Fix off by one in treat err as bug

src/librustc_errors/lib.rs

@@ -869,7 +869,10 @@ impl HandlerInner {
     }
 
     fn delay_span_bug(&mut self, sp: impl Into<MultiSpan>, msg: &str) {
-        if self.treat_err_as_bug() {
+        // This is technically `self.treat_err_as_bug()` but `delay_span_bug` is called before
+        // incrementing `err_count` by one, so we need to +1 the comparing.
+        // FIXME: Would be nice to increment err_count in a more coherent way.
+        if self.flags.treat_err_as_bug.map(|c| self.err_count() + 1 >= c).unwrap_or(false) {
             // FIXME: don't abort here if report_delayed_bugs is off
             self.span_bug(sp, msg);
         }

src/test/run-make-fulldeps/treat-err-as-bug/Makefile

@@ -3,3 +3,5 @@
 all:
 	$(RUSTC) err.rs -Z treat-err-as-bug 2>&1 \
 	    | $(CGREP) "panicked at 'aborting due to \`-Z treat-err-as-bug=1\`'"
+	$(RUSTC) delay_span_bug.rs -Z treat-err-as-bug 2>&1 \
+	    | $(CGREP) "panicked at 'aborting due to \`-Z treat-err-as-bug=1\`'"

src/test/run-make-fulldeps/treat-err-as-bug/delay_span_bug.rs

@@ -0,0 +1,4 @@
+#![feature(rustc_attrs)]
+
+#[rustc_error(delay_span_bug_from_inside_query)]
+fn main() {}