Keep track of the start of the argument block of a closure

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

@@ -409,6 +409,7 @@ impl<'a> State<'a> {
                 ref fn_decl,
                 ref body,
                 fn_decl_span: _,
+                fn_arg_span: _,
             }) => {
                 self.print_closure_binder(binder);
                 self.print_movability(movability);