Compile Multiple Modules with RUSTC (Not Cargo)?


So here I am. On my way to learning the basics of Rust where it seems I already have a complaint/problem. I think the problem stems from the lack of my ability to google the answer (I even found a link to “How to compile cargo without cargo” as funny to me as that was), but lets see what the community has to say.

Often while I was learning c/c++ (I still am) I found it really easy to just type out some lines of code and send it to the compiler to give me something to run. EG:
clang++ foo.cpp -o bar

Now I have been using this same method with the Rust compiler to provide me the same quick feedback. I have no problem using cargo, but it seems like the program has a default for meduim/large dev projects in mind right out of the gate. I have no real reason to use cargo to compile a 500 line or less piece of code it seems. I still don’t use many dependencies for my small projects, but when I build a project the folder containing the project balloons to well over 200 mb. With good reason, sure. I just feel there is no reason at this time for what I am doing…

So now to my question.

How the heck to we just use rustc to compile multiple modules into something I can run and test. I just want to run the command, let rustc chew on my code a little bit, and have it spit out a file.

Also, does rustc still depend on the main function lying in a file called, or can I specify:
and have all the dependencies lie in…
Do I still need a in this case?

I understand the community really promoting cargo. I can see how useful it is, just not for me right now.

Any help would be appreciated.


If your crate root is called, you run rustc


It would seem that simple IMHO.

Now lets say I have an additional module called, in I add the line
use foo;

I get back an error from the compiler saying:
warning: unused import: foo
12 | use foo;
| ^^^
= note: #[warn(unused_imports)] on by default

Maybe I should have clarified that in my original question. I thought I did, but guess not.


Um, please take a closer look at that


So clearly I’m just importing the module incorrect then? Seems simple enough.

Guess I’ll hit the book once again. Must have missed something simple


To define a module, you have the following code:

mod foo;

and rustc will look for either:

├ //or


├ //or

There were some newer rules introduced in Rust 2018, but I can’t recall them, but either way the above will work


I guess to boil it down it is most simple form. Here is a screenshot of exactly where I am going wrong. I guess I just don’t understand for this evening. Rust certainly has me seriously qustioning my ability.


No, no you have it wrong, in you just put

mod filename;

And this will tell the compiler that the module is in a file (or directory) named and the contents of should look like

pub struct Understand { //...

Rustc will only compile the files it needs to, and will ignore all files it doesn’t have a mod filename for.
You could also choose to use an explicit mod like what you thought you were supposed to use in your sub-file in your image
Sorry for the lack of better formatting or examples, mobile isn’t very comfortable

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Oh my gosh. If that isn’t the supidest/most cleaver way of doing that syntax wise. I really appreciate your time OptimisticPeach. I think my main issue was, in my mind I was thinking of the file as a module. Not how the compiler would see it. This makes sense to me.


You can run cargo build --verbose to learn how cargo does it.

Btw, cargo is a pretty thin layer if you don’t use dependencies, so it’s fine even for 1-line projects IMHO.

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