Fix pretty printing of never pattern match arms.

compiler/rustc_ast_pretty/src/pprust/state/expr.rs

@@ -876,6 +876,7 @@ fn print_arm(&mut self, arm: &ast::Arm) {
                 }
             }
         } else {
+            self.end(); // Close the ibox for the pattern.
             self.word(",");
         }
         self.end(); // Close enclosing cbox.

tests/pretty/never-pattern.pp

@@ -12,7 +12,6 @@ use ::std::prelude::rust_2015::*;
 #[macro_use]
 extern crate std;
 
-fn f(x: Result<u32, !>) {
-    _ = match x { Ok(x) => x, Err(!) , }; }
+fn f(x: Result<u32, !>) { _ = match x { Ok(x) => x, Err(!) , }; }
 
-    fn main() {}
+fn main() {}