# Crates and source files

> **<sup>Syntax</sup>**\
> _Crate_ :\
> &nbsp;&nbsp; [_InnerAttribute_]<sup>\*</sup>\
> &nbsp;&nbsp; [_Item_]<sup>\*</sup>

> Note: Although Rust, like any other language, can be implemented by an
> interpreter as well as a compiler, the only existing implementation is a
> compiler, and the language has always been designed to be compiled. For these
> reasons, this section assumes a compiler.

Rust's semantics obey a *phase distinction* between compile-time and
run-time.[^phase-distinction] Semantic rules that have a *static
interpretation* govern the success or failure of compilation, while
semantic rules that have a *dynamic interpretation* govern the behavior of the
program at run-time.

The compilation model centers on artifacts called _crates_. Each compilation
processes a single crate in source form, and if successful, produces a single
crate in binary form: either an executable or some sort of
library.[^cratesourcefile]

A _crate_ is a unit of compilation and linking, as well as versioning,
distribution, and runtime loading. A crate contains a _tree_ of nested
[module] scopes. The top level of this tree is a module that is
anonymous (from the point of view of paths within the module) and any item
within a crate has a canonical [module path] denoting its location
within the crate's module tree.

The Rust compiler is always invoked with a single source file as input, and
always produces a single output crate. The processing of that source file may
result in other source files being loaded as modules. Source files have the
extension `.rs`.

A Rust source file describes a module, the name and location of which &mdash;
in the module tree of the current crate &mdash; are defined from outside the
source file: either by an explicit [_Module_][module] item in a referencing
source file, or by the name of the crate itself. Every source file is a
module, but not every module needs its own source file: [module
definitions][module] can be nested within one file.

Each source file contains a sequence of zero or more [_Item_] definitions, and
may optionally begin with any number of [attributes]
that apply to the containing module, most of which influence the behavior of
the compiler. The anonymous crate module can have additional attributes that
apply to the crate as a whole.

> **Note**: The file's contents may be preceded by a [shebang].

```rust
// Specify the crate name.
#![crate_name = "projx"]

// Specify the type of output artifact.
#![crate_type = "lib"]

// Turn on a warning.
// This can be done in any module, not just the anonymous crate module.
#![warn(non_camel_case_types)]
```

## Main Functions

A crate that contains a `main` [function] can be compiled to an executable. If a
`main` function is present, it must take no arguments, must not declare any
[trait or lifetime bounds], must not have any [where clauses], and its return
type must implement the [`Termination`] trait.

```rust
fn main() {}
```
```rust
fn main() -> ! {
    std::process::exit(0);
}
```
```rust
fn main() -> impl std::process::Termination {
    std::process::ExitCode::SUCCESS
}
```

The `main` function may be an import, e.g. from an external crate or from the current one.

```rust
mod foo {
    pub fn bar() {
        println!("Hello, world!");
    }
}
use foo::bar as main;
```

> **Note**: Types with implementations of [`Termination`] in the standard library include:
>
> * `()`
> * [`!`]
> * [`Infallible`]
> * [`ExitCode`]
> * `Result<T, E> where T: Termination, E: Debug`

<!-- If the previous section needs updating (from "must take no arguments"
  onwards, also update it in the testing.md file -->

### The `no_main` attribute

The *`no_main` [attribute]* may be applied at the crate level to disable
emitting the `main` symbol for an executable binary. This is useful when some
other object being linked to defines `main`.

## The `crate_name` attribute

The *`crate_name` [attribute]* may be applied at the crate level to specify the
name of the crate with the [_MetaNameValueStr_] syntax.

```rust
#![crate_name = "mycrate"]
```

The crate name must not be empty, and must only contain [Unicode alphanumeric]
or `_` (U+005F) characters.

[^phase-distinction]: This distinction would also exist in an interpreter.
    Static checks like syntactic analysis, type checking, and lints should
    happen before the program is executed regardless of when it is executed.

[^cratesourcefile]: A crate is somewhat analogous to an *assembly* in the
    ECMA-335 CLI model, a *library* in the SML/NJ Compilation Manager, a *unit*
    in the Owens and Flatt module system, or a *configuration* in Mesa.

[Unicode alphanumeric]: char::is_alphanumeric
[`!`]: types/never.md
[_InnerAttribute_]: attributes.md
[_Item_]: items.md
[_MetaNameValueStr_]: attributes.md#meta-item-attribute-syntax
[`ExitCode`]: std::process::ExitCode
[`Infallible`]: std::convert::Infallible
[`Termination`]: std::process::Termination
[attribute]: attributes.md
[attributes]: attributes.md
[function]: items/functions.md
[module]: items/modules.md
[module path]: paths.md
[shebang]: input-format.md#shebang-removal
[trait or lifetime bounds]: trait-bounds.md
[where clauses]: items/generics.md#where-clauses

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