Avoid printing 'where' keyword in empty where-clause

A 'where' keyword with an empty where-clause is not supported before
Rust 1.16.

    struct S where {}

Do some convenient legalization by omitting the 'where' keyword if
there are no predicates in the where-clause. If the user really wants
the WhereClause printed faithfully they can print 'where_token' and
'predicates' themselves directly.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/generics.rs b/src/generics.rs
index b86f77dc..13a850bb 100644
--- a/src/generics.rs
+++ b/src/generics.rs
@@ -950,8 +950,10 @@ mod printing {
 
     impl ToTokens for WhereClause {
         fn to_tokens(&self, tokens: &mut Tokens) {
-            self.where_token.to_tokens(tokens);
-            self.predicates.to_tokens(tokens);
+            if !self.predicates.is_empty() {
+                self.where_token.to_tokens(tokens);
+                self.predicates.to_tokens(tokens);
+            }
         }
     }