lib.rs

trait ArgsExtensionMethods {
    fn getOutputFilename(self) -> Option<String>;
//    fn removeOutputFile(&self) -> Args;
//    fn removeNoStartFiles(&self) -> Args;
//    fn removeWlSwitches(&self) -> Args;
}
//
impl<T> ArgsExtensionMethods for T where T: IntoIterator<Item=String> {
    fn getOutputFilename(self) -> Option<String> {
        self.skip_while(|ref el| *el != "-o").next()
    }
}

pub fn libMain<T>(args: T) where T: IntoIterator<Item=String>
{
}

#[cfg(test)]
mod tests;

--

error: no method named `skip_while` found for type `T` in the current scope
  --> src/lib.rs:17:14
   |
17 |         self.skip_while(|ref el| *el != "-o").next()
   |              ^^^^^^^^^^
   |
   = note: the method `skip_while` exists but the following trait bounds were not satisfied: `T : std::iter::Iterator`
   = help: items from traits can only be used if the trait is implemented and in scope; the following trait defines an item `skip_while`, perhaps you need to implement it:
   = help: candidate #1: `std::iter::Iterator`

main.rs

use std::env::Args;

extern crate xargo_lld_wrapper;

fn main() {
    lib::libMain(std::env::args());
}

rustc output

Working config: lib.rs/test.rs/app.rs

/Users/bRad/.cargo/bin/cargo test --color=always
   Compiling xargo-lld-wrapper v0.1.0 (file:///Users/bRad/Development/bg/experiments/rust/rpi3/xargo-lld-wrapper)
warning: unused variable: `args`
  --> src/lib.rs:23:16
   |
23 | pub fn libMain(args: Args)
   |                ^^^^
   |
   = note: #[warn(unused_variables)] on by default

warning: unused import: `super::*;`
 --> src/tests.rs:1:5
  |
1 | use super::*;
  |     ^^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

    Finished dev [unoptimized + debuginfo] target(s) in 1.54 secs
     Running target/debug/deps/xargo_lld_wrapper-dc5583a5b836a9c8

running 1 test
test tests::app_call_with_no_params_succeeds ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

   Doc-tests xargo-lld-wrapper

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured


Process finished with exit code 0

rustc output 2

Exactly same project, renaming 
Renaming app.rs -> main.rs:

/Users/bRad/.cargo/bin/cargo test --color=always
   Compiling xargo-lld-wrapper v0.1.0 (file:///Users/bRad/Development/bg/experiments/rust/rpi3/xargo-lld-wrapper)
error: cannot declare a new module at this location
  --> src/lib.rs:28:5
   |
28 | mod tests;
   |     ^^^^^
   |
note: maybe move this module `src` to its own directory via `src/mod.rs`
  --> src/lib.rs:28:5
   |
28 | mod tests;
   |     ^^^^^
note: ... or maybe `use` the module `tests` instead of possibly redeclaring it
  --> src/lib.rs:28:5
   |
28 | mod tests;
   |     ^^^^^

error: aborting due to previous error

error: Could not compile `xargo-lld-wrapper`.

To learn more, run the command again with --verbose.

Process finished with exit code 101