fix underscore in slice patterns are removed (rust-lang#3719)

Author: rchaser53

src/patterns.rs

@@ -133,7 +133,7 @@ impl Rewrite for Pat {
                     .iter()
                     .map(|p| {
                         if let Some(rw) = p.rewrite(context, shape) {
-                            format!("{}", if rw == "_" { "" } else { &rw })
+                            rw
                         } else {
                             format!("{}", context.snippet(p.span))
                         }

tests/target/issue-3718.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let x: &[i32] = &[2, 2];
+    match x {
+        [_a, _] => println!("Wrong username or password"),
+        _ => println!("Logged in"),
+    }
+}