Rust Book - Chapter 7.1

If you run rustc my_program.rs, the Rust compiler considers the file you passed to be a crate ^[1]

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  Fig. 1 - Structure of a Rust crate.
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[Crate] --> [Binary]
[Crate] --> [Library]


note left of Binary: programs that \ncan be compiled \nto an executable
note bottom of Library: no main function, \ncan't be compiled to an executable. \nDefines functionality \nintended to be shared with multiple programs
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We see from Fig. 1, that a crate can either a Library or a Binary crate.
A package can contain as many binary crates as you like, but at most only one library crate. A package must contain at least one crate, whether that's library or binary.

crate is used interchangeably with the concept of a library. When someone says crate, they usually mean library crate

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  Fig. 2 - Structure of a package
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package "Package" {
    [Cargo.toml]
    [Crate1]
    [Crate2]
    [CrateN]
}

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What happens when we create a new package?

When we use cargo to create a new project, we see that it created a binary application

$cargo new my-project
     Created binary (application) `my-project` package

~/projects/rust/the-book main ?14 >

Examining the content of Cargo.toml, we see that there's no mention of src/main.rs. Why? This is because Cargo follows a convention that src/main.rs is the crate root of a binary crate with the same name as the package. Cargo passes the crate root files to rustc to buld the binary.

Here, our package contains only 1 binary crate named my-project. If a package contains src/main.rs and src/lib.rs, it has two crates: a binary and a library, both with the same name as the package. A package can have multiple binary crates by placing files in the src/bin directory: each file will be a separate binary crate.

[package]
name = "my-project"
version = "0.1.0"
edition = "2021"

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

[dependencies]