Seemingly infinite loop in translation item collection

[alexcrichton]

When compiling this file with cargo build --test eventual in that repository (at that revision), the compiler seemingly hangs. The stack trace after awhile is:

#0  0x00007f6d96fb5d59 in _$LT$rustc..ty..sty..TypeVariants$LT$$u27$tcx$GT$$u20$as$u20$core..cmp..PartialEq$GT$::eq::hdc97fea3d2a9ec36 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#1  0x00007f6d96fb4fbb in _$LT$std..collections..hash..set..HashSet$LT$T$C$$u20$S$GT$$GT$::get::h2e6fecee3d8a00a0 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#2  0x00007f6d96fb3b1c in rustc::ty::context::CtxtInterners::intern_ty::he404503a59fde7c2 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#3  0x00007f6d96dbf104 in rustc::ty::fold::TypeFolder::fold_ty::hd091a959fe9e93da () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#4  0x00007f6d96dbfd30 in rustc::ty::fold::TypeFolder::fold_substs::h5354c2cb141ecad0 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#5  0x00007f6d96dbf210 in rustc::ty::fold::TypeFolder::fold_ty::hd091a959fe9e93da () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#6  0x00007f6d96dc010a in _$LT$collections..vec..Vec$LT$T$GT$$u20$as$u20$core..iter..traits..FromIterator$LT$T$GT$$GT$::from_iter::h4ef495e38b3b392f () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#7  0x00007f6d96dc0067 in rustc::ty::fold::TypeFoldable::fold_with::hca4e6c5f1855cc24 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#8  0x00007f6d96dbf447 in rustc::ty::fold::TypeFolder::fold_ty::hd091a959fe9e93da () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#9  0x00007f6d96dbfd30 in rustc::ty::fold::TypeFolder::fold_substs::h5354c2cb141ecad0 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#10 0x00007f6d96dbf210 in rustc::ty::fold::TypeFolder::fold_ty::hd091a959fe9e93da () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#11 0x00007f6d96dbfc76 in rustc::ty::fold::TypeFolder::fold_substs::h5354c2cb141ecad0 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#12 0x00007f6d96dbf210 in rustc::ty::fold::TypeFolder::fold_ty::hd091a959fe9e93da () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#13 0x00007f6d96dbfc76 in rustc::ty::fold::TypeFolder::fold_substs::h5354c2cb141ecad0 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#14 0x00007f6d96dbf432 in rustc::ty::fold::TypeFolder::fold_ty::hd091a959fe9e93da () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#15 0x00007f6d96dbfd30 in rustc::ty::fold::TypeFolder::fold_substs::h5354c2cb141ecad0 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#16 0x00007f6d96dbf210 in rustc::ty::fold::TypeFolder::fold_ty::hd091a959fe9e93da () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#17 0x00007f6d96dbfc76 in rustc::ty::fold::TypeFolder::fold_substs::h5354c2cb141ecad0 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#18 0x00007f6d96dbf210 in rustc::ty::fold::TypeFolder::fold_ty::hd091a959fe9e93da () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#19 0x00007f6d96dbfc76 in rustc::ty::fold::TypeFolder::fold_substs::h5354c2cb141ecad0 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#20 0x00007f6d96dbf432 in rustc::ty::fold::TypeFolder::fold_ty::hd091a959fe9e93da () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#21 0x00007f6d96dbfd30 in rustc::ty::fold::TypeFolder::fold_substs::h5354c2cb141ecad0 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#22 0x00007f6d96f2fc81 in rustc::ty::fold::TypeFoldable::fold_with::h2226d0564c2753aa () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#23 0x00007f6d96f2f77a in rustc::ty::fold::TypeFoldable::fold_with::h5a066b9faadb555a () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#24 0x00007f6d96f575c9 in rustc::infer::higher_ranked::_$LT$impl$u20$rustc..infer..InferCtxt$LT$$u27$a$C$$u20$$u27$gcx$C$$u20$$u27$tcx$GT$$GT$::plug_leaks::h31ea4f9ecee7e01a () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#25 0x00007f6d96f5896d in rustc::traits::select::SelectionContext::impl_or_trait_obligations::h5cb331c959adfea1 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#26 0x00007f6d96f58f1c in rustc::traits::select::SelectionContext::confirm_impl_candidate::_$u7b$$u7b$closure$u7d$$u7d$::h123590ceb6b13c52 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#27 0x00007f6d96f58b53 in rustc::infer::InferCtxt::in_snapshot::h5b64a26f07e0118e () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#28 0x00007f6d96f44254 in rustc::traits::select::SelectionContext::confirm_candidate::h1588d067a35baf64 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#29 0x00007f6d96f2b48b in rustc::traits::select::SelectionContext::select::h748107d7ba3e7cab () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#30 0x00007f6d96f31bd5 in rustc::traits::project::opt_normalize_projection_type::hdcd0601b73e88ce7 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#31 0x00007f6d96f1a161 in rustc::traits::project::normalize_projection_type::h42ddc46289d47331 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#32 0x00007f6d96d90248 in _$LT$rustc..traits..project..AssociatedTypeNormalizer$LT$$u27$a$C$$u20$$u27$b$C$$u20$$u27$gcx$C$$u20$$u27$tcx$GT$$u20$as$u20$rustc..ty..fold..TypeFolder$LT$$u27$gcx$C$$u20$$u27$tcx$GT$$GT$::fold_ty::hb538441763e332d4 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#33 0x00007f6d96d94493 in rustc::ty::fold::TypeFolder::fold_substs::hcb429e71c727d725 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#34 0x00007f6d96d8fb5b in _$LT$rustc..traits..project..AssociatedTypeNormalizer$LT$$u27$a$C$$u20$$u27$b$C$$u20$$u27$gcx$C$$u20$$u27$tcx$GT$$u20$as$u20$rustc..ty..fold..TypeFolder$LT$$u27$gcx$C$$u20$$u27$tcx$GT$$GT$::fold_ty::hb538441763e332d4 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#35 0x00007f6d96d94493 in rustc::ty::fold::TypeFolder::fold_substs::hcb429e71c727d725 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#36 0x00007f6d96d8fb5b in _$LT$rustc..traits..project..AssociatedTypeNormalizer$LT$$u27$a$C$$u20$$u27$b$C$$u20$$u27$gcx$C$$u20$$u27$tcx$GT$$u20$as$u20$rustc..ty..fold..TypeFolder$LT$$u27$gcx$C$$u20$$u27$tcx$GT$$GT$::fold_ty::hb538441763e332d4 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#37 0x00007f6d96d8ef54 in rustc::traits::project::normalize_with_depth::h1d7e5f812d0949c5 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#38 0x00007f6d96d8e56b in _$LT$$RF$$u27$gcx$u20$rustc..ty..TyS$LT$$u27$gcx$GT$$u20$as$u20$rustc..infer..TransNormalize$LT$$u27$gcx$GT$$GT$::trans_normalize::hc0ed6aa5590a9a37 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc-8102e29f.so
#39 0x00007f6d97e3db13 in rustc::infer::_$LT$impl$u20$rustc..ty..context..TyCtxt$LT$$u27$a$C$$u20$$u27$tcx$C$$u20$$u27$tcx$GT$$GT$::normalize_associated_type::ha192860e5464ab44 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#40 0x00007f6d97ea01cc in _$LT$rustc_trans..collector..MirNeighborCollector$LT$$u27$a$C$$u20$$u27$tcx$GT$$u20$as$u20$rustc..mir..visit..Visitor$LT$$u27$tcx$GT$$GT$::visit_lvalue::h3adecd3a6db5869e () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#41 0x00007f6d97ea1984 in _$LT$rustc_trans..collector..MirNeighborCollector$LT$$u27$a$C$$u20$$u27$tcx$GT$$u20$as$u20$rustc..mir..visit..Visitor$LT$$u27$tcx$GT$$GT$::visit_terminator_kind::hd7becdaf225d43f1 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#42 0x00007f6d97ea00cf in rustc::mir::visit::Visitor::visit_mir::hed913172065cc16c () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#43 0x00007f6d97e99a0b in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#44 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#45 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#46 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#47 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#48 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#49 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#50 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#51 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#52 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#53 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#54 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#55 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#56 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#57 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#58 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#59 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#60 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#61 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#62 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#63 0x00007f6d97e9a747 in rustc_trans::collector::collect_items_rec::hbad9b62cfe28ce98 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#64 0x00007f6d97e5fb34 in rustc_trans::base::collect_and_partition_translation_items::_$u7b$$u7b$closure$u7d$$u7d$::ha31f4a78f586b30e () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#65 0x00007f6d97e4cc4a in rustc_trans::base::trans_crate::h999c47a06d45abea () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_trans-8102e29f.so
#66 0x00007f6d9ac39e50 in rustc_driver::driver::phase_4_translate_to_llvm::had910d0e745bc40f () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-8102e29f.so
#67 0x00007f6d9ac36e8e in rustc_driver::driver::compile_input::_$u7b$$u7b$closure$u7d$$u7d$::hf187cb470aad2bf2 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-8102e29f.so
#68 0x00007f6d9ac335ee in rustc_driver::driver::phase_3_run_analysis_passes::_$u7b$$u7b$closure$u7d$$u7d$::hd565ad56c5876a16 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-8102e29f.so
#69 0x00007f6d9ac2cdba in rustc::ty::context::TyCtxt::create_and_enter::habef58c7230d34f9 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-8102e29f.so
#70 0x00007f6d9abf7f60 in rustc_driver::driver::compile_input::hfd60b020f6d0208d () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-8102e29f.so
#71 0x00007f6d9abe3e85 in rustc_driver::run_compiler::h884d01d12eb76bbb () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-8102e29f.so
#72 0x00007f6d9abe0f5f in std::panicking::try::call::hd72cf79141f67e60 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-8102e29f.so
#73 0x00007f6d9a717c4c in __rust_try () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libstd-8102e29f.so
#74 0x00007f6d9a717bef in __rust_maybe_catch_panic () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libstd-8102e29f.so
#75 0x00007f6d9abe1a45 in _$LT$F$u20$as$u20$alloc..boxed..FnBox$LT$A$GT$$GT$::call_box::h589d2091babf223a () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-8102e29f.so
#76 0x00007f6d9a7078b5 in std::sys::thread::Thread::new::thread_start::h9c883b6d445ece46 () from /home/alex/.multirust/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libstd-8102e29f.so
#77 0x00007f6d9225b182 in start_thread (arg=0x7f6d907ff700) at pthread_create.c:312
#78 0x00007f6d9a34547d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

And the logs towards the end look like this: https://gist.github.com/alexcrichton/08e46aaef1c170e74752ce1d75287a02

cc @michaelwoerister, looks to be hanging during translation item collections

[alexcrichton]

Ok, managed to reduce it to this:

pub trait Future: 'static {
    type Item;
    type Error;

    fn boxed(self) -> Box<Future<Item=Self::Item, Error=Self::Error>>
        where Self: Sized
    {
        Box::new(self)
    }

    fn map(self) -> Map<Self>
        where Self: Sized,
    {
        loop {}
    }

    fn forget(self) where Self: Sized {
        self.map().boxed();
    }
}

// =============================================================================

pub struct Map<A> {
    _future: A,
}

impl<A> Future for Map<A>
    where A: Future,
{
    type Item = ();
    type Error = A::Error;
}

// =============================================================================

pub struct Promise;

impl Future for Promise {
    type Item = ();
    type Error = ();
}

// =============================================================================

fn foo() -> Promise {
    loop {}
}

fn main() {
    foo().boxed();
}

If rustc is run over that, it takes awhile but eventually spits out

error: overflow evaluating the requirement `<Promise as Future>::Error` [--explain E0275]
note: consider adding a `#![recursion_limit="128"]` attribute to your crate

error: aborting due to previous error

This is in theory just a reduced down version of the original crate, so presumably it was just going to take a very long time to say that before?

Haven't quite figured out how this program is causing the recursion limit to get hit...

[alexcrichton]

@eddyb managed to reduce further:

pub trait Future: 'static {
    fn forget(self) where Self: Sized {
        Box::new(Map(self)) as Box<Future>;
    }
}

pub struct Map<A>(A);
impl<A: Future> Future for Map<A> {}

pub struct Promise;
impl Future for Promise {}

fn main() {
    Promise.forget();
}