How do I tell Cargo to build files other than main.rs?

Put other.rs file into bin subfolder of src folder (./src/bin/other.rs). And run cargo build --bin other or cargo run --bin other

The Rust compiler compiles all the files at the same time to build a crate, which is either an executable or a library. To add files to your crate, add mod items to your crate root (here, main.rs) or to other modules:

mod boolean_example;
mod function_goodbye_world;
mod listdir;
mod looping;
mod pattern_match;
mod write_to_file;

To access items defined in another module from your crate root, you must qualify that item with the module name. For example, if you have a function named foo in module looping, you must refer to it as looping::foo.

You can also add use statements to import names in the module's scope. For example, if you add use looping::foo;, then you can just use foo to refer to looping::foo.

For more information, see Separating Modules into Different Files in The Rust Programming Language.