This is the very minimum required to use the module_name scope:

use my_crate::module_name; 

Here, you are telling the compiler that module_name exists and that, as long as the function name exists in the symbols, it can be used. However, in order to use module_name, the function will need to be preceded by module_name. For example, to use the print_me(f32) function, which exists in module_name, you will have the following:

let some_text = module_name::print_me(10.1f32);   

The module_name:: has to be added to tell the compiler to use the module_name scope rather than the current scope of the application.