Fix spacing and ordering of words in pretty printed Impl

Author: dtolnay

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1285,14 +1285,17 @@ impl<'a> State<'a> {
                 self.print_visibility(&item.vis);
                 self.print_defaultness(defaultness);
                 self.print_unsafety(unsafety);
-                self.word_nbsp("impl");
-                self.print_constness(constness);
+                self.word("impl");
 
-                if !generics.params.is_empty() {
+                if generics.params.is_empty() {
+                    self.nbsp();
+                } else {
                     self.print_generic_params(&generics.params);
                     self.space();
                 }
 
+                self.print_constness(constness);
+
                 if let ast::ImplPolarity::Negative(_) = polarity {
                     self.word("!");
                 }

src/test/ui/macros/stringify.rs

@@ -603,7 +603,7 @@ fn test_item() {
         stringify_item!(
             impl<T> Struct<T> {}
         ),
-        "impl <T> Struct<T> {}", // FIXME
+        "impl<T> Struct<T> {}",
     );
     assert_eq!(
         stringify_item!(
@@ -615,7 +615,7 @@ fn test_item() {
         stringify_item!(
             impl<T> const Trait for T {}
         ),
-        "impl const <T> Trait for T {}", // FIXME
+        "impl<T> const Trait for T {}",
     );
     assert_eq!(
         stringify_item!(