Modules and Crates

Organize code with modules, paths, and crate visibility. Results appear in the same fence: same-line // comments when short, multiline // blocks below the sample when not.

mod Tree

Declare modules; files map to paths.

mod net {
    pub fn ping() -> &'static str { "pong" }
}
net::ping()  // "pong"

use Paths

Bring paths into scope; rename with as.

use std::collections::HashMap as Map;
let m: Map<i32, i32> = Map::new();
m.len()  // 0

pub Visibility

pub, pub(crate), pub(super) control access.

mod a {
    pub(crate) fn f() -> i32 { 1 }
}
a::f()  // 1

crate vs super

crate:: from root; super:: parent module.

mod outer {
    pub fn x() -> i32 { 2 }
    pub mod inner {
        pub fn y() -> i32 { super::x() }
    }
}
outer::inner::y()  // 2

Re-export pub use

Expose child items at a higher path.

mod raw { pub fn f() -> i32 { 3 } }
pub use raw::f;
f()  // 3

Inline Module File

mod foo; loads foo.rs or foo/mod.rs - structure only note.

// src/lib.rs: mod foo;
// src/foo.rs: pub fn f() {}

External Crate use

Dependencies from Cargo.toml via extern crate paths.

// use serde::Serialize;
// after: serde = "1" in Cargo.toml

Prelude Style

Re-export common items in a prelude module.

mod prelude {
    pub use std::collections::HashMap;
}
use prelude::*;
HashMap::<u8, u8>::new().len()  // 0

cfg Conditional

Compile items only on some targets/features.

#[cfg(test)]
fn only_test() -> i32 { 1 }
// available under cargo test

Cargo Features Note

Optional deps gated by features in Cargo.toml.

// [features]
// full = ["dep:serde"]
// enable with: cargo build --features full

Workspace Note

Multi-crate repos use [workspace] members.

// [workspace]
// members = ["crates/*"]

lib vs bin

lib.rs library root; main.rs binary root.

// src/lib.rs exports; src/main.rs calls the lib

Path Attribute

#[path = "..."] overrides module file path - use sparingly.

// #[path = "alt.rs"]
// mod m;

Glob use

use module::* imports public names - keep limited.

mod m { pub const X: i32 = 1; }
use m::*;
X  // 1

Self in use

use self::child::Item relative to current module.

mod child { pub const N: i32 = 9; }
use self::child::N;
N  // 9

Related

• Cargo and Testing - cargo commands and tests
• Syntax and Basics - items inside modules
• Traits and Generics - re-export traits

External Related Links

• The Cargo Book - Official guide and reference for Cargo.
• crates.io - Official crate registry.
• Edition Guide - Differences between Rust editions.
• The Rust Programming Language, 2nd Edition (Amazon) - Steve Klabnik & Carol Nichols.
• Rust in Action (Manning) - Tim McNamara.

Stack versions: Rust 1.97.0 · edition 2024 · Tokio 1.x · serde 1.0