Unresolved import in Rust. Can't find crate [duplicate]

Question

I have 4 files:

main.rs

mod bar;

fn main() {
    let v = vec![1, 2, 3];
    println!("Hello, world!");
}

lib.rs

pub mod foo;
pub mod bar;

foo.rs

pub fn say_foo() {

}

bar.rs

use crate::foo;

fn bar() {
    foo::say_foo();
}

When I run cargo run I get an error saying:

error[E0432]: unresolved import `crate::foo`
 --> src/bar.rs:1:5
  |
1 | use crate::foo;
  |     ^^^^^^^^^^ no `foo` in the root

Could someone explain to me how to fix this? A bit more broadly: how does module lookup work when there's a main.rs and a lib.rs?

Edit: Adding mod foo to main.rs fixes the issue. But I don't understand this -- I was under the impression the lib.rs was the place that "exposed" all of my modules? Why do I have to declare the module in main.rs as well?

My Cargo.toml:

[package]
name = "hello-world"
version = "0.1.0"
authors = ["[email protected]>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]

Answer 1

Let's start from the beginning. Look at the Package Layout chapter in The Cargo Book. As you can see, your package can contain lot of stuff:

• a binary (something you can run) or multiple binaries,
• a single library (shared code),
• example(s),
• benchmark(s),
• integration tests.

Package layout

Not all of the possibilities are listed here, just the binary / library combinations.

A binary

This is an example of a package with single binary. Entry point is the main function in the src/main.rs.

Cargo.toml:

[package]
name = "hallo"
version = "0.1.0"
edition = "2018"

src/main.rs:

fn main() {
    println!("Hallo, Rust here!")
}

$ cargo run
Hallo, Rust here!

A library

This is an example of a package with a library. Libraries don't have entry points, you can't run them. They're used for functionality sharing.

Cargo.toml:

[package]
name = "hallo"
version = "0.1.0"
edition = "2018"

src/lib.rs:

pub fn foo() {
    println!("Hallo, Rust library here!")
}

$ cargo run
error: a bin target must be available for `cargo run`

Do you see anything in the Cargo.toml file about a binary or a library? No. The reason is that I've followed the Package Layout and the cargo knows where to look for things.

A binary and a library

This is an example of a package with a binary and a library.

Cargo.toml:

[package]
name = "hallo"
version = "0.1.0"
edition = "2018"

src/lib.rs:

pub const GREETING: &'static str = "Hallo, Rust library here!";

src/main.rs:

use hallo::GREETING;

fn main() {
    println!("{}", GREETING);
}

Same question, do you see anything in the Cargo.toml file about a binary or a library? No.

This package contains two things:

• a binary (root src/main.rs, entry point src/main.rs::main),
• a library (root src/lib.rs, shared code).

A library can be referenced from the binary via use hallo::... where the hallo is this package name (Cargo.toml -> [package] -> name).

Your problem

Cargo.toml:

[package]
name = "hallo"
version = "0.1.0"
edition = "2018"

Same package layout

A library part

src/lib.rs:

pub mod bar;
pub mod foo;

src/foo.rs:

pub fn say_foo() {
    println!("Foo");
}

src/bar.rs:

use crate::foo;

pub fn bar() {
    foo::say_foo();
}

crate refers to src/lib.rs, because we're in the context of our library here.

Treat it as a standalone unit and refer to it via use hallo::...; from the outside world.

A binary part

src/main.rs:

use hallo::bar::bar;

fn main() {
    bar();
}

Here we're just using our library.

Without a library

Same code, but lib.rs was renamed to utils.rs and (foo|bar).rs files were moved to the src/utils/ folder.

src/utils.rs:

pub mod bar;
pub mod foo;

src/utils/foo.rs:

pub fn say_foo() {
    println!("Foo");
}

src/utils/bar.rs:

use super::foo;
// or use crate::utils::foo;

pub fn bar() {
    foo::say_foo();
}

We can use crate here as well, but because we're in the context of our binary, the path differs.

src/main.rs:

use utils::bar::bar;

mod utils;

fn main() {
    bar();
}

Here we just declared another module (utils) and we're using it.

Summary

Cargo.toml content:

[package]
name = "hallo"
version = "0.1.0"
edition = "2018"

If there's a src/main.rs file, you're basically saying this:

[package]
name = "hallo"
version = "0.1.0"
edition = "2018"

[[bin]]
name = "hallo"
src = "src/main.rs"

If there's a src/lib.rs file, you're basically saying this:

[package]
name = "hallo"
version = "0.1.0"
edition = "2018"

[lib]
name = "hallo"
path = "src/lib.rs"

If there're both of them, you're basically saying this:

[package]
name = "hallo"
version = "0.1.0"
edition = "2018"

[[bin]]
name = "hallo"
path = "src/main.rs"

[lib]
name = "hallo"
path = "src/lib.rs"

Documentation

• Package Layout
• The Manifest Format
• Managing Growing Projects with Packages, Crates, and Modules