How to build a Rust binary executable - rust

Question

I am trying to build a single Rust binary executable. In the src directory I have four files:
main.rs:
use fasta_multiple_cmp::get_filenames;
fn main() {
get_filenames();
}
mod.rs:
pub mod fasta_multiple_cmp;
pub mod build_sequences_matrix;
fasta_multiple_cmp.rs:
pub mod fasta_multiple_cmp {
...
pub fn get_filenames() {
let args: Vec<String> = env::args().collect();
...
build_sequences_matrix.rs:
pub mod build_sequences_matrix {
use simple_matrix::Matrix;
...
Cargo told me:
src/main.rs:3:5
|
3 | use fasta_multiple_cmp::get_filenames;
| ^^^^^^^^^^^^^^^^^^ use of undeclared crate or module `fasta_multiple_cmp
I believed I understood some little things, but ther I'm lost. What is going on?
Thaks for any hint!

Answer 1

Your original code wasn't working because Rust's use is looking for a crate to import from, not just a file in the same directory.
If you want to separate logic into other files, you will need to break apart your project into both a binary and into a library (aka - a crate that your binary can use for imports).
This can be done in your Cargo.toml as:
[package]
edition = "2018"
name = "my_project"
version = "1.2.3"
... your other settings ...
[[bin]]
edition = "2018"
name = "whatever_name_for_binary"
path = "src/main.rs"
This means that your binary will be compiled into a file called "whatever_name_for_binary" in your target folder. It also means your non-binary files act as a library, and can be imported by your binary by using use my_project::some_module;
Because your project will now contain both a library and binary, you will need to specify when you want to run/compile the binary.
You do this by running: cargo run --bin whatever_name_for_binary
For more information, see the Rust Book's Chapter on Modules.