A crate is a compilation unit in Rust. Whenever rustc some_file.rs is called,
some_file.rs is treated as the crate file. If some_file.rs has mod
declarations in it, then the contents of the module files will get merged with
the crate file before running the compiler over it. In other words, modules
do not get compiled individually, only crates get compiled.
A crate can be compiled into a binary or into a library. By default, rustc
will produce a binary from a crate. This behavior can be overridden by passing
the --crate-type flag to rustc.