rust-lang / rust-lang/rust

Post-mono cycle ICEs Miri and next-solver

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A-coinduction A-miri A-trait-system C-bug I-ICE T-types WG-trait-system-refactor
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Description

trait Apply {
    type Output<T: Trait>: Trait;
}
struct Identity;
impl Apply for Identity {
    type Output<T: Trait> = T;
}

struct Thing<A: Apply>(A);

trait Trait {
    fn method() {}
}

impl<A: Apply> Trait for Thing<A> where <A as Apply>::Output<Self>: Trait {}

/*
effectively:
impl Trait for Thing<Identity> where Thing<Identity>: Trait {}
*/

fn weird<A: Apply>() {
   <Thing<A> as Trait>::method();
}

fn main() {
    weird::<Identity>();
}

In the above code, each indivuidual trait bound holds pre-monomorphization. However, after monomorphization, the required trait bound is Thing<Identity>: Trait. Trying to prove this bound runs into an inductive cycle.

In normal compilation, we get the following post-mono error (ran on the playground with version 1.94.0-nightly (2025-12-29 56f24e00c62ec2db9a8b)):

error[E0275]: overflow evaluating the requirement `Thing<Identity>: Trait`
  |
note: required by a bound in `Apply::Output`
 --> src/main.rs:2:20
  |
2 |     type Output<T: Trait>: Trait;
  |                    ^^^^^ required by this bound in `Apply::Output`

For more information about this error, try `rustc --explain E0275`.
error: could not compile `playground` (bin "playground") due to 1 previous error

However, running the code with Miri gives the following ICE (ran on the playground with Miri version 0.1.0 (2025-12-29 56f24e00c6)):

Output with Miri 
error[E0275]: overflow evaluating the requirement `Thing<Identity>: Trait`
  |
note: required by a bound in `Apply::Output`
 --> src/main.rs:2:20
  |
2 |     type Output<T: Trait>: Trait;
  |                    ^^^^^ required by this bound in `Apply::Output`


Miri caused an ICE during evaluation. Here's the interpreter backtrace at the time of the panic:
note: the place in the program where the ICE was triggered
  --> src/main.rs:23:4
   |
23 |    <Thing<A> as Trait>::method();
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: BACKTRACE:
   = note: inside `weird::<Identity>` at src/main.rs:23:4: 23:33
note: inside `main`
  --> src/main.rs:27:5
   |
27 |     weird::<Identity>();
   |     ^^^^^^^^^^^^^^^^^^^
   = note: inside `<fn() as std::ops::FnOnce<()>>::call_once - shim(fn())` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5: 250:71
   = note: inside `std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/backtrace.rs:166:18: 166:21
   = note: inside closure at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:206:18: 206:75
   = note: inside `std::ops::function::impls::<impl std::ops::FnOnce<()> for &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>::call_once` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:287:13: 287:31
   = note: inside `std::panicking::catch_unwind::do_call::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:581:40: 581:43
   = note: inside `std::panicking::catch_unwind::<i32, &dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:544:19: 544:88
   = note: inside `std::panic::catch_unwind::<&dyn std::ops::Fn() -> i32 + std::marker::Sync + std::panic::RefUnwindSafe, i32>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:359:14: 359:40
   = note: inside closure at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:175:24: 175:49
   = note: inside `std::panicking::catch_unwind::do_call::<{closure@std::rt::lang_start_internal::{closure#0}}, isize>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:581:40: 581:43
   = note: inside `std::panicking::catch_unwind::<isize, {closure@std::rt::lang_start_internal::{closure#0}}>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panicking.rs:544:19: 544:88
   = note: inside `std::panic::catch_unwind::<{closure@std::rt::lang_start_internal::{closure#0}}, isize>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:359:14: 359:40
   = note: inside `std::rt::lang_start_internal` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:171:5: 193:7
   = note: inside `std::rt::lang_start::<()>` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/rt.rs:205:5: 210:6

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0275`.

Additionally, running the code with -Znext-solver gives the following ICE (ran on godbolt with version 1.94.0-nightly (7b5cde737 2025-12-27) and changing fn main to pub fn main):

Output with next-solver 
error: internal compiler error: /rustc-dev/7b5cde7370b82fecf700071dda98c3fbc7d41fae/compiler/rustc_middle/src/ty/instance.rs:579:21: failed to resolve instance for <Thing<Identity> as Trait>::method
  --> <source>:23:4
   |
23 |    <Thing<A> as Trait>::method();
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


thread 'rustc' (3) panicked at /rustc-dev/7b5cde7370b82fecf700071dda98c3fbc7d41fae/compiler/rustc_middle/src/ty/instance.rs:579:21:
Box<dyn Any>
stack backtrace:
   0:     0x7228bb653393 - <<std[d04b43a2428f6e7c]::sys::backtrace::BacktraceLock>::print::DisplayBacktrace as core[5301abcae13ad7a]::fmt::Display>::fmt
   1:     0x7228bbc10b88 - core[5301abcae13ad7a]::fmt::write
   2:     0x7228bb669aa6 - <std[d04b43a2428f6e7c]::sys::stdio::unix::Stderr as std[d04b43a2428f6e7c]::io::Write>::write_fmt
   3:     0x7228bb629c48 - std[d04b43a2428f6e7c]::panicking::default_hook::{closure#0}
   4:     0x7228bb6472d3 - std[d04b43a2428f6e7c]::panicking::default_hook
   5:     0x7228ba621aca - std[d04b43a2428f6e7c]::panicking::update_hook::<alloc[966711e2d4248474]::boxed::Box<rustc_driver_impl[d4ed2211832a1e45]::install_ice_hook::{closure#1}>>::{closure#0}
   6:     0x7228bb6475b2 - std[d04b43a2428f6e7c]::panicking::panic_with_hook
   7:     0x7228ba65e371 - std[d04b43a2428f6e7c]::panicking::begin_panic::<rustc_errors[cb139601f36bb398]::ExplicitBug>::{closure#0}
   8:     0x7228ba64e866 - std[d04b43a2428f6e7c]::sys::backtrace::__rust_end_short_backtrace::<std[d04b43a2428f6e7c]::panicking::begin_panic<rustc_errors[cb139601f36bb398]::ExplicitBug>::{closure#0}, !>
   9:     0x7228ba64be3c - std[d04b43a2428f6e7c]::panicking::begin_panic::<rustc_errors[cb139601f36bb398]::ExplicitBug>
  10:     0x7228ba67c2b1 - <rustc_errors[cb139601f36bb398]::diagnostic::BugAbort as rustc_errors[cb139601f36bb398]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  11:     0x7228bac0693c - <rustc_errors[cb139601f36bb398]::DiagCtxtHandle>::span_bug::<rustc_span[41c12b187d0c00e0]::span_encoding::Span, alloc[966711e2d4248474]::string::String>
  12:     0x7228bac2b636 - rustc_middle[b12d616c0d1b1d17]::util::bug::opt_span_bug_fmt::<rustc_span[41c12b187d0c00e0]::span_encoding::Span>::{closure#0}
  13:     0x7228bac2b7e2 - rustc_middle[b12d616c0d1b1d17]::ty::context::tls::with_opt::<rustc_middle[b12d616c0d1b1d17]::util::bug::opt_span_bug_fmt<rustc_span[41c12b187d0c00e0]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  14:     0x7228bac1c6db - rustc_middle[b12d616c0d1b1d17]::ty::context::tls::with_context_opt::<rustc_middle[b12d616c0d1b1d17]::ty::context::tls::with_opt<rustc_middle[b12d616c0d1b1d17]::util::bug::opt_span_bug_fmt<rustc_span[41c12b187d0c00e0]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  15:     0x7228b904f0a8 - rustc_middle[b12d616c0d1b1d17]::util::bug::span_bug_fmt::<rustc_span[41c12b187d0c00e0]::span_encoding::Span>
  16:     0x7228bc027975 - <rustc_middle[b12d616c0d1b1d17]::ty::instance::Instance>::expect_resolve
  17:     0x7228bc3da1d5 - <rustc_monomorphize[93af97dab7a3fd6a]::collector::MirUsedCollector as rustc_middle[b12d616c0d1b1d17]::mir::visit::Visitor>::visit_terminator
  18:     0x7228bc3d3a4a - rustc_monomorphize[93af97dab7a3fd6a]::collector::items_of_instance
  19:     0x7228bc3d343a - rustc_query_impl[7dbdc15bcc8f2d5]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7dbdc15bcc8f2d5]::query_impl::items_of_instance::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b12d616c0d1b1d17]::query::erase::Erased<[u8; 32usize]>>
  20:     0x7228bc830195 - rustc_query_system[ff548912004eb624]::query::plumbing::try_execute_query::<rustc_query_impl[7dbdc15bcc8f2d5]::DynamicConfig<rustc_query_system[ff548912004eb624]::query::caches::DefaultCache<(rustc_middle[b12d616c0d1b1d17]::ty::instance::Instance, rustc_middle[b12d616c0d1b1d17]::mir::mono::CollectionMode), rustc_middle[b12d616c0d1b1d17]::query::erase::Erased<[u8; 32usize]>>, false, false, false>, rustc_query_impl[7dbdc15bcc8f2d5]::plumbing::QueryCtxt, false>
  21:     0x7228bc82fdbf - rustc_query_impl[7dbdc15bcc8f2d5]::query_impl::items_of_instance::get_query_non_incr::__rust_end_short_backtrace
  22:     0x7228bd073e6e - rustc_monomorphize[93af97dab7a3fd6a]::collector::collect_items_rec
  23:     0x7228bd078042 - rustc_monomorphize[93af97dab7a3fd6a]::collector::collect_items_rec
  24:     0x7228bbf0dba1 - rustc_monomorphize[93af97dab7a3fd6a]::collector::collect_crate_mono_items::{closure#1}::{closure#0}
  25:     0x7228bc38e658 - rustc_monomorphize[93af97dab7a3fd6a]::partitioning::collect_and_partition_mono_items
  26:     0x7228bd028624 - rustc_query_impl[7dbdc15bcc8f2d5]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7dbdc15bcc8f2d5]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b12d616c0d1b1d17]::query::erase::Erased<[u8; 24usize]>>
  27:     0x7228bd028609 - <rustc_query_impl[7dbdc15bcc8f2d5]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2} as core[5301abcae13ad7a]::ops::function::FnOnce<(rustc_middle[b12d616c0d1b1d17]::ty::context::TyCtxt, ())>>::call_once
  28:     0x7228bd02815e - rustc_query_system[ff548912004eb624]::query::plumbing::try_execute_query::<rustc_query_impl[7dbdc15bcc8f2d5]::DynamicConfig<rustc_query_system[ff548912004eb624]::query::caches::SingleCache<rustc_middle[b12d616c0d1b1d17]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[7dbdc15bcc8f2d5]::plumbing::QueryCtxt, false>
  29:     0x7228bd027f03 - rustc_query_impl[7dbdc15bcc8f2d5]::query_impl::collect_and_partition_mono_items::get_query_non_incr::__rust_end_short_backtrace
  30:     0x7228bc7fde9f - rustc_codegen_ssa[6bd2bc7e2f8a2c6d]::back::symbol_export::exported_generic_symbols_provider_local
  31:     0x7228bcb6f275 - rustc_query_impl[7dbdc15bcc8f2d5]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[7dbdc15bcc8f2d5]::query_impl::exported_generic_symbols::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b12d616c0d1b1d17]::query::erase::Erased<[u8; 16usize]>>
  32:     0x7228bcb6f24b - <rustc_query_impl[7dbdc15bcc8f2d5]::query_impl::exported_generic_symbols::dynamic_query::{closure#2} as core[5301abcae13ad7a]::ops::function::FnOnce<(rustc_middle[b12d616c0d1b1d17]::ty::context::TyCtxt, rustc_span[41c12b187d0c00e0]::def_id::CrateNum)>>::call_once
  33:     0x7228bcb6d0a6 - rustc_query_system[ff548912004eb624]::query::plumbing::try_execute_query::<rustc_query_impl[7dbdc15bcc8f2d5]::DynamicConfig<rustc_data_structures[6f877b1072984333]::vec_cache::VecCache<rustc_span[41c12b187d0c00e0]::def_id::CrateNum, rustc_middle[b12d616c0d1b1d17]::query::erase::Erased<[u8; 16usize]>, rustc_query_system[ff548912004eb624]::dep_graph::graph::DepNodeIndex>, false, false, false>, rustc_query_impl[7dbdc15bcc8f2d5]::plumbing::QueryCtxt, false>
  34:     0x7228bcb6cbcc - rustc_query_impl[7dbdc15bcc8f2d5]::query_impl::exported_generic_symbols::get_query_non_incr::__rust_end_short_backtrace
  35:     0x7228bc460a0d - <rustc_metadata[15381d64219637fd]::rmeta::encoder::EncodeContext>::encode_crate_root
  36:     0x7228bcfe0612 - <rustc_metadata[15381d64219637fd]::rmeta::encoder::encode_metadata::{closure#3} as core[5301abcae13ad7a]::ops::function::FnOnce<(rustc_middle[b12d616c0d1b1d17]::ty::context::TyCtxt, &std[d04b43a2428f6e7c]::path::Path)>>::call_once
  37:     0x7228bcfdd853 - rustc_metadata[15381d64219637fd]::rmeta::encoder::encode_metadata
  38:     0x7228bcbe9b7f - rustc_metadata[15381d64219637fd]::fs::encode_and_write_metadata
  39:     0x7228bcfd1ee8 - <rustc_interface[84bae928b5c4a14c]::queries::Linker>::codegen_and_build_linker
  40:     0x7228bcfcf6dc - <rustc_interface[84bae928b5c4a14c]::passes::create_and_enter_global_ctxt<core[5301abcae13ad7a]::option::Option<rustc_interface[84bae928b5c4a14c]::queries::Linker>, rustc_driver_impl[d4ed2211832a1e45]::run_compiler::{closure#0}::{closure#2}>::{closure#2} as core[5301abcae13ad7a]::ops::function::FnOnce<(&rustc_session[34b11767d146bd]::session::Session, rustc_middle[b12d616c0d1b1d17]::ty::context::CurrentGcx, alloc[966711e2d4248474]::sync::Arc<rustc_data_structures[6f877b1072984333]::jobserver::Proxy>, &std[d04b43a2428f6e7c]::sync::once_lock::OnceLock<rustc_middle[b12d616c0d1b1d17]::ty::context::GlobalCtxt>, &rustc_data_structures[6f877b1072984333]::sync::worker_local::WorkerLocal<rustc_middle[b12d616c0d1b1d17]::arena::Arena>, &rustc_data_structures[6f877b1072984333]::sync::worker_local::WorkerLocal<rustc_hir[bd2e92b33866e02d]::Arena>, rustc_driver_impl[d4ed2211832a1e45]::run_compiler::{closure#0}::{closure#2})>>::call_once::{shim:vtable#0}
  41:     0x7228bcdf2358 - rustc_interface[84bae928b5c4a14c]::interface::run_compiler::<(), rustc_driver_impl[d4ed2211832a1e45]::run_compiler::{closure#0}>::{closure#1}
  42:     0x7228bcd926ce - std[d04b43a2428f6e7c]::sys::backtrace::__rust_begin_short_backtrace::<rustc_interface[84bae928b5c4a14c]::util::run_in_thread_with_globals<rustc_interface[84bae928b5c4a14c]::util::run_in_thread_pool_with_globals<rustc_interface[84bae928b5c4a14c]::interface::run_compiler<(), rustc_driver_impl[d4ed2211832a1e45]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>
  43:     0x7228bcd924a0 - <std[d04b43a2428f6e7c]::thread::lifecycle::spawn_unchecked<rustc_interface[84bae928b5c4a14c]::util::run_in_thread_with_globals<rustc_interface[84bae928b5c4a14c]::util::run_in_thread_pool_with_globals<rustc_interface[84bae928b5c4a14c]::interface::run_compiler<(), rustc_driver_impl[d4ed2211832a1e45]::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}, ()>::{closure#1} as core[5301abcae13ad7a]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  44:     0x7228bcd97df8 - <std[d04b43a2428f6e7c]::sys::thread::unix::Thread>::new::thread_start
  45:     0x7228b6694ac3 - <unknown>
  46:     0x7228b67268c0 - <unknown>
  47:                0x0 - <unknown>

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: please attach the file at `/app/rustc-ice-2025-12-30T02_26_48-1.txt` to your bug report

note: rustc 1.94.0-nightly (7b5cde737 2025-12-27) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C llvm-args=--x86-asm-syntax=intel --crate-type rlib -Z next-solver

query stack during panic:
#0 [items_of_instance] collecting items used by `weird::<Identity>`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
... and 1 other queries... use `env RUST_BACKTRACE=1` to see the full query stack
error: aborting due to 1 previous error

See also https://github.com/rust-lang/rust/issues/149898

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First steps

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Research direction

Reproduce the provided program with normal compilation, Miri, and -Znext-solver, comparing the reported overflow with both ICEs. Start by reading rustc_middle/src/ty/instance.rs around Instance::expect_resolve and the rustc_monomorphize collector frames shown in the backtrace. Done means the reproducer no longer triggers an internal compiler error and emits a supported diagnostic instead.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
25/100

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