Fix HIR pretty-printing of fns with just a variadic arg.

Avoid the extraneous comma.

Author: nnethercote

compiler/rustc_hir_pretty/src/lib.rs

@@ -2165,7 +2165,9 @@ impl<'a> State<'a> {
  s.end();
  });
  if decl.c_variadic {
- self.word(", ");
+ if !decl.inputs.is_empty() {
+ self.word(", ");
+ }
  print_arg(self, None);
  self.word("...");
  }

tests/pretty/hir-fn-variadic.pp

@@ -20,8 +20,8 @@
  fn g3(_: extern "C" fn(u8, va: ...)) { }
  fn g4(_: extern "C" fn(u8, ...)) { }
 
- fn g5(_: extern "C" fn(, va: ...)) { }
- fn g6(_: extern "C" fn(, ...)) { }
+ fn g5(_: extern "C" fn(va: ...)) { }
+ fn g6(_: extern "C" fn(...)) { }
 
  {
  let _ =
@@ -39,13 +39,13 @@
  {
  let _ =
  {
- unsafe extern "C" fn f5(, va: ...) { }
+ unsafe extern "C" fn f5(va: ...) { }
  };
  };
  {
  let _ =
  {
- unsafe extern "C" fn f6(, _: ...) { }
+ unsafe extern "C" fn f6(_: ...) { }
  };
  };
 }