XXX: drop checking

Author: nnethercote

compiler/rustc_ast_pretty/src/pp.rs

@@ -241,11 +241,22 @@ struct BufEntry {
 // opens/closes. (It is still possible to mess up, e.g. by opening a box and
 // then closing it on one path while not closing it on another.)
 #[must_use]
-pub struct BoxMarker;
+pub struct BoxMarker {
+    ended: bool,
+}
 
 impl !Clone for BoxMarker {}
 impl !Copy for BoxMarker {}
 
+impl Drop for BoxMarker {
+    fn drop(&mut self) {
+        if !self.ended {
+            // njn: make this debug assertion?
+            panic!("BoxMarker not ended");
+        }
+    }
+}
+
 impl Printer {
     pub fn new() -> Self {
         Printer {
@@ -291,17 +302,18 @@ impl Printer {
         }
         let right = self.buf.push(BufEntry { token: Token::Begin(token), size: -self.right_total });
         self.scan_stack.push_back(right);
-        BoxMarker
+        BoxMarker { ended: false }
     }
 
     // This is is where `BoxMarker`s are consumed.
-    fn scan_end(&mut self, _b: BoxMarker) {
+    fn scan_end(&mut self, mut b: BoxMarker) {
         if self.scan_stack.is_empty() {
             self.print_end();
         } else {
             let right = self.buf.push(BufEntry { token: Token::End, size: -1 });
             self.scan_stack.push_back(right);
         }
+        b.ended = true;
     }
 
     fn scan_break(&mut self, token: BreakToken) {