gistfile1.txt

enum PyrsErr {
  InternalErr
}

...

pub fn init() -> Result<*mut Struct_ppyData, PyrsErr> {
  unsafe {
    return Ok(ppy_init());
  }
  return Err(InternalErr); // <-- Works fine
}

gistfile2.txt

enum PyrsErr {
  InternalErr
}

...

pub fn init() -> Result<*mut Struct_ppyData, PyrsErr> {
  unsafe {
    return Ok(ppy_init());
  }
  Err(InternalErr);  // <--- See error below
} 

gistfile3.txt

pyrs.rs:28:3: 28:19 error: cannot determine a type for this expression: unconstrained type
pyrs.rs:28   Err(InternalErr);
             ^~~~~~~~~~~~~~~~
error: aborting due to previous error