Discovered this locally when loading https://reviewboard.mozilla.org/r/149970/diff/11#index_header in a debug build. Not sure exactly how benign it is, since this assertion was added since I last looked at this code. DEBUG:style::invalidation::element::invalidator: > Invalidation matched, next: Invalidation(.diff-container), (NextSibling) DEBUG:style::invalidation::element::invalidator: StyleTreeInvalidator::invalidate_descendants(<span class="reviewer-name reviewer-ship-it review-granted"> (0x7f0fcef24ee0)) DEBUG:style::invalidation::element::invalidator: > [Invalidation(:first-child.diff-container), Invalidation(:not(.diff-container)), Invalidation(:first-child.diff-container), Invalidation(:not(.diff-container))] DEBUG:style::invalidation::element::invalidator: TreeStyleInvalidator::process_invalidation(<_moz_generated_content_after> (0x7f0fe0a38d30), Invalidation(:first-child.diff-container)) DEBUG:style::invalidation::element::invalidator: TreeStyleInvalidator::process_invalidation(<_moz_generated_content_after> (0x7f0fe0a38d30), Invalidation(:not(.diff-container))) DEBUG:style::invalidation::element::invalidator: > Invalidation matched, next: Invalidation(.diff-container), (NextSibling) DEBUG:style::invalidation::element::invalidator: TreeStyleInvalidator::process_invalidation(<_moz_generated_content_after> (0x7f0fe0a38d30), Invalidation(:first-child.diff-container)) DEBUG:style::invalidation::element::invalidator: TreeStyleInvalidator::process_invalidation(<_moz_generated_content_after> (0x7f0fe0a38d30), Invalidation(:not(.diff-container))) DEBUG:style::invalidation::element::invalidator: > Invalidation matched, next: Invalidation(.diff-container), (NextSibling) DEBUG:style::invalidation::element::invalidator: StyleTreeInvalidator::invalidate_descendants(<_moz_generated_content_after> (0x7f0fe0a38d30)) DEBUG:style::invalidation::element::invalidator: > [Invalidation(:first-child.diff-container), Invalidation(:not(.diff-container)), Invalidation(:first-child.diff-container), Invalidation(:not(.diff-container))] thread '<unnamed>' panicked at 'pseudos can't generate sibling invalidations, since using them in other position that isn't the rightmost part of the selector is invalid (for now at least)', /files/mozilla/mc/q/servo/components/style/invalidation/element/invalidator.rs:269 note: Run with `RUST_BACKTRACE=1` for a backtrace. Redirecting call to abort() to mozalloc_abort

It's bening... This assertion was written assuming that we'd only match ::before/::after against the corresponding pseudo, but that's of course not what we do, so it matches :first-child too... Worst case we invalidate that element's style, which seems fine. I guess I can just drop the assertion, since pseudos are NAC roots, and thus they have no siblings to invalidate.

I'm lying, in this case the relevant selector is: .with-commit-msg-filediff :not(.diff-container) + .diff-container .sidebyside .filename-row th a:after. We're invalidating the :not(.diff-container), presumably because some ancestor changed the "with-commit-msg-filediff" class. Some descendant of that element happens to be a ::before pseudo-element, so it matches :not(.diff-container), and creates the sibling invalidation. No big deal, since it's dropped on the floor, but it can't match anything anyway. I can build a crashtest tomorrow and check-in the assertion-removal with a comment pointing to the other comment that mentions that we could save work gathering invalidations just for pseudos (which incidentally would make the assertion hold). Leaving ni? for that.

Comment on attachment 8887691 [details] style: Remove bogus assertion. https://reviewboard.mozilla.org/r/158588/#review163914 ::: servo/components/style/invalidation/element/invalidator.rs:267 (Diff revision 1) > // Roots of NAC subtrees can indeed generate sibling invalidations, but > // they can be just ignored, since they have no siblings. > - debug_assert!(child.implemented_pseudo_element().is_none() || Can you mention in the comment here that we can end up checking invalidations against elements that normally would not match these selectors, such as element-backed pseudos, or other NAC (when the invalidation came from a rule in a document level style sheet), but that we just over-invalidate here rather than bothering to check these conditions.

Comment on attachment 8887692 [details] Bug 1381682: Crashtest. https://reviewboard.mozilla.org/r/158590/#review163916

Servo bits landed at https://github.com/servo/servo/pull/17781, the test should autoland when autoland is reopened.

Pushed by ecoal95@gmail.com: https://hg.mozilla.org/integration/autoland/rev/0f0faf30c690 Crashtest. r=heycam