Rustdoc regression (overflow evaluating the requirement …)

[weiznich]

Code

I tried this code:

pub trait Query {}

pub trait AsQuery {
    type Query;
}

impl<T: Query> AsQuery for T {
    type Query = T;
}

pub trait SelectDsl<Selection> {
    type Output;
}

impl<T, Selection> SelectDsl<Selection> for T
where
    T: AsQuery,
    T::Query: SelectDsl<Selection>,
{
    type Output = <T::Query as SelectDsl<Selection>>::Output;
}

pub type Select<Source, Selection> = <Source as SelectDsl<Selection>>::Output;

I expected to see this happen: cargo doc --no-deps succeeds

Instead, this happened: Running cargo doc --no-deps failed with the following error message:

error[E0275]: overflow evaluating the requirement `<Source as SelectDsl<Selection>>::Output == _`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0275`.
error: could not document `rustdoc_regression`

(Note that compiling/checking that code via cargo run/cargo check will just work fine)

Version it worked on

It most recently worked on: rustdoc 1.50.0-nightly (b48cafd9e 2020-11-25)

Version with regression

rustdoc --version:

rustdoc 1.50.0-nightly (72da5a9d8 2020-11-26)
binary: rustdoc
commit-hash: 72da5a9d85a522b11e80d0fdd1fd95247d442604
commit-date: 2020-11-26
host: x86_64-unknown-linux-gnu

If you know when this regression occurred, please add a line like below, replacing {channel} with one of stable, beta, or nightly.

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

[weiznich]

Probably caused by #77467, so cc @jyn514

[pksunkara]

Side question: Can we add diesel to rust test suite?

[Aaron1011]

This also happens when documenting a crate with typenum as a dependency.

It's caused by this code in rustdoc:

rust/src/librustdoc/clean/mod.rs

Lines 1511 to 1518 in 361543d

	// Try to normalize `<X as Y>::T` to a type
	let lifted = ty.lift_to_tcx(cx.tcx).unwrap();
	let normalized = cx.tcx.infer_ctxt().enter(|infcx| {
	infcx
	.at(&ObligationCause::dummy(), cx.param_env.get())
	.normalize(lifted)
	.map(|resolved| infcx.resolve_vars_if_possible(resolved.value))
	});

When we call infcx.at(...).normalize, trait overflow errors result in an error being reported and compilation being aborted, instead of being propagated back to the caller.

[jyn514]

Backtrace

error[E0275]: overflow evaluating the requirement `<Source as SelectDsl<Selection>>::Output == _`

thread 'rustc' panicked at 'aborting due to `-Z treat-err-as-bug=1`', compiler/rustc_errors/src/lib.rs:990:27
stack backtrace:
   0: std::panicking::begin_panic
             at ./library/std/src/panicking.rs:519:12
   1: rustc_errors::HandlerInner::panic_if_treat_err_as_bug
             at ./compiler/rustc_errors/src/lib.rs:990:27
   2: rustc_errors::HandlerInner::bump_err_count
             at ./compiler/rustc_errors/src/lib.rs:980:9
   3: rustc_errors::HandlerInner::emit_diagnostic
             at ./compiler/rustc_errors/src/lib.rs:796:13
   4: rustc_errors::Handler::emit_diagnostic
             at ./compiler/rustc_errors/src/lib.rs:715:9
   5: rustc_errors::diagnostic_builder::DiagnosticBuilder::emit
             at ./compiler/rustc_errors/src/diagnostic_builder.rs:101:9
   6: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtExt>::report_overflow_error
             at ./compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs:206:9
   7: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtExt>::report_overflow_error_cycle
             at ./compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs:222:9
   8: <rustc_trait_selection::traits::fulfill::FulfillProcessor as rustc_data_structures::obligation_forest::ObligationProcessor>::process_backedge
             at ./compiler/rustc_trait_selection/src/traits/fulfill.rs:321:13
   9: rustc_data_structures::obligation_forest::ObligationForest<O>::find_cycles_from_node
             at ./compiler/rustc_data_structures/src/obligation_forest/mod.rs:598:21
  10: rustc_data_structures::obligation_forest::ObligationForest<O>::find_cycles_from_node
             at ./compiler/rustc_data_structures/src/obligation_forest/mod.rs:591:25
  11: rustc_data_structures::obligation_forest::ObligationForest<O>::process_cycles
             at ./compiler/rustc_data_structures/src/obligation_forest/mod.rs:573:17
  12: rustc_data_structures::obligation_forest::ObligationForest<O>::process_obligations
             at ./compiler/rustc_data_structures/src/obligation_forest/mod.rs:478:13
  13: rustc_trait_selection::traits::fulfill::FulfillmentContext::select
             at ./compiler/rustc_trait_selection/src/traits/fulfill.rs:133:17
  14: <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_where_possible
             at ./compiler/rustc_trait_selection/src/traits/fulfill.rs:235:9
  15: rustc_infer::infer::canonical::query_response::<impl rustc_infer::infer::InferCtxt>::make_query_response
             at ./compiler/rustc_infer/src/infer/canonical/query_response.rs:115:27
  16: rustc_infer::infer::canonical::query_response::<impl rustc_infer::infer::InferCtxt>::make_canonicalized_query_response
             at ./compiler/rustc_infer/src/infer/canonical/query_response.rs:61:30
  17: <rustc_infer::infer::InferCtxtBuilder as rustc_trait_selection::infer::InferCtxtBuilderExt>::enter_canonical_trait_query::{{closure}}
             at ./compiler/rustc_trait_selection/src/infer.rs:129:17
  18: rustc_infer::infer::InferCtxtBuilder::enter_with_canonical::{{closure}}
             at ./compiler/rustc_infer/src/infer/mod.rs:576:13
  19: rustc_infer::infer::InferCtxtBuilder::enter
             at ./compiler/rustc_infer/src/infer/mod.rs:583:9
  20: rustc_infer::infer::InferCtxtBuilder::enter_with_canonical
             at ./compiler/rustc_infer/src/infer/mod.rs:573:9
  21: <rustc_infer::infer::InferCtxtBuilder as rustc_trait_selection::infer::InferCtxtBuilderExt>::enter_canonical_trait_query
             at ./compiler/rustc_trait_selection/src/infer.rs:123:9
  22: rustc_traits::normalize_projection_ty::normalize_projection_ty
             at ./compiler/rustc_traits/src/normalize_projection_ty.rs:24:5
  23: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::normalize_projection_ty>::compute
             at ./compiler/rustc_middle/src/ty/query/plumbing.rs:377:17
  24: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
             at ./compiler/rustc_query_system/src/dep_graph/graph.rs:301:14
  25: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
             at ./compiler/rustc_query_system/src/dep_graph/graph.rs:200:9
  26: rustc_query_system::query::plumbing::force_query_with_job::{{closure}}::{{closure}}
             at ./compiler/rustc_query_system/src/query/plumbing.rs:608:17
  27: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}::{{closure}}
             at ./compiler/rustc_middle/src/ty/query/plumbing.rs:73:74
  28: stacker::maybe_grow
             at /home/joshua/.local/lib/cargo/registry/src/github.com-1ecc6299db9ec823/stacker-0.1.12/src/lib.rs:55:9
  29: rustc_data_structures::stack::ensure_sufficient_stack
             at ./compiler/rustc_data_structures/src/stack.rs:16:5
  30: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}
             at ./compiler/rustc_middle/src/ty/query/plumbing.rs:73:17
  31: rustc_middle::ty::context::tls::enter_context::{{closure}}
             at ./compiler/rustc_middle/src/ty/context.rs:1739:50
  32: rustc_middle::ty::context::tls::set_tlv
             at ./compiler/rustc_middle/src/ty/context.rs:1723:9
  33: rustc_middle::ty::context::tls::enter_context
             at ./compiler/rustc_middle/src/ty/context.rs:1739:9
  34: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}
             at ./compiler/rustc_middle/src/ty/query/plumbing.rs:72:13
  35: rustc_middle::ty::context::tls::with_related_context::{{closure}}
             at ./compiler/rustc_middle/src/ty/context.rs:1783:13
  36: rustc_middle::ty::context::tls::with_context::{{closure}}
             at ./compiler/rustc_middle/src/ty/context.rs:1767:40
  37: rustc_middle::ty::context::tls::with_context_opt
             at ./compiler/rustc_middle/src/ty/context.rs:1756:22
  38: rustc_middle::ty::context::tls::with_context
             at ./compiler/rustc_middle/src/ty/context.rs:1767:9
  39: rustc_middle::ty::context::tls::with_related_context
             at ./compiler/rustc_middle/src/ty/context.rs:1780:9
  40: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query
             at ./compiler/rustc_middle/src/ty/query/plumbing.rs:61:9
  41: rustc_query_system::query::plumbing::force_query_with_job::{{closure}}
             at ./compiler/rustc_query_system/src/query/plumbing.rs:598:9
  42: rustc_query_system::query::plumbing::with_diagnostics
             at ./compiler/rustc_query_system/src/query/plumbing.rs:302:18
  43: rustc_query_system::query::plumbing::force_query_with_job
             at ./compiler/rustc_query_system/src/query/plumbing.rs:597:51
  44: rustc_query_system::query::plumbing::try_execute_query
             at ./compiler/rustc_query_system/src/query/plumbing.rs:426:16
  45: rustc_query_system::query::plumbing::get_query_impl::{{closure}}
             at ./compiler/rustc_query_system/src/query/plumbing.rs:645:23
  46: <rustc_query_system::query::caches::DefaultCache<K,V> as rustc_query_system::query::caches::QueryCache>::lookup
             at ./compiler/rustc_query_system/src/query/caches.rs:114:79
  47: rustc_query_system::query::plumbing::try_get_cached
             at ./compiler/rustc_query_system/src/query/plumbing.rs:379:5
  48: rustc_query_system::query::plumbing::get_query_impl
             at ./compiler/rustc_query_system/src/query/plumbing.rs:637:5
  49: rustc_query_system::query::plumbing::get_query
             at ./compiler/rustc_query_system/src/query/plumbing.rs:739:5
  50: rustc_middle::ty::query::TyCtxtAt::normalize_projection_ty
             at ./compiler/rustc_middle/src/ty/query/plumbing.rs:487:17
  51: rustc_middle::ty::query::<impl rustc_middle::ty::context::TyCtxt>::normalize_projection_ty
             at ./compiler/rustc_middle/src/ty/query/plumbing.rs:448:17
  52: <rustc_trait_selection::traits::query::normalize::QueryNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty::{{closure}}
             at ./compiler/rustc_trait_selection/src/traits/query/normalize.rs:172:23
  53: <rustc_trait_selection::traits::query::normalize::QueryNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty
             at ./compiler/rustc_trait_selection/src/traits/query/normalize.rs:111:19
  54: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for &rustc_middle::ty::TyS>::fold_with
             at ./compiler/rustc_middle/src/ty/structural_impls.rs:969:9
  55: <rustc_infer::infer::at::At as rustc_trait_selection::traits::query::normalize::AtExt>::normalize
             at ./compiler/rustc_trait_selection/src/traits/query/normalize.rs:65:22
  56: rustdoc::clean::normalize::{{closure}}
             at ./src/librustdoc/clean/mod.rs:1514:9
  57: rustc_infer::infer::InferCtxtBuilder::enter
             at ./compiler/rustc_infer/src/infer/mod.rs:583:9
  58: rustdoc::clean::normalize
             at ./src/librustdoc/clean/mod.rs:1513:22
  59: rustdoc::clean::clean_qpath
             at ./src/librustdoc/clean/mod.rs:1404:45
  60: <rustc_hir::hir::Ty as rustdoc::clean::Clean<rustdoc::clean::types::Type>>::clean
             at ./src/librustdoc/clean/mod.rs:1477:32
  61: <(&rustc_hir::hir::Item,core::option::Option<rustc_span::symbol::Ident>) as rustdoc::clean::Clean<alloc::vec::Vec<rustdoc::clean::types::Item>>>::clean::{{closure}}
             at ./src/librustdoc/clean/mod.rs:2005:38
  62: rustdoc::core::DocContext::with_param_env
             at ./src/librustdoc/core.rs:88:19
  63: <(&rustc_hir::hir::Item,core::option::Option<rustc_span::symbol::Ident>) as rustdoc::clean::Clean<alloc::vec::Vec<rustdoc::clean::types::Item>>>::clean
             at ./src/librustdoc/clean/mod.rs:1987:9
  64: <rustdoc::doctree::Module as rustdoc::clean::Clean<rustdoc::clean::types::Item>>::clean::{{closure}}
             at ./src/librustdoc/clean/mod.rs:235:48
  65: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once
             at ./library/core/src/ops/function.rs:280:13
  66: core::option::Option<T>::map
             at ./library/core/src/option.rs:453:29
  67: <core::iter::adapters::map::Map<I,F> as core::iter::traits::iterator::Iterator>::next
             at ./library/core/src/iter/adapters/map.rs:99:9
  68: <core::iter::adapters::fuse::Fuse<I> as core::iter::adapters::fuse::FuseImpl<I>>::next
             at ./library/core/src/iter/adapters/fuse.rs:407:9
  69: <core::iter::adapters::fuse::Fuse<I> as core::iter::traits::iterator::Iterator>::next
             at ./library/core/src/iter/adapters/fuse.rs:65:9
  70: <core::iter::adapters::flatten::FlattenCompat<I,U> as core::iter::traits::iterator::Iterator>::next
             at ./library/core/src/iter/adapters/flatten.rs:268:19
  71: <core::iter::adapters::flatten::Flatten<I> as core::iter::traits::iterator::Iterator>::next
             at ./library/core/src/iter/adapters/flatten.rs:169:9
  72: alloc::vec::Vec<T,A>::extend_desugared
             at ./library/alloc/src/vec.rs:2618:35
  73: <alloc::vec::Vec<T,A> as alloc::vec::SpecExtend<T,I>>::spec_extend
             at ./library/alloc/src/vec.rs:2541:9
  74: <alloc::vec::Vec<T,A> as core::iter::traits::collect::Extend<T>>::extend
             at ./library/alloc/src/vec.rs:2251:9
  75: <rustdoc::doctree::Module as rustdoc::clean::Clean<rustdoc::clean::types::Item>>::clean
             at ./src/librustdoc/clean/mod.rs:235:9
  76: rustdoc::clean::utils::krate
             at ./src/librustdoc/clean/utils.rs:44:22
  77: rustdoc::core::run_global_ctxt::{{closure}}
             at ./src/librustdoc/core.rs:560:53
  78: rustc_data_structures::profiling::VerboseTimingGuard::run
             at ./compiler/rustc_data_structures/src/profiling.rs:570:9
  79: rustc_session::utils::<impl rustc_session::session::Session>::time
             at ./compiler/rustc_session/src/utils.rs:9:9
  80: rustdoc::core::run_global_ctxt
             at ./src/librustdoc/core.rs:560:21
  81: rustdoc::core::run_core::{{closure}}::{{closure}}::{{closure}}::{{closure}}
             at ./src/librustdoc/core.rs:469:21
  82: rustc_interface::passes::QueryContext::enter::{{closure}}
             at ./compiler/rustc_interface/src/passes.rs:725:42
  83: rustc_middle::ty::context::tls::enter_context::{{closure}}
             at ./compiler/rustc_middle/src/ty/context.rs:1739:50
  84: rustc_middle::ty::context::tls::set_tlv
             at ./compiler/rustc_middle/src/ty/context.rs:1723:9
  85: rustc_middle::ty::context::tls::enter_context
             at ./compiler/rustc_middle/src/ty/context.rs:1739:9
  86: rustc_interface::passes::QueryContext::enter
             at ./compiler/rustc_interface/src/passes.rs:725:9
  87: rustdoc::core::run_core::{{closure}}::{{closure}}::{{closure}}
             at ./src/librustdoc/core.rs:468:17
  88: rustc_data_structures::profiling::VerboseTimingGuard::run
             at ./compiler/rustc_data_structures/src/profiling.rs:570:9
  89: rustc_session::utils::<impl rustc_session::session::Session>::time
             at ./compiler/rustc_session/src/utils.rs:9:9
  90: rustdoc::core::run_core::{{closure}}::{{closure}}
             at ./src/librustdoc/core.rs:467:46
  91: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
             at ./compiler/rustc_interface/src/queries.rs:415:19
  92: rustdoc::core::run_core::{{closure}}
             at ./src/librustdoc/core.rs:426:9
  93: rustc_interface::interface::create_compiler_and_run::{{closure}}
             at ./compiler/rustc_interface/src/interface.rs:196:13
  94: rustc_span::with_source_map
             at ./compiler/rustc_span/src/lib.rs:765:5
  95: rustc_interface::interface::create_compiler_and_run
             at ./compiler/rustc_interface/src/interface.rs:190:5
  96: rustdoc::core::run_core
             at ./src/librustdoc/core.rs:425:5
  97: rustdoc::main_options
             at ./src/librustdoc/lib.rs:531:53
  98: rustdoc::main_args::{{closure}}
             at ./src/librustdoc/lib.rs:464:17
  99: rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals::{{closure}}::{{closure}}
             at ./compiler/rustc_interface/src/util.rs:152:13
 100: scoped_tls::ScopedKey<T>::set
             at /home/joshua/.local/lib/cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137:9
 101: rustc_span::with_session_globals
             at ./compiler/rustc_span/src/lib.rs:94:5
 102: rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals::{{closure}}
             at ./compiler/rustc_interface/src/util.rs:150:9
 103: rustc_interface::util::scoped_thread::{{closure}}
             at ./compiler/rustc_interface/src/util.rs:125:24
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

So we need to change

rust/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

Line 208 in 361543d

bug!();

not to abort.

This comment was added in 8bd8466:

rust/compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

Lines 171 to 176 in 361543d

	/// Reports that an overflow has occurred and halts compilation. We
	/// halt compilation unconditionally because it is important that
	/// overflows never be masked -- they basically represent computations
	/// whose result could not be truly determined and thus we can't say
	/// if the program type checks or not -- and they are unusual
	/// occurrences in any case.

@nikomatsakis would it maybe be possible for the caller to say whether it wants to abort or not?

[jyn514]

Actually I guess the issue is that the type length limit code shouldn't be called at all, otherwise it will still emit an error even if it doesn't abort.

[jyn514]

I guess in the meantime #77467 should probably be reverted :(

[Aaron1011]

@jyn514: Simply disabling the bug!() call would just cause us to try to continue compilation after an error has already been reported. Rustdoc would then need to suppress this error somehow (since it's a "speculative" error, we don't want to show it to the user).

I think one solution would be to run the normalization in TraitQueryMode::Canonical, which propagates overflow error to the caller instead of reporting a user-facing error. Unfortunately, the normalization code creates a SelectionContext behind several layers of indirection, so passing along a TraitQueryMode could be fairly ugly.

[Aaron1011]

As a side note, this ended up putting the CI workflow for https://github.com/rust-lang/cargo in a broken state, as this overflow occurs when trying to document cargo on the latest nightly. See rust-lang/cargo#8906 for an example of this.

If we have the CI budget, I think it would be a good idea to generate (but not upload) the docs for tools, to ensure that this kind of problem can't happen again.