rustdoc: ICE: synthetic auto trait impls: `unexpected result when selecting […]` involving const in super trait bounds & item bounds

[donkeyteethUX]

Problem

Minimal example

pub const N: usize = 1;

pub struct MapType<K: Supertrait<V>, V> {
    _array: K::Array,
}

pub trait Subtrait: Supertrait<[u8; N]> {}

pub trait Supertrait<V> {
    type Array: AnotherTrait<V>;
}

pub trait AnotherTrait<V> {
    const LENGTH: usize;
}

pub struct Container<S: Subtrait> {
    _x: MapType<S, [u8; N]>,
}

What I expected: cargo doc works.
What happens:

thread 'rustc' panicked at 'Unexpected result when selecting Container<S> Obligation(predicate=Binder(ProjectionPredicate(ProjectionTy { substs: [S, [u8; 1]], item_def_id: DefId(0:12 ~ panic[490c]::Supertrait::Array) }, Term::Ty(_)), []), depth=2)', compiler/rustc_trait_selection/src/traits/auto_trait.rs:750:33
stack backtrace:
   0:     0x7f1849b6c9a0 - std::backtrace_rs::backtrace::libunwind::trace::he615646ea344481f
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7f1849b6c9a0 - std::backtrace_rs::backtrace::trace_unsynchronized::h6ea8eaac68705b9c
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f1849b6c9a0 - std::sys_common::backtrace::_print_fmt::h7ac486a935ce0bf7
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/sys_common/backtrace.rs:65:5
   3:     0x7f1849b6c9a0 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h1b5a095d3db2e28f
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f1849bc898e - core::fmt::write::h445545b92224a1cd
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/core/src/fmt/mod.rs:1209:17
   5:     0x7f1849b5cb15 - std::io::Write::write_fmt::h55a43474c6520b00
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/io/mod.rs:1682:15
   6:     0x7f1849b6c765 - std::sys_common::backtrace::_print::h65d20526fdb736b0
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/sys_common/backtrace.rs:47:5
   7:     0x7f1849b6c765 - std::sys_common::backtrace::print::h6555fbe12a1cc41b
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/sys_common/backtrace.rs:34:9
   8:     0x7f1849b6f56f - std::panicking::default_hook::{{closure}}::hbdf58083140e7ac6
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/panicking.rs:267:22
   9:     0x7f1849b6f2aa - std::panicking::default_hook::haef8271c56b74d85
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/panicking.rs:286:9
  10:     0x7f1849b6fd78 - std::panicking::rust_panic_with_hook::hfd45b6b6c12d9fa5
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/panicking.rs:688:13
  11:     0x7f1849b6fb17 - std::panicking::begin_panic_handler::{{closure}}::hf591e8609a75bd4b
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/panicking.rs:579:13
  12:     0x7f1849b6ce4c - std::sys_common::backtrace::__rust_end_short_backtrace::h81899558795e4ff7
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/sys_common/backtrace.rs:137:18
  13:     0x7f1849b6f832 - rust_begin_unwind
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/panicking.rs:575:5
  14:     0x7f1849bc5373 - core::panicking::panic_fmt::h4235fa9b4675b332
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/core/src/panicking.rs:65:14
  15:     0x7f184d8733ea - <rustc_trait_selection[5aec47e3fc81d5cc]::traits::auto_trait::AutoTraitFinder>::evaluate_predicates
  16:     0x5652762e9602 - <rustc_trait_selection[5aec47e3fc81d5cc]::traits::auto_trait::AutoTraitFinder>::find_auto_trait_generics::<rustdoc[85586d594f3d7f89]::clean::types::Generics, <rustdoc[85586d594f3d7f89]::clean::auto_trait::AutoTraitFinder>::generate_for_trait::{closure#0}>
  17:     0x5652761e01d3 - <rustdoc[85586d594f3d7f89]::clean::auto_trait::AutoTraitFinder>::generate_for_trait
  18:     0x565276195391 - <alloc[d987cf4402e5b40a]::vec::Vec<rustdoc[85586d594f3d7f89]::clean::types::Item> as alloc[d987cf4402e5b40a]::vec::spec_from_iter::SpecFromIter<rustdoc[85586d594f3d7f89]::clean::types::Item, core[b97a30f8df81432d]::iter::adapters::filter_map::FilterMap<alloc[d987cf4402e5b40a]::vec::into_iter::IntoIter<rustc_span[14998722174c1bca]::def_id::DefId>, <rustdoc[85586d594f3d7f89]::clean::auto_trait::AutoTraitFinder>::get_auto_trait_impls::{closure#0}>>>::from_iter
  19:     0x5652761e0f0f - <rustdoc[85586d594f3d7f89]::clean::auto_trait::AutoTraitFinder>::get_auto_trait_impls
  20:     0x56527613a501 - rustdoc[85586d594f3d7f89]::clean::utils::get_auto_trait_and_blanket_impls
  21:     0x56527626b0d8 - <rustdoc[85586d594f3d7f89]::passes::collect_trait_impls::SyntheticImplCollector as rustdoc[85586d594f3d7f89]::visit::DocVisitor>::visit_item
  22:     0x56527626b21a - <rustdoc[85586d594f3d7f89]::passes::collect_trait_impls::SyntheticImplCollector as rustdoc[85586d594f3d7f89]::visit::DocVisitor>::visit_item
  23:     0x5652762683d5 - rustdoc[85586d594f3d7f89]::passes::collect_trait_impls::collect_trait_impls
  24:     0x5652762351fd - <rustc_session[87e5e219eb43521c]::session::Session>::time::<rustdoc[85586d594f3d7f89]::clean::types::Crate, rustdoc[85586d594f3d7f89]::core::run_global_ctxt::{closure#7}>
  25:     0x56527604c78c - rustdoc[85586d594f3d7f89]::core::run_global_ctxt
  26:     0x565276236e0b - <rustc_interface[ee1a3f92e887e004]::passes::QueryContext>::enter::<rustdoc[85586d594f3d7f89]::main_args::{closure#1}::{closure#0}::{closure#1}, core[b97a30f8df81432d]::result::Result<(), rustc_errors[21897ed46328f955]::ErrorGuaranteed>>
  27:     0x5652761a6fa4 - <rustc_interface[ee1a3f92e887e004]::interface::Compiler>::enter::<rustdoc[85586d594f3d7f89]::main_args::{closure#1}::{closure#0}, core[b97a30f8df81432d]::result::Result<(), rustc_errors[21897ed46328f955]::ErrorGuaranteed>>
  28:     0x56527602aa33 - std[bbad73ae434e23e5]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[ee1a3f92e887e004]::util::run_in_thread_pool_with_globals<rustc_interface[ee1a3f92e887e004]::interface::run_compiler<core[b97a30f8df81432d]::result::Result<(), rustc_errors[21897ed46328f955]::ErrorGuaranteed>, rustdoc[85586d594f3d7f89]::main_args::{closure#1}>::{closure#0}, core[b97a30f8df81432d]::result::Result<(), rustc_errors[21897ed46328f955]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[b97a30f8df81432d]::result::Result<(), rustc_errors[21897ed46328f955]::ErrorGuaranteed>>
  29:     0x5652763065ea - <<std[bbad73ae434e23e5]::thread::Builder>::spawn_unchecked_<rustc_interface[ee1a3f92e887e004]::util::run_in_thread_pool_with_globals<rustc_interface[ee1a3f92e887e004]::interface::run_compiler<core[b97a30f8df81432d]::result::Result<(), rustc_errors[21897ed46328f955]::ErrorGuaranteed>, rustdoc[85586d594f3d7f89]::main_args::{closure#1}>::{closure#0}, core[b97a30f8df81432d]::result::Result<(), rustc_errors[21897ed46328f955]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[b97a30f8df81432d]::result::Result<(), rustc_errors[21897ed46328f955]::ErrorGuaranteed>>::{closure#1} as core[b97a30f8df81432d]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  30:     0x7f1849b79843 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h4273f95ec44459b3
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/alloc/src/boxed.rs:1987:9
  31:     0x7f1849b79843 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h70f28fa4ddc269e5
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/alloc/src/boxed.rs:1987:9
  32:     0x7f1849b79843 - std::sys::unix::thread::Thread::new::thread_start::h85a9c16b988e2bd0
                               at /rustc/69f9c33d71c871fc16ac445211281c6e7a340943/library/std/src/sys/unix/thread.rs:108:17
  33:     0x7f1849894b43 - start_thread
                               at ./nptl/./nptl/pthread_create.c:442:8
  34:     0x7f1849926a00 - clone3
                               at ./misc/../sysdeps/unix/sysv/linux/x86_64/clone3.S:81
  35:                0x0 - <unknown>

Steps

cargo doc (check & build work)

Possible Solution(s)

No response

Notes

No response

Version

rustc 1.66.0 (69f9c33d7 2022-12-12)
binary: rustc
commit-hash: 69f9c33d71c871fc16ac445211281c6e7a340943
commit-date: 2022-12-12
host: x86_64-unknown-linux-gnu
release: 1.66.0
LLVM version: 15.0.2

[ehuss]

Thanks for the report! I have transferred this to rust-lang/rust as that is where rustdoc issues are tracked.

[GuillaumeGomez]

Seems to be a compiler bug. Adding the relevant tag.

[donkeyteethUX]

Although the MCVE looks a bit esoteric, it actually comes from using EnumMap in an fairly straightforward way. I wouldn't be surprised if others have/will run into this.

[megakorre]

Ran cargo-bisect-rustc on this and found it regressed in d49e7e7
The PR (#103279) states it's a revert of a previous commit. So maybe it was broken before the commit that needed reverting.

searched nightlies: from nightly-2022-10-23 to nightly-2023-02-20
regressed nightly: nightly-2022-10-27
searched commit range: https://github.com/rust-lang/rust/compare/bed4ad65bf7a1cef39e3d66b3670189581b3b073...1898c34e923bad763e723c68dd9f23a09f9eb0fc
regressed commit: https://github.com/rust-lang/rust/commit/d49e7e7fa13479c11a3733824c78e280e391288b

[fmease]

@rustbot label A-auto-traits A-synthetic-impls