Thanks clippy!

src/main.rs

@@ -52,7 +52,7 @@ fn main() {
         std::process::exit(1);
     }
 
-    if matches.free.len() == 0 {
+    if matches.free.is_empty() {
         println!("Missing operand: INPUT OUTPUT");
         println!("Try `{} --help` for more information.", program);
         std::process::exit(1);
@@ -62,16 +62,16 @@ fn main() {
         let input = &matches.free[0];
         let output = &matches.free[1];
 
-        compress(&input, &output);
+        compress(input, output);
     } else if matches.opt_present("d") {
         let input = &matches.free[0];
         let output = &matches.free[1];
 
-        extract(&input, &output)
+        extract(input, output)
     }
 }
 
-fn compress(input: &String, output: &String) {
+fn compress(input: &str, output: &str) {
     let mut output_file = fs::File::create(output).unwrap();
 
     let input_file = fs::File::open(input).unwrap();
@@ -104,7 +104,7 @@ fn compress(input: &String, output: &String) {
     }
 }
 
-fn extract(input: &String, output: &String) {
+fn extract(input: &str, output: &str) {
     let mut input_file = fs::File::open(input).unwrap();
     let n_bytes = input_file.metadata().unwrap().len() as u64;
 
@@ -118,7 +118,7 @@ fn extract(input: &String, output: &String) {
     loop {
         let size = input_file.read(&mut buf).unwrap();
 
-        if size <= 0 {
+        if size == 0 {
             break;
         }