feat(stream): Add access to previous tokens

Author: epage

src/stream/token.rs

@@ -51,6 +51,15 @@ where
         let start = self.initial.checkpoint();
         self.input.reset(&start);
     }
+
+    /// Iterate over consumed tokens starting with the last emitted
+    ///
+    /// This is intended to help build up appropriate context when reporting errors.
+    #[inline]
+    pub fn previous_tokens(&self) -> impl Iterator<Item = &'t T> {
+        let offset = self.input.offset_from(&self.initial);
+        self.initial[0..offset].iter().rev()
+    }
 }
 
 /// Track locations by implementing [`Location`] on the Token.