clone, etc.).
• Things outside the prelude (like Display) need the full path or a use.