rustdoc: "Unexpected predicate" when using const_generics.

[rot256]

Code

When running cargo doc within a library containing a struct holding the Hasher from the blake3 crate:

#![feature(const_generics)]
#![allow(incomplete_features)]

use blake3::Hasher;

pub struct Foo {
    hasher: Hasher,
}

It results in rustdoc crashing with Unexpected predicate Foo ConstEquate(Const { ty: usize, val: Unevaluated(WithOptConstParam { did: DefId(15:602 ~ blake3[dd2c]::Hasher[0]::cv_stack[0]::{{constant}}[0]), const_param_did: None }, [], None) }, Const { ty: usize, val: Value(Scalar(0x0000000000000037)) }).

The full library code (including Cargo.toml and Cargo.lock)

The behaviour is only triggered when the "Hasher" struct is encapsulated in another struct, nor does it occur if const_generics is not enabled. The blake3 crate does not make use of const generics.

Meta

I am running the recent nightly toolchain:

rustc --version --verbose:

rustc 1.47.0-nightly (6c8927b0c 2020-07-26)
binary: rustc
commit-hash: 6c8927b0cf80ceee19386026cf9d7fd4fd9d486f
commit-date: 2020-07-26
host: x86_64-unknown-linux-gnu
release: 1.47.0-nightly
LLVM version: 10.0

Error output

$ RUST_BACKTRACE=full rustdoc --edition=2018 --crate-type lib --crate-name doc_bug src/lib.rs -o ./target/doc --error-format=json --json=diagnostic-rendered-ansi -L dependency=./target/debug/deps --extern blake3=./target/debug/deps/libblake3-543e2512cd3d94ea.rmeta  thread 'rustc' panicked at 'Unexpected predicate Foo ConstEquate(Const { ty: usize, val: Unevaluated(WithOptConstParam { did: DefId(15:602 ~ blake3[dd2c]::Hasher[0]::cv_stack[0]::{{constant}}[0]), const_param_did: None }, [], None) }, Const { ty: usize, val: Value(Scalar(0x0000000000000037)) })', src/librustc_trait_selection/traits/auto_trait.rs:801:22
stack backtrace:
   0:     0x7f8532360215 - backtrace::backtrace::libunwind::trace::h75aedf5f78e5147f
                               at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1:     0x7f8532360215 - backtrace::backtrace::trace_unsynchronized::h18fb73c9ac9ae753
                               at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2:     0x7f8532360215 - std::sys_common::backtrace::_print_fmt::h65f97470ff13ec84
                               at src/libstd/sys_common/backtrace.rs:78
   3:     0x7f8532360215 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hee061c54ddc9f024
                               at src/libstd/sys_common/backtrace.rs:59
   4:     0x7f853239c41c - core::fmt::write::hfbd2baad61ed21a8
                               at src/libcore/fmt/mod.rs:1117
   5:     0x7f8532351f52 - std::io::Write::write_fmt::h72f9bd227f40dc62
                               at src/libstd/io/mod.rs:1508
   6:     0x7f85323650b0 - std::sys_common::backtrace::_print::h2d2cd8fe02feb5fa
                               at src/libstd/sys_common/backtrace.rs:62
   7:     0x7f85323650b0 - std::sys_common::backtrace::print::h801b12991252ba7c
                               at src/libstd/sys_common/backtrace.rs:49
   8:     0x7f85323650b0 - std::panicking::default_hook::{{closure}}::h25fc1fbf3b63b5c8
                               at src/libstd/panicking.rs:198
   9:     0x7f8532364dfc - std::panicking::default_hook::h62c897957a5e0f26
                               at src/libstd/panicking.rs:217
  10:     0x7f8532ae9719 - rustc_driver::report_ice::h54e3b417084b44b0
  11:     0x7f8532365828 - std::panicking::rust_panic_with_hook::hb8a276f163c59810
                               at src/libstd/panicking.rs:530
  12:     0x7f85323653fb - rust_begin_unwind
                               at src/libstd/panicking.rs:437
  13:     0x7f853236536b - std::panicking::begin_panic_fmt::h085a1206d7d8b985
                               at src/libstd/panicking.rs:391
  14:     0x7f8534c15cb7 - rustc_trait_selection::traits::auto_trait::AutoTraitFinder::evaluate_nested_obligations::hbbf5d06450f1333f
  15:     0x7f8534c1309c - rustc_trait_selection::traits::auto_trait::AutoTraitFinder::evaluate_predicates::h9a6435bf8beb4278
  16:     0x555cd7f03ae6 - rustc_infer::infer::InferCtxtBuilder::enter::h4a537f659c968915
  17:     0x555cd7f2a025 - rustc_trait_selection::traits::auto_trait::AutoTraitFinder::find_auto_trait_generics::h6deecb901d30d42c
  18:     0x555cd7d1ea2f - core::ops::function::impls::<impl core::ops::function::FnMut<A> for &mut F>::call_mut::h99a4206d7f972dd8
  19:     0x555cd7fa0f27 - <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter::h20ff2f15605a4d5a
  20:     0x555cd7de5d35 - rustdoc::clean::utils::get_auto_trait_and_blanket_impls::h7e25d90c0fd8ac60
  21:     0x555cd7dec643 - <rustdoc::passes::collect_trait_impls::SyntheticImplCollector as rustdoc::fold::DocFolder>::fold_item::hb38a36b935282f90
  22:     0x555cd7f9f78c - <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter::hf6d04521ed2a610e
  23:     0x555cd7de8e6e - rustdoc::fold::DocFolder::fold_inner_recur::h466652c9451b0c85
  24:     0x555cd7dec2d5 - <rustdoc::passes::collect_trait_impls::SyntheticImplCollector as rustdoc::fold::DocFolder>::fold_item::hb38a36b935282f90
  25:     0x555cd7dea025 - rustdoc::passes::collect_trait_impls::collect_trait_impls::h541e7b3ea032b9a3
  26:     0x555cd7ea8a8f - rustc_middle::ty::context::tls::enter_global::h53bbe2472ef4ec19
  27:     0x555cd7d1c6ce - rustc_interface::interface::create_compiler_and_run::h09163cc735ce1db9
  28:     0x555cd800d861 - rustdoc::core::run_core::h2c398e47a2530502
  29:     0x555cd7f22b30 - <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::haba510a0466e0ae6
  30:     0x555cd7f05992 - rustc_driver::catch_fatal_errors::h55a0028df8fc48bd
  31:     0x555cd7d10109 - scoped_tls::ScopedKey<T>::set::h1ec574fa30dfcac7
  32:     0x555cd8004492 - rustc_ast::attr::with_session_globals::h40b92d81c6d9cab3
  33:     0x555cd7d1d450 - std::sys_common::backtrace::__rust_begin_short_backtrace::h2998d973725ba510
  34:     0x555cd7ee72be - core::ops::function::FnOnce::call_once{{vtable.shim}}::ha96eb98837a026a5
  35:     0x7f8532373d3a - <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once::h3e41d894970cea2f
                               at /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/liballoc/boxed.rs:1074
  36:     0x7f8532373d3a - <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once::h9a318bb00b7f8871
                               at /rustc/6c8927b0cf80ceee19386026cf9d7fd4fd9d486f/src/liballoc/boxed.rs:1074
  37:     0x7f8532373d3a - std::sys::unix::thread::Thread::new::thread_start::hf522342530b04cb6
                               at src/libstd/sys/unix/thread.rs:87
  38:     0x7f85322a3f27 - start_thread
                               at ./nptl/pthread_create.c:479
  39:     0x7f85321b931f - __clone
                               at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
  40:                0x0 - <unknown>

error: internal compiler error: unexpected panic

note: 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-compiler&template=ice.md

note: rustc 1.47.0-nightly (6c8927b0c 2020-07-26) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib

query stack during panic:
end of query stack