rustdoc: ICE: synthetic auto trait impls: unable to fulfill trait […] for […] due to ambiguity involving to unconstrained param in impl

[bvanjoi]

Code

rustdoc code.rs

// code.rs
trait MyTrait {}
struct Inner;
struct Outer(Inner);

unsafe impl<Q> Send for Inner
where
  Q: MyTrait 
{}

Meta

rustc --version --verbose:

rustc 1.72.0-nightly (fe7454bf4 2023-06-19)
binary: rustc
commit-hash: fe7454bf439c93cbe9ac8a8f7fcfacd5a40244c2
commit-date: 2023-06-19
host: aarch64-apple-darwin
release: 1.72.0-nightly
LLVM version: 16.0.5

Error output

<output>

Backtrace

thread 'rustc' panicked at 'Unable to fulfill trait DefId(2:2683 ~ core[0f69]::marker::Send) for 'Outer': [FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<_ as std::marker::Sized>, polarity:Positive), []), depth=2),Ambiguity), FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<_ as MyTrait>, polarity:Positive), []), depth=2),Ambiguity)]', /rustc/fe7454bf439c93cbe9ac8a8f7fcfacd5a40244c2/compiler/rustc_trait_selection/src/traits/auto_trait.rs:186:13
stack backtrace:
   0:        0x1015c40f4 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h98bf6f1b8a3176b4
   1:        0x10161475c - core::fmt::write::h32b4394822667739
   2:        0x1015b999c - std::io::Write::write_fmt::h19e6965a4f1b2789
   3:        0x1015c3f4c - std::sys_common::backtrace::print::h75a318905bc68cf1
   4:        0x1015c6ac8 - std::panicking::default_hook::{{closure}}::hd804ed281d7e08a7
   5:        0x1015c6858 - std::panicking::default_hook::hf1f7a911542630ee
   6:        0x109b456cc - rustc_driver_impl[d5bfd45cb53215fb]::install_ice_hook::{closure#0}
   7:        0x1015c7248 - std::panicking::rust_panic_with_hook::h74c8b87d4371442c
   8:        0x1015c701c - std::panicking::begin_panic_handler::{{closure}}::hd7ff9b18e4ab7574
   9:        0x1015c44d4 - std::sys_common::backtrace::__rust_end_short_backtrace::hdc7391160fc74630
  10:        0x1015c6d88 - _rust_begin_unwind
  11:        0x10163f924 - core::panicking::panic_fmt::hb1c27fe21c4f1f37
  12:        0x100300f28 - <rustc_trait_selection[3bfe20a7ab0ad0d3]::traits::auto_trait::AutoTraitFinder>::find_auto_trait_generics::<rustdoc[e386f36fe3828e32]::clean::types::Generics, <rustdoc[e386f36fe3828e32]::clean::auto_trait::AutoTraitFinder>::generate_for_trait::{closure#0}>
  13:        0x1004030b0 - <rustdoc[e386f36fe3828e32]::clean::auto_trait::AutoTraitFinder>::generate_for_trait
  14:        0x100445ad4 - <alloc[480d25cc58e20263]::vec::Vec<rustdoc[e386f36fe3828e32]::clean::types::Item> as alloc[480d25cc58e20263]::vec::spec_from_iter::SpecFromIter<rustdoc[e386f36fe3828e32]::clean::types::Item, core[f69943f7197f08d]::iter::adapters::filter_map::FilterMap<alloc[480d25cc58e20263]::vec::into_iter::IntoIter<rustc_span[131a45c9b4032fd]::def_id::DefId>, <rustdoc[e386f36fe3828e32]::clean::auto_trait::AutoTraitFinder>::get_auto_trait_impls::{closure#0}>>>::from_iter
  15:        0x100403698 - <rustdoc[e386f36fe3828e32]::clean::auto_trait::AutoTraitFinder>::get_auto_trait_impls
  16:        0x100395f54 - rustdoc[e386f36fe3828e32]::clean::utils::get_auto_trait_and_blanket_impls
  17:        0x10052afa0 - <rustdoc[e386f36fe3828e32]::passes::collect_trait_impls::SyntheticImplCollector as rustdoc[e386f36fe3828e32]::visit::DocVisitor>::visit_item
  18:        0x10052b070 - <rustdoc[e386f36fe3828e32]::passes::collect_trait_impls::SyntheticImplCollector as rustdoc[e386f36fe3828e32]::visit::DocVisitor>::visit_item
  19:        0x10052c36c - <rustdoc[e386f36fe3828e32]::passes::collect_trait_impls::SyntheticImplCollector as rustdoc[e386f36fe3828e32]::visit::DocVisitor>::visit_crate
  20:        0x10039e61c - <rustc_session[b33ec7bea72ab9c0]::session::Session>::time::<alloc[480d25cc58e20263]::vec::Vec<rustdoc[e386f36fe3828e32]::clean::types::Item>, rustdoc[e386f36fe3828e32]::passes::collect_trait_impls::collect_trait_impls::{closure#0}>
  21:        0x100528cfc - rustdoc[e386f36fe3828e32]::passes::collect_trait_impls::collect_trait_impls
  22:        0x10039e914 - <rustc_session[b33ec7bea72ab9c0]::session::Session>::time::<rustdoc[e386f36fe3828e32]::clean::types::Crate, rustdoc[e386f36fe3828e32]::core::run_global_ctxt::{closure#7}>
  23:        0x100526c94 - rustdoc[e386f36fe3828e32]::core::run_global_ctxt
  24:        0x10039ec58 - <rustc_session[b33ec7bea72ab9c0]::session::Session>::time::<(rustdoc[e386f36fe3828e32]::clean::types::Crate, rustdoc[e386f36fe3828e32]::config::RenderOptions, rustdoc[e386f36fe3828e32]::formats::cache::Cache), rustdoc[e386f36fe3828e32]::main_args::{closure#1}::{closure#0}::{closure#0}::{closure#0}>
  25:        0x10037c4ac - <rustc_middle[69929678d7438a39]::ty::context::GlobalCtxt>::enter::<rustdoc[e386f36fe3828e32]::main_args::{closure#1}::{closure#0}::{closure#0}, core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>>
  26:        0x1003cfb54 - <rustc_interface[f4474a023a28a1e1]::interface::Compiler>::enter::<rustdoc[e386f36fe3828e32]::main_args::{closure#1}::{closure#0}, core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>>
  27:        0x10047b0b4 - rustc_span[131a45c9b4032fd]::set_source_map::<core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>, rustc_interface[f4474a023a28a1e1]::interface::run_compiler<core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>, rustdoc[e386f36fe3828e32]::main_args::{closure#1}>::{closure#0}::{closure#0}>
  28:        0x100533b38 - <scoped_tls[14441d0542f530d4]::ScopedKey<rustc_span[131a45c9b4032fd]::SessionGlobals>>::set::<rustc_interface[f4474a023a28a1e1]::interface::run_compiler<core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>, rustdoc[e386f36fe3828e32]::main_args::{closure#1}>::{closure#0}, core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>>
  29:        0x1003ad540 - std[305047cfbd8f31b0]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[f4474a023a28a1e1]::util::run_in_thread_pool_with_globals<rustc_interface[f4474a023a28a1e1]::interface::run_compiler<core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>, rustdoc[e386f36fe3828e32]::main_args::{closure#1}>::{closure#0}, core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>>
  30:        0x100495fb0 - <<std[305047cfbd8f31b0]::thread::Builder>::spawn_unchecked_<rustc_interface[f4474a023a28a1e1]::util::run_in_thread_pool_with_globals<rustc_interface[f4474a023a28a1e1]::interface::run_compiler<core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>, rustdoc[e386f36fe3828e32]::main_args::{closure#1}>::{closure#0}, core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[f69943f7197f08d]::result::Result<(), rustc_span[131a45c9b4032fd]::ErrorGuaranteed>>::{closure#1} as core[f69943f7197f08d]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  31:        0x1015cfa6c - std::sys::unix::thread::Thread::new::thread_start::hdf5e17a1fe30b937
  32:        0x191d6bfa8 - __pthread_joiner_wake

error: the compiler unexpectedly panicked. this is a bug.

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

note: rustc 1.72.0-nightly (fe7454bf4 2023-06-19) running on aarch64-apple-darwin

query stack during panic:
end of query stack

[fmease]

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

T-compiler [next to T-rustdoc]

The bug originates in AutoTraitFinder which lives in rustc, so having both team labels is valid.

[apiraino]

Bisection seems to point to nightly-2022-10-24, one of these:

commit[0] 2022-10-22: Auto merge of #103240 - BelovDV:issue-102290, r=petrochenkov
commit[1] 2022-10-23: Auto merge of #102660 - camsteffen:uninhabited-perf, r=oli-obk
commit[2] 2022-10-23: Auto merge of #103426 - matthiaskrgr:rollup-n6dqdy8, r=matthiaskrgr
commit[3] 2022-10-23: Auto merge of #103345 - Nilstrieb:diag-flat, r=compiler-errors
commit[4] 2022-10-23: Auto merge of #103431 - Dylan-DPC:rollup-oozfo89, r=Dylan-DPC
commit[5] 2022-10-23: Auto merge of #101403 - bjorn3:dylib_lto, r=Mark-Simulacrum
commit[6] 2022-10-23: Auto merge of #103137 - dtolnay:readdir, r=Mark-Simulacrum

[fmease]

Out of those, #103368 looks the most likely (from Auto merge of #103426 - matthiaskrgr:rollup-n6dqdy8, r=matthiaskrgr).

[fmease]

I can no longer reproduce this. Fixed: ICE → errors. Probably fixed by one of those PRs that made rustdoc run more of rustc's passes.