large number of obligation produced in async code causing compiler stuck

[skyzh]

Code

I tried this code:

risingwavelabs/risingwave#5119

Sorry for not having a very good small reproduce of this issue. Basically we found our code will stuck when compiling after upgrading compiler to nightly-2022-10-16. I suspect this is related to a not-so-recent change of trait normalization -> #102651.

I modified normalize.rs in the Rust compiler on the latest master and find that some part of code is producing a large number of obligations. The highest one hits 4052 obligations.

Digging it into a little bit, we found that the root cause is the OrderedMergeIteratorInner + UnorderedMergeIteratorInner, which produces ~50 obligations. And after some static dispatching with generic parameters, it exploded to 4052. For example, when we define something like IteratorUnion<IteratorType1, IteratorType2, IteratorType3, IteratorType4>, we will find about 4x50 obligations at the call site. If the code is wrapped in yet another enum that specializes some generics to two concrete types, it doubles again.

Anyway, one interesting observation that can workaround the bug is here:

https://github.com/risingwavelabs/risingwave/blob/a0b2a119d6b5843bde33f9937b0f1bed3e80e0c8/src/storage/src/hummock/iterator/merge_inner.rs#L306-L308
https://github.com/risingwavelabs/risingwave/blob/a0b2a119d6b5843bde33f9937b0f1bed3e80e0c8/src/storage/src/hummock/iterator/merge_inner.rs#L265-L267

Once we erase the concrete future type and replace it with BoxFuture, the compiler no longer stuck and there won't be a large number of obligations.

fn unsafe_boxed_static_future<F, O>(f: F) -> BoxFuture<'static, O>
where
    F: Future<Output = O> + Send,
{
    let b = f.boxed();
    unsafe { std::mem::transmute::<BoxFuture<'_, O>, BoxFuture<'static, O>>(b) }
}


        let f = unsafe_boxed_static_future(node.iter.next());

        match f.await {

Interestingly, any of the below changes will cause obligations to increase again:

moving the function call into the match will increase number of obligations

        match unsafe_boxed_static_future(node.iter.next()).await {

the original code before bumping toolchain

        match node.iter.next().await {

When the compiler stuck, it is resolving obligations like follows, which might take ~1sec at each call site that awaits a future including a MergeIterator.

normalize::<rustc_middle::ty::subst::GenericArg>: with 52 obligations
result=Ok([static generator@src/storage/src/hummock/iterator/mod.rs:381:55: 386:6])
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, ()>>, usize>, hummock::iterator::concat_inner::ConcatIteratorInner<hummock::sstable::forward_sstable_iterator::SstableIterator>, hummock::sstable::forward_sstable_iterator::SstableIterator>, ()>, ReStatic), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, ()>>, usize>, hummock::iterator::concat_inner::ConcatIteratorInner<hummock::sstable::forward_sstable_iterator::SstableIterator>, hummock::sstable::forward_sstable_iterator::SstableIterator>, ()>, RePlaceholder(Placeholder { universe: U2, name: BrAnon(3) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, ()>>, usize>, hummock::iterator::concat_inner::ConcatIteratorInner<hummock::sstable::forward_sstable_iterator::SstableIterator>, hummock::sstable::forward_sstable_iterator::SstableIterator>, RePlaceholder(Placeholder { universe: U3, name: BrAnon(3) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, RePlaceholder(Placeholder { universe: U4, name: BrAnon(2) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, ()>>, usize>, ReStatic), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, ()>>, usize>, RePlaceholder(Placeholder { universe: U4, name: BrAnon(4) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, ()>>, RePlaceholder(Placeholder { universe: U5, name: BrAnon(3) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, RePlaceholder(Placeholder { universe: U6, name: BrAnon(2) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, RePlaceholder(Placeholder { universe: U7, name: BrAnon(2) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::forward_sstable_iterator::SstableIterator, RePlaceholder(Placeholder { universe: U7, name: BrAnon(4) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Forward>, RePlaceholder(Placeholder { universe: U7, name: BrAnon(6) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Forward>, RePlaceholder(Placeholder { universe: U7, name: BrAnon(8) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, ()>, ReStatic), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, ()>, RePlaceholder(Placeholder { universe: U6, name: BrAnon(4) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::HummockIteratorUnion<hummock::iterator::Forward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, hummock::sstable::forward_sstable_iterator::SstableIterator>, RePlaceholder(Placeholder { universe: U8, name: BrAnon(3) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Forward>, RePlaceholder(Placeholder { universe: U9, name: BrAnon(2) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::forward_sstable_iterator::SstableIterator, RePlaceholder(Placeholder { universe: U9, name: BrAnon(4) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Forward>, RePlaceholder(Placeholder { universe: U9, name: BrAnon(6) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Forward>, RePlaceholder(Placeholder { universe: U9, name: BrAnon(8) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Forward>, RePlaceholder(Placeholder { universe: U6, name: BrAnon(6) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Forward>, RePlaceholder(Placeholder { universe: U6, name: BrAnon(8) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::concat_inner::ConcatIteratorInner<hummock::sstable::forward_sstable_iterator::SstableIterator>, RePlaceholder(Placeholder { universe: U4, name: BrAnon(6) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::forward_sstable_iterator::SstableIterator, RePlaceholder(Placeholder { universe: U10, name: BrAnon(1) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::forward_sstable_iterator::SstableIterator, RePlaceholder(Placeholder { universe: U11, name: BrAnon(11) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::forward_sstable_iterator::SstableIterator, RePlaceholder(Placeholder { universe: U11, name: BrAnon(12) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::forward_sstable_iterator::SstableIterator, RePlaceholder(Placeholder { universe: U4, name: BrAnon(8) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, ()>>, usize>, hummock::iterator::concat_inner::ConcatIteratorInner<hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, ()>, ReStatic), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, ()>>, usize>, hummock::iterator::concat_inner::ConcatIteratorInner<hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, ()>, RePlaceholder(Placeholder { universe: U12, name: BrAnon(3) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, ()>>, usize>, hummock::iterator::concat_inner::ConcatIteratorInner<hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, RePlaceholder(Placeholder { universe: U13, name: BrAnon(3) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, RePlaceholder(Placeholder { universe: U14, name: BrAnon(2) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, ()>>, usize>, ReStatic), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, ()>>, usize>, RePlaceholder(Placeholder { universe: U14, name: BrAnon(4) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, ()>>, RePlaceholder(Placeholder { universe: U15, name: BrAnon(3) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, RePlaceholder(Placeholder { universe: U16, name: BrAnon(2) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, RePlaceholder(Placeholder { universe: U17, name: BrAnon(2) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::backward_sstable_iterator::BackwardSstableIterator, RePlaceholder(Placeholder { universe: U17, name: BrAnon(4) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Backward>, RePlaceholder(Placeholder { universe: U17, name: BrAnon(6) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Backward>, RePlaceholder(Placeholder { universe: U17, name: BrAnon(8) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, ()>, ReStatic), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::merge_inner::MergeIteratorInner<hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, ()>, RePlaceholder(Placeholder { universe: U16, name: BrAnon(4) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::HummockIteratorUnion<hummock::iterator::Backward, hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, RePlaceholder(Placeholder { universe: U18, name: BrAnon(3) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::shared_buffer::shared_buffer_batch::SharedBufferBatchIterator<hummock::iterator::Backward>, RePlaceholder(Placeholder { universe: U19, name: BrAnon(2) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::backward_sstable_iterator::BackwardSstableIterator, RePlaceholder(Placeholder { universe: U19, name: BrAnon(4) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Backward>, RePlaceholder(Placeholder { universe: U19, name: BrAnon(6) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Backward>, RePlaceholder(Placeholder { universe: U19, name: BrAnon(8) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Backward>, RePlaceholder(Placeholder { universe: U16, name: BrAnon(6) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::PhantomHummockIterator<hummock::iterator::Backward>, RePlaceholder(Placeholder { universe: U16, name: BrAnon(8) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::iterator::concat_inner::ConcatIteratorInner<hummock::sstable::backward_sstable_iterator::BackwardSstableIterator>, RePlaceholder(Placeholder { universe: U14, name: BrAnon(6) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::backward_sstable_iterator::BackwardSstableIterator, RePlaceholder(Placeholder { universe: U20, name: BrAnon(1) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::backward_sstable_iterator::BackwardSstableIterator, RePlaceholder(Placeholder { universe: U21, name: BrAnon(11) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::backward_sstable_iterator::BackwardSstableIterator, RePlaceholder(Placeholder { universe: U21, name: BrAnon(12) })), []), depth=0)
- Obligation(predicate=Binder(OutlivesPredicate(hummock::sstable::backward_sstable_iterator::BackwardSstableIterator, RePlaceholder(Placeholder { universe: U14, name: BrAnon(8) })), []), depth=0)

Version it worked on

nightly-07-29

Version with regression

5a0f454c021d9617c6c37ed2f703bf4a8f41c410

nightly-2022-10-16

Backtrace

Backtrace

<backtrace>

[skyzh]

Probably also related to RPITIT, as we make a intensive use of zero-overhead async trait with GAT:

impl<I: HummockIterator> MergeIteratorNext for OrderedMergeIteratorInner<I> {
    type HummockResultFuture<'a> = impl Future<Output = HummockResult<()>> + 'a;

    fn next_inner(&mut self) -> Self::HummockResultFuture<'_> {
        self.next_inner_inner()
    }
}

[compiler-errors]

I don't think #102651 changed behavior at all, but cc @oli-obk just in case I missed something. This probably could use bisection for a sudden spike in compile time anyways to find out where this actually regressed.

[compiler-errors]

I wonder if this was due to #100980.

[skyzh]

I’ll do a bisect tomorrow 🤣

[skyzh]

... but probably will take more time. #100980 is merged before #102700 which fixes a compiler bug. I'll need to bypass #101750 before bisecting into that date.

[compiler-errors]

It's probably possible to just revert 43119d6 and check if that makes the program no longer slow to compile.

[skyzh]

Yeah no prob, I'll have a try.

[skyzh]

I reverted 43119d6 over latest master 6e95b6d and it worked perfectly 🤣 Thanks!

[compiler-errors]

By "worked perfectly", you mean the slowness is gone? In that case, this issue probably needs minimization and I can then look at why the fix in #100980 is incomplete.