rust-lang / rust-lang/rust

Infinite loop in type inference

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A-trait-system C-bug I-hang T-compiler
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Rust
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Description

I tried this code:

struct F<A, O>(fn(A) -> O);

trait Fun {
    type O;
    type A;
}

impl<A, O> Fun for F<A, O> {
    type O = O;
    type A = A;
}

trait Gen {
    type B;
}

fn a() {
    b(F(c))
}

fn b<G, B>(_: G)
where
    G: Fun<A = B>,
    <G as Fun>::O: Gen<B = B>,
{
}

fn c<T>(_: T) -> impl Gen {}

Rust Explorer

I expected to see this happen: This code should fail to compile with errors.

Instead, this happened: Rustc enters a infinite loop and uses memory until the user or OS kills it.

Meta

rustc --version --verbose:

rustc 1.81.0-nightly (8fcd4dd08 2024-06-18)
binary: rustc
commit-hash: 8fcd4dd08e2ba3e922d917d819ba0be066bdb005
commit-date: 2024-06-18
host: x86_64-unknown-linux-gnu
release: 1.81.0-nightly
LLVM version: 18.1.7

cargo bisect-rustc

Regression in nightly-2023-12-06

The following backtrace was generated by attaching to a running rustc instance and dumping the call stack as it was when the debugger was attached.

Backtrace 

Thread 3 (Thread 0x7f3869fff6c0 (LWP 14473) "rustc"):
#0  0x00007f3873eee2a5 in <rustc_middle[fecc9a0d5ad25b76]::ty::context::CtxtInterners>::intern_ty () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#1  0x00007f38778b7207 in <rustc_infer[87ef6f2195437645]::infer::InferCtxt>::next_ty_var_in_universe () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#2  0x00007f387746420e in <rustc_infer[87ef6f2195437645]::infer::relate::generalize::Generalizer as rustc_type_ir[596bd8e1208b8c9]::relate::TypeRelation<rustc_middle[fecc9a0d5ad25b76]::ty::context::TyCtxt>>::tys::{closure#0} () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#3  0x00007f3877454d3d in <rustc_infer[87ef6f2195437645]::infer::relate::type_relating::TypeRelating as rustc_type_ir[596bd8e1208b8c9]::relate::TypeRelation<rustc_middle[fecc9a0d5ad25b76]::ty::context::TyCtxt>>::tys () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#4  0x00007f38740a7e71 in <rustc_trait_selection[32fdd2c471292ab4]::traits::fulfill::FulfillProcessor as rustc_data_structures[4cb81b6577327383]::obligation_forest::ObligationProcessor>::process_obligation () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#5  0x00007f3877408b1f in <rustc_data_structures[4cb81b6577327383]::obligation_forest::ObligationForest<rustc_trait_selection[32fdd2c471292ab4]::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection[32fdd2c471292ab4]::traits::fulfill::FulfillProcessor> () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#6  0x00007f38743d03d0 in <rustc_hir_typeck[a8fa972dab75dcd1]::fn_ctxt::FnCtxt>::confirm_builtin_call () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#7  0x00007f387803bc7e in <rustc_hir_typeck[a8fa972dab75dcd1]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#8  0x00007f3878036f14 in <rustc_hir_typeck[a8fa972dab75dcd1]::fn_ctxt::FnCtxt>::check_block_with_expected () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#9  0x00007f387803c421 in <rustc_hir_typeck[a8fa972dab75dcd1]::fn_ctxt::FnCtxt>::check_expr_with_expectation_and_args () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#10 0x00007f38776da6ee in rustc_hir_typeck[a8fa972dab75dcd1]::check::check_fn () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#11 0x00007f38776d067e in rustc_hir_typeck[a8fa972dab75dcd1]::typeck () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#12 0x00007f38776d00fb in rustc_query_impl[606e62fa16b19c12]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[606e62fa16b19c12]::query_impl::typeck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[fecc9a0d5ad25b76]::query::erase::Erased<[u8; 8: usize]>> () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#13 0x00007f38775bceee in rustc_query_system[ea7bd5a7968c1f8]::query::plumbing::try_execute_query::<rustc_query_impl[606e62fa16b19c12]::DynamicConfig<rustc_query_system[ea7bd5a7968c1f8]::query::caches::VecCache<rustc_span[2c9242bd6e97c416]::def_id::LocalDefId, rustc_middle[fecc9a0d5ad25b76]::query::erase::Erased<[u8; 8: usize]>>, false: bool, false: bool, false: bool>, rustc_query_impl[606e62fa16b19c12]::plumbing::QueryCtxt, false: bool> () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#14 0x00007f38775bb9eb in rustc_query_impl[606e62fa16b19c12]::query_impl::typeck::get_query_non_incr::__rust_end_short_backtrace () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#15 0x00007f38775bb621 in <rustc_middle[fecc9a0d5ad25b76]::hir::map::Map>::par_body_owners::<rustc_hir_analysis[738c88335a519906]::check_crate::{closure#4}>::{closure#0} () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#16 0x00007f38775ba0c7 in rustc_hir_analysis[738c88335a519906]::check_crate () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#17 0x00007f3877e14687 in rustc_interface[98f3030eb7c197c2]::passes::analysis () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#18 0x00007f3877e141c7 in rustc_query_impl[606e62fa16b19c12]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[606e62fa16b19c12]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[fecc9a0d5ad25b76]::query::erase::Erased<[u8; 1: usize]>> () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#19 0x00007f3878238725 in rustc_query_system[ea7bd5a7968c1f8]::query::plumbing::try_execute_query::<rustc_query_impl[606e62fa16b19c12]::DynamicConfig<rustc_query_system[ea7bd5a7968c1f8]::query::caches::SingleCache<rustc_middle[fecc9a0d5ad25b76]::query::erase::Erased<[u8; 1: usize]>>, false: bool, false: bool, false: bool>, rustc_query_impl[606e62fa16b19c12]::plumbing::QueryCtxt, false: bool> () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#20 0x00007f387823848f in rustc_query_impl[606e62fa16b19c12]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#21 0x00007f38780dc512 in rustc_interface[98f3030eb7c197c2]::interface::run_compiler::<core[4db8349ac6a0c205]::result::Result<(), rustc_span[2c9242bd6e97c416]::ErrorGuaranteed>, rustc_driver_impl[df262410c7f7944d]::run_compiler::{closure#0}>::{closure#1} () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#22 0x00007f3878091d49 in std[82c32ede93c8c228]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[98f3030eb7c197c2]::util::run_in_thread_with_globals<rustc_interface[98f3030eb7c197c2]::util::run_in_thread_pool_with_globals<rustc_interface--Type <RET> for more, q to quit, c to continue without paging--
[98f3030eb7c197c2]::interface::run_compiler<core[4db8349ac6a0c205]::result::Result<(), rustc_span[2c9242bd6e97c416]::ErrorGuaranteed>, rustc_driver_impl[df262410c7f7944d]::run_compiler::{closure#0}>::{closure#1}, core[4db8349ac6a0c205]::result::Result<(), rustc_span[2c9242bd6e97c416]::ErrorGuaranteed>>::{closure#0}, core[4db8349ac6a0c205]::result::Result<(), rustc_span[2c9242bd6e97c416]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[4db8349ac6a0c205]::result::Result<(), rustc_span[2c9242bd6e97c416]::ErrorGuaranteed>> () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#23 0x00007f3878091afa in <<std[82c32ede93c8c228]::thread::Builder>::spawn_unchecked_<rustc_interface[98f3030eb7c197c2]::util::run_in_thread_with_globals<rustc_interface[98f3030eb7c197c2]::util::run_in_thread_pool_with_globals<rustc_interface[98f3030eb7c197c2]::interface::run_compiler<core[4db8349ac6a0c205]::result::Result<(), rustc_span[2c9242bd6e97c416]::ErrorGuaranteed>, rustc_driver_impl[df262410c7f7944d]::run_compiler::{closure#0}>::{closure#1}, core[4db8349ac6a0c205]::result::Result<(), rustc_span[2c9242bd6e97c416]::ErrorGuaranteed>>::{closure#0}, core[4db8349ac6a0c205]::result::Result<(), rustc_span[2c9242bd6e97c416]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[4db8349ac6a0c205]::result::Result<(), rustc_span[2c9242bd6e97c416]::ErrorGuaranteed>>::{closure#2} as core[4db8349ac6a0c205]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#24 0x00007f38797908eb in alloc::boxed::{impl#48}::call_once<(), dyn core::ops::function::FnOnce<(), Output=()>, alloc::alloc::Global> () at library/alloc/src/boxed.rs:2062
#25 alloc::boxed::{impl#48}::call_once<(), alloc::boxed::Box<dyn core::ops::function::FnOnce<(), Output=()>, alloc::alloc::Global>, alloc::alloc::Global> () at library/alloc/src/boxed.rs:2062
#26 std::sys::pal::unix::thread::{impl#2}::new::thread_start () at library/std/src/sys/pal/unix/thread.rs:108
#27 0x00007f3872aae068 in ?? () from /lib64/libc.so.6
#28 0x00007f3872b2f0bc in ?? () from /lib64/libc.so.6

Thread 2 (Thread 0x7f386a7ff6c0 (LWP 14472) "ctrl-c"):
#0  0x00007f3872b1d74e in read () from /lib64/libc.so.6
#1  0x00007f38781e2f12 in nix[891d26199dd147b8]::unistd::read () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#2  0x00007f38781e2e36 in std[82c32ede93c8c228]::sys::backtrace::__rust_begin_short_backtrace::<ctrlc[b7c9252397e4647d]::set_handler_inner<rustc_driver_impl[df262410c7f7944d]::install_ctrlc_handler::{closure#0}>::{closure#0}, ()> () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#3  0x00007f38781e2d61 in <<std[82c32ede93c8c228]::thread::Builder>::spawn_unchecked_<ctrlc[b7c9252397e4647d]::set_handler_inner<rustc_driver_impl[df262410c7f7944d]::install_ctrlc_handler::{closure#0}>::{closure#0}, ()>::{closure#2} as core[4db8349ac6a0c205]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#4  0x00007f38797908eb in alloc::boxed::{impl#48}::call_once<(), dyn core::ops::function::FnOnce<(), Output=()>, alloc::alloc::Global> () at library/alloc/src/boxed.rs:2062
#5  alloc::boxed::{impl#48}::call_once<(), alloc::boxed::Box<dyn core::ops::function::FnOnce<(), Output=()>, alloc::alloc::Global>, alloc::alloc::Global> () at library/alloc/src/boxed.rs:2062
#6  std::sys::pal::unix::thread::{impl#2}::new::thread_start () at library/std/src/sys/pal/unix/thread.rs:108
#7  0x00007f3872aae068 in ?? () from /lib64/libc.so.6
#8  0x00007f3872b2f0bc in ?? () from /lib64/libc.so.6

Thread 1 (Thread 0x7f387967f580 (LWP 14471) "rustc"):
#0  0x00007f3872aaabd6 in ?? () from /lib64/libc.so.6
#1  0x00007f3872aafa83 in ?? () from /lib64/libc.so.6
#2  0x00007f3879790a41 in std::sys::pal::unix::thread::Thread::join () at library/std/src/sys/pal/unix/thread.rs:272
#3  0x00007f38781fde6a in rustc_driver_impl[df262410c7f7944d]::run_compiler () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#4  0x00007f38781fcbb8 in rustc_driver_impl[df262410c7f7944d]::main () from /home/konnor/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-802b4aa580d71496.so
#5  0x000056535a57ac07 in rustc_main[2a842e528cc0f9db]::main ()
#6  0x000056535a57abf3 in std[82c32ede93c8c228]::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()> ()
#7  0x000056535a57abe9 in <std[82c32ede93c8c228]::rt::lang_start<()>::{closure#0} as core[4db8349ac6a0c205]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} ()
#8  0x00007f3879768662 in core::ops::function::impls::{impl#2}::call_once<(), (dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe)> () at library/core/src/ops/function.rs:284
#9  std::panicking::try::do_call<&(dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe), i32> () at library/std/src/panicking.rs:553
#10 std::panicking::try<i32, &(dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe)> () at library/std/src/panicking.rs:517
#11 std::panic::catch_unwind<&(dyn core::ops::function::Fn<(), Output=i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe), i32> () at library/std/src/panic.rs:350
#12 std::rt::lang_start_internal::{closure#2} () at library/std/src/rt.rs:141
#13 std::panicking::try::do_call<std::rt::lang_start_internal::{closure_env#2}, isize> () at library/std/src/panicking.rs:553
#14 std::panicking::try<isize, std::rt::lang_start_internal::{closure_env#2}> () at library/std/src/panicking.rs:517
#15 std::panic::catch_unwind<std::rt::lang_start_internal::{closure_env#2}, isize> () at library/std/src/panic.rs:350
--Type <RET> for more, q to quit, c to continue without paging--
#16 std::rt::lang_start_internal () at library/std/src/rt.rs:141
#17 0x000056535a57ad27 in main ()

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the minimal example with the reported nightly regression, then inspect the backtrace entry points in rustc_hir_typeck::fn_ctxt, rustc_trait_selection::traits::fulfill, and rustc_infer::infer::relate::generalize. The fix should make rustc terminate with compilation errors instead of looping and exhausting memory; add a regression test for the example.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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