style: Try to collect some more crash information so that we can diagnose this better.

Differential Revision: https://phabricator.services.mozilla.com/D59704

components/style/rule_tree/mod.rs

@@ -136,6 +136,27 @@ impl StyleSource {
         let _ = write!(writer, "  -> {:?}", self.read(guard).declarations());
     }
 
+    // This is totally unsafe, should be removed when we figure out the cause of
+    // bug 1607553.
+    unsafe fn dump_unchecked<W: Write>(&self, writer: &mut W) {
+        if let Some(ref rule) = self.0.as_first() {
+            let rule = rule.read_unchecked();
+            let _ = write!(writer, "{:?}", rule.selectors);
+        }
+        let _ = write!(writer, "  -> {:?}", self.read_unchecked().declarations());
+    }
+
+    // This is totally unsafe, should be removed when we figure out the cause of
+    // bug 1607553.
+    #[inline]
+    unsafe fn read_unchecked(&self) -> &PropertyDeclarationBlock {
+        let block: &Locked<PropertyDeclarationBlock> = match self.0.borrow() {
+            ArcUnionBorrow::First(ref rule) => &rule.get().read_unchecked().block,
+            ArcUnionBorrow::Second(ref block) => block.get(),
+        };
+        block.read_unchecked()
+    }
+
     /// Read the style source guard, and obtain thus read access to the
     /// underlying property declaration block.
     #[inline]
@@ -1720,7 +1741,23 @@ impl Drop for StrongRuleNode {
         }
 
         if cfg!(debug_assertions) || crate::gecko_bindings::structs::GECKO_IS_NIGHTLY {
-            assert!(node.children.read().is_empty());
+            let children = node.children.read();
+            if !children.is_empty() {
+                let mut crash_str = vec![];
+                unsafe {
+                    // Try to unsafely collect some information of this before
+                    // crashing the process.
+                    if let Some(ref s) = node.source {
+                        s.dump_unchecked(&mut crash_str);
+                        crash_str.push(b'\n');
+                    }
+                    children.each(|child| {
+                        (*child.ptr()).source.as_ref().unwrap().dump_unchecked(&mut crash_str);
+                        crash_str.push(b'\n');
+                    });
+                }
+                panic!("Children left in the rule tree on drop: {}", String::from_utf8_lossy(&crash_str).trim());
+            }
         }
 
         if node.parent.is_none() {