Can a package contain multiple binary crates? Binary crates are files with main() so does that mean it can have multiple mains

On this link:

I found this where it says :
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 a library or binary crate.

Previously it says that a binary crate is one that has a main.. does that mean i can have as many mains as i want? Please explain to me.

One package can have zero or one library crates and any number of binary crates. Each of these crates has a crate root, which is a file.

By default, cargo detects src/ as a library crate root. It detects src/ as a binary crate root, and will build it into an executable with the same name as the package. Cargo also automatically detects src/bin/*.rs as binary crate roots, and they will be compiled into executables with the same name as the .rs file. You can also use src/bin/*/ and it will use the directory name under bin as the executable name.

You can also edit Cargo.toml to use any file as a crate root ( lib | bin ).


got it. i understand. i am new to rust coming from python and c so things are on my nerves a little.
Hope it wasn't too stupid. lol
thank you very much. i appreciate it .

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I'm a newer to learn Rust and I am very confused about the crates concept after reading the official docs and asking the ChatGPT 3.5.

The information I got:

  • A binary crate must contain a main function.
  • A cargo package can contain at most only one library crate.
  • A crate may contain multiple modules.

The main questions:

  1. When I organize the modules with several .rs files in a cargo repo, are they library crates since there are no main functions in them?
  2. A crate may contain multiple modules, may a module contain a crate?
  3. What's the meaning of "A crate is the smallest amount of code that the Rust compiler considers at a time." Must a crate be compiled to an executable binary or a library binary file?

Please help me and thank you for any opinions!

Nope. A single file is not always a crate at all, some files (actually, most of them in real projects, I think) don't even make sense in isolation, since they require some other part of the crate they're in.

No. Each crate is essentially "main module + everything it (recursively) declares as submodules" (with mod foo; or mod foo { ... } items).

More or less, yes, but note the "library binary file" might mean different things depending on how exactly it was compiled. It might be a Rust library, meant only for the consumption by another compilation stages. as a simple Rust dependency; in this case, it might not contain the actual executable code - e.g. if everything inside is generic and will be codegenerated later, when the generic parameters are filled in. It might be an ordinary static or dynamic library, like in C, - in this case it will contain the executable code, since, well, that's what the consumer (some non-Rust code) will need.

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@Cerber-Ursi Thanks a lot for your kind reply!

However, I am still a little confused with the relationship of .rs files and crate. I don't know when .rs files with mod foo items become crates.

It seems that ChatGPT 3.5 thinks a .rs file is just a crate, I don't know if it is right:

Q: When I organize the modules with several .rs files in a cargo repo, are they library crates since there are no main functions in them?
ChatGPT: Yes, when you organize modules with several .rs files in a Cargo project and none of them contain a fn main() function, they are considered library crates. Each .rs file is treated as a module, and when compiled together, they form a library crate.
A library crate in Rust is a crate that doesn't have a fn main() function and is intended to be used as a library by other Rust code. Library crates provide reusable functionality that can be imported and used in other projects. So, if you have multiple .rs files organized as modules in your project, and none of them have a fn main() function, they are indeed library crates.

Please consider the following scenarios:

  • a. There are several .rs files with some mod foo items in them. For each .rs file, there are also some test functions with #[test] attribute in them. There is a src/ but no src/ file in this repo.
  • b. There are several .rs files with some mod foo items in them. For each .rs file, there is a main function in it with some test code, just like what I did in some other languages, like Python, which doesn't have a built-in test suite. There is a src/ but no src/ file in this repo.

How many crates are there in each scenario?

My answer:

  • For a, a binary crate (src/ with several library crates (other .rs files with mod foo in them).
  • For b, a main binary crate (src/ with several other binary crates (other .rs files with main functions).

BTW, it is strange to me if there is only a single create in scenario a and no other crates there because for scenario b it means that more binary crates may come up by just adding main functions in the .rs files.

Is my answer right or not? If there are any issues with my answer, please feel free to let me know and thank you for your patience.

When the root .rs file is passed as argument to rustc, this file and everything it imports as modules are together interpreted as a single crate. Normally, cargo calls rustc for you - in this case, the root file is either src/, src/ or something you've explicitly mentioned in Cargo.toml.

So, from the Rust language point of view, the question of "is this a crate?" is meaningless unless compiler is invoked. From the Rust infrastructure point of view (that is, when Cargo is involved) - the crate always begins with one of the files mentioned before.

According to this:

src/ is the root of binary crate. Every file imported by it, directly or indirectly, is a module in the same crate. Every file which is not imported isn't part of any crate at all.

By default - same thing: src/ is a root of binary crate, everything else is not in a crate. However, here you can explicitly ask for Cargo to treat some of these files as crates, by listing them in Cargo.toml. Then, there will be one binary crate per Cargo.toml entry, plus one for src/

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It's not, every sentence in the answer is incorrect to varying degrees except for "Library crates provide reusable functionality that can be imported and used in other projects". Most of the sentences are completely wrong. I wouldn't rely on it for any subject where you can't fact check the answers.

Source files, modules and crates are all on different levels of abstraction.

Source files contain the source code. The default extension for them is .rs but it's not strictly necessary.

# Cargo.toml
name = "some-package"
version = "0.1.0"
edition = "2021"

name = "some-bin"
path = "src/some-file"
// src/some-file
#[path = "non-rs-file"]
mod some_module;

fn main() {
// src/another-file
pub fn hi() {

This compiles and runs despite not having any .rs files. Of course, there's no point in doing this in this case, or the vast majority of other cases, and sticking with the default src/ and src/ without is best. But the point is that .rs files are just a convention/the default, not a fundamental requirement.

If source files are used to organise your code, modules are used to organise your functions and types. You don't need to define any modules in your code, but it's usually better to organise your functions and types into modules to make things more manageable, just like it's better to split your source code across multiple files. Usually, your modules will be structured similar to your files (mod foo corresponds to etc.) but this isn't required.

Crates are, basically, a collection of modules. Starting from the crate root (often src/ or src/, the crate comprises of all the modules that can be reached from the crate root. Unless you modify Cargo.toml to define other crates (like I did with [[bin]] in the earlier example), there's only the main binary crate from src/, library crate from src/, extra binary crates in src/bin (for example, a src/bin/ defines a foo binary crate), and integration test crates in tests (similar to bin, a tests/ defines a foo test crate).

Assuming a default Cargo.toml and no files in src/bin/ or tests, there's just one crate, with src/ being the crate root. It doesn't matter what other files there are or what their contents are.

Same as before, because the other files and their contents don't change how many crates there are (except for the specific exceptions listed).

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@Cerber-Ursi I see, thank you very much for your time and patience!

@Heliozoa Thank you for your code sample and patience! I learned more than just this from your answer.

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