From 1e4f6d56836e9f9867bb8b05efc2fffceab6b423 Mon Sep 17 00:00:00 2001
From: Ariel Ben-Yehuda <arielb1@mail.tau.ac.il>
Date: Thu, 21 Jul 2016 20:12:04 +0300
Subject: [PATCH] rustc_errors: fix a few bugs

---
 src/librustc_errors/lib.rs | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/librustc_errors/lib.rs b/src/librustc_errors/lib.rs
index 6a48f65714c..610e5647d6d 100644
--- a/src/librustc_errors/lib.rs
+++ b/src/librustc_errors/lib.rs
@@ -531,10 +531,12 @@ pub fn struct_fatal<'a>(&'a self, msg: &str) -> DiagnosticBuilder<'a> {
         DiagnosticBuilder::new(self, Level::Fatal, msg)
     }
 
-    pub fn cancel(&mut self, err: &mut DiagnosticBuilder) {
+    pub fn cancel(&self, err: &mut DiagnosticBuilder) {
         if err.level == Level::Error || err.level == Level::Fatal {
-            assert!(self.has_errors());
-            self.err_count.set(self.err_count.get() + 1);
+            self.err_count.set(
+                self.err_count.get().checked_sub(1)
+                    .expect("cancelled an error but err_count is 0")
+            );
         }
         err.cancel();
     }
-- 
2.44.0