Handle auto traits & negative impls

We don't pass field types to Chalk yet though, so the auto trait inference won't
be correct.

crates/ra_syntax/src/ast/extensions.rs

@@ -170,6 +170,10 @@ impl ast::ImplBlock {
         let second = types.next();
         (first, second)
     }
+
+    pub fn is_negative(&self) -> bool {
+        self.syntax().children_with_tokens().any(|t| t.kind() == EXCL)
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -348,3 +352,9 @@ impl ast::WherePred {
             .find(|it| it.kind() == LIFETIME)
     }
 }
+
+impl ast::TraitDef {
+    pub fn is_auto(&self) -> bool {
+        self.syntax().children_with_tokens().any(|t| t.kind() == AUTO_KW)
+    }
+}