# Crates and source files

> Syntax
> Crate :
> UTF8BOM?
> SHEBANG?
> InnerAttribute*
> Item*

> Lexer
> UTF8BOM : `\uFEFF`
> SHEBANG : `#!` ~[`[` `\n`] ~`\n`*

> 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.1 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.2

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 — in the module tree of the current crate — are defined from outside the source file: either by an explicit `mod` 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 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.

    # #![allow(unused_variables)]
    #fn main() {
    // 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)]
    #}

The optional UTF8 byte order mark (UTF8BOM production) indicates that the file is encoded in UTF8. It can only occur at the beginning of the file and is ignored by the compiler.

A source file can have a shebang (SHEBANG production), which indicates to the operating system what program to use to execute this file. It serves essentially to treat the source file as an executable script. The shebang can only occur at the beginning of the file (but after the optional UTF8BOM). It is ignored by the compiler. For example:

    #!/usr/bin/env rustx

    fn main() {
        println!("Hello!");
    }

## Preludes and `no_std`

All crates have a prelude that automatically inserts names from a specific module, the prelude module, into scope of each module and an `extern crate` into the crate root module. By default, the standard prelude is used. The linked crate is `std` and the prelude module is `std::prelude::v1`.

The prelude can be changed to the core prelude by using the `no_std` attribute on the root crate module. The linked crate is `core` and the prelude module is `core::prelude::v1`. Using the core prelude over the standard prelude is useful when either the crate is targeting a platform that does not support the standard library or is purposefully not using the capabilities of the standard library. Those capabilities are mainly dynamic memory allocation (e.g. `Box` and `Vec`) and file and network capabilities (e.g. `std::fs` and `std::io`).

Warning: Using `no_std` does not prevent the standard library from being linked in. It is still valid to put `extern crate std;` into the crate and dependencies can also link it in.

## 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 be one of the following:

- `()`
- `Result<(), E> where E: Error`

> Note: The implementation of which return types are allowed is determined by the unstable `Termination` trait.

1

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.

2

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.