Implement dyn Trait syntax

src/libsyntax/parse/token.rs

@@ -258,6 +258,12 @@ impl Token {
         }
     }
 
+    /// Returns `true` if the token can appear at the start of a generic bound.
+    pub fn can_begin_bound(&self) -> bool {
+        self.is_path_start() || self.is_lifetime() || self.is_keyword(keywords::For) ||
+        self == &Question || self == &OpenDelim(Paren)
+    }
+
     /// Returns `true` if the token is any literal
     pub fn is_lit(&self) -> bool {
         match *self {