ICE: assertion failed: !ty.has_vars_bound_at_or_above() , glacier fixed/76826.rs with --emit=mir -Zpolymorphize=on

[matthiaskrgr]

Code

code from glacier fixed/76826.rs

#![feature(generic_associated_types)]

pub trait Iter {
    type Item<'a> where Self: 'a;

    fn next<'a>(&'a mut self) -> Option<Self::Item<'a>>;

    fn for_each<F>(mut self, mut f: F)
        where Self: Sized, F: for<'a> FnMut(Self::Item<'a>)
    {
        while let Some(item) = self.next() {
            f(item);
        }
    }
}

pub struct Windows<T> {
    items: Vec<T>,
    start: usize,
    len: usize,
}

impl<T> Windows<T> {
    pub fn new(items: Vec<T>, len: usize) -> Self {
        Self { items, start: 0, len }
    }
}

impl<T> Iter for Windows<T> {
    type Item<'a> where T: 'a = &'a mut [T];

    fn next<'a>(&'a mut self) -> Option<Self::Item<'a>> {
        let slice = self.items.get_mut(self.start..self.start + self.len)?;
        self.start += 1;
        Some(slice)
    }
}

fn main() {
    Windows::new(vec![1, 2, 3, 4, 5], 3)
        .for_each(|slice| println!("{:?}", slice));
}

Meta

rustc --version --verbose:

rustc 1.55.0-nightly (74ef0c3e4 2021-07-16)
binary: rustc
commit-hash: 74ef0c3e404cc72c08b2d1e14506f90d9e877269
commit-date: 2021-07-16
host: x86_64-unknown-linux-gnu
release: 1.55.0-nightly
LLVM version: 12.0.1

Error output

rustc glacier/fixed/76826.rs --emit=mir -Zpolymorphize=on

thread 'rustc' panicked at 'assertion failed: !ty.has_vars_bound_at_or_above(ty::DebruijnIndex::from_usize(self.universes.len()))', compiler/rustc_trait_selection/src/traits/query/normalize.rs:232:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

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

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

note: rustc 1.55.0-nightly (74ef0c3e4 2021-07-16) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z polymorphize=on

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `std::option::Option<<Windows<T> as Iter>::Item<'_>>`
end of query stack

Backtrace

thread 'rustc' panicked at 'assertion failed: !ty.has_vars_bound_at_or_above(ty::DebruijnIndex::from_usize(self.universes.len()))', compiler/rustc_trait_selection/src/traits/query/normalize.rs:232:17
stack backtrace:
   0:     0x7f22a1f2af00 - std::backtrace_rs::backtrace::libunwind::trace::h0f5cd2ee8b0d7274
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/../../backtrace/src/backtrace/libunwind.rs:90:5
   1:     0x7f22a1f2af00 - std::backtrace_rs::backtrace::trace_unsynchronized::h06905b5aeda069a1
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f22a1f2af00 - std::sys_common::backtrace::_print_fmt::h4fe4c7c875072f30
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7f22a1f2af00 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hcc0746f004a9b7ef
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/sys_common/backtrace.rs:46:22
   4:     0x7f22a1f98cec - core::fmt::write::h9a6d9c74526a6c1b
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/core/src/fmt/mod.rs:1115:17
   5:     0x7f22a1f1c655 - std::io::Write::write_fmt::h23dab4cc9ce72ee2
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/io/mod.rs:1663:15
   6:     0x7f22a1f2ec1b - std::sys_common::backtrace::_print::h173dc702502d65d2
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/sys_common/backtrace.rs:49:5
   7:     0x7f22a1f2ec1b - std::sys_common::backtrace::print::h61bd27c4742ba817
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/sys_common/backtrace.rs:36:9
   8:     0x7f22a1f2ec1b - std::panicking::default_hook::{{closure}}::hcaae87f0495ae613
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/panicking.rs:208:50
   9:     0x7f22a1f2e6f1 - std::panicking::default_hook::h0538e728ee080db0
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/panicking.rs:225:9
  10:     0x7f22a27051c1 - rustc_driver::DEFAULT_HOOK::{{closure}}::{{closure}}::h432ae30fc20e6372
  11:     0x7f22a1f2f449 - std::panicking::rust_panic_with_hook::h3039e236b6ca482c
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/panicking.rs:626:17
  12:     0x7f22a1f2eed7 - std::panicking::begin_panic_handler::{{closure}}::h884fbab544ffd91c
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/panicking.rs:517:13
  13:     0x7f22a1f2b3fc - std::sys_common::backtrace::__rust_end_short_backtrace::hdaf2e18ba3d91210
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/sys_common/backtrace.rs:141:18
  14:     0x7f22a1f2ee69 - rust_begin_unwind
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/panicking.rs:515:5
  15:     0x7f22a1ef9e71 - core::panicking::panic_fmt::hcf5f6d96e1dd7099
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/core/src/panicking.rs:92:14
  16:     0x7f22a1ef9dbd - core::panicking::panic::hd695e3b1d0dd4ef4
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/core/src/panicking.rs:50:5
  17:     0x7f22a409d4b5 - <rustc_trait_selection::traits::query::normalize::QueryNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty::hc95a20e7cdd22597
  18:     0x7f22a403fa30 - rustc_middle::ty::fold::TypeFoldable::fold_with::h954fc24bdeb2e96e
  19:     0x7f22a408d66a - rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for &rustc_middle::ty::TyS>::super_fold_with::h8ceca8e018b93ffa
  20:     0x7f22a409c4f6 - <rustc_trait_selection::traits::query::normalize::QueryNormalizer as rustc_middle::ty::fold::TypeFolder>::fold_ty::hc95a20e7cdd22597
  21:     0x7f22a3899970 - <rustc_infer::infer::at::At as rustc_trait_selection::traits::query::normalize::AtExt>::normalize::hb71d72f0ba35c667
  22:     0x7f22a388c99d - rustc_infer::infer::InferCtxtBuilder::enter::hab199ae1631b39e8
  23:     0x7f22a389af68 - core::ops::function::FnOnce::call_once::h24f16da279ed7bd7
  24:     0x7f22a3b1154d - rustc_query_system::query::plumbing::get_query_impl::hd825fc255c29519f
  25:     0x7f22a3b5c53f - <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::normalize_generic_arg_after_erasing_regions::h062c0c4d5027d100
  26:     0x7f22a41841d0 - rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder::normalize_generic_arg_after_erasing_regions::hcd821ce068baf57a
  27:     0x7f22a36095cb - rustc_middle::ty::util::fold_list::h128f20764f893854
  28:     0x7f22a3660f3b - rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for rustc_middle::ty::sty::Binder<T>>::super_fold_with::hb4b89ad0eb294be5
  29:     0x7f22a36216a4 - rustc_middle::ty::fold::TypeFolder::fold_binder::h3d44401b2d50c4c0
  30:     0x7f22a359dbb5 - rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::normalize_erasing_regions::hb7a1178a524e7b35
  31:     0x7f22a41777ec - rustc_middle::ty::layout::<impl rustc_middle::ty::instance::Instance>::fn_sig_for_fn_abi::hd97e417fc8900b14
  32:     0x7f22a3863487 - <rustc_target::abi::call::FnAbi<&rustc_middle::ty::TyS> as rustc_middle::ty::layout::FnAbiExt<C>>::of_instance::h01bf068a1677b1a2
  33:     0x7f22a38571ba - rustc_codegen_llvm::mono_item::<impl rustc_codegen_ssa::traits::declare::PreDefineMethods for rustc_codegen_llvm::context::CodegenCx>::predefine_fn::h9ffd9900040301bc
  34:     0x7f22a3869af3 - rustc_codegen_llvm::base::compile_codegen_unit::module_codegen::hefd9993fd5d8f6af
  35:     0x7f22a42abc07 - rustc_query_system::dep_graph::graph::DepGraph<K>::with_task::h109c11c2d2b2fe9e
  36:     0x7f22a42c31f7 - rustc_codegen_llvm::base::compile_codegen_unit::hd128bb1380560c35
  37:     0x7f22a42b03c6 - <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate::hc25561f12c31c9af
  38:     0x7f22a423246c - rustc_interface::passes::QueryContext::enter::h21fac079e595ecfb
  39:     0x7f22a422a3a6 - rustc_interface::queries::Queries::ongoing_codegen::h26b62c4c2f9af25f
  40:     0x7f22a42091bf - rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter::hd3df2f2fd8ee2f9c
  41:     0x7f22a41f7ecc - rustc_span::with_source_map::h4320e6c7562b1c81
  42:     0x7f22a420a32a - rustc_interface::interface::create_compiler_and_run::h8b004a10f044574a
  43:     0x7f22a41fd219 - scoped_tls::ScopedKey<T>::set::hc495f050953f2646
  44:     0x7f22a41f9fdb - std::sys_common::backtrace::__rust_begin_short_backtrace::hf6d64874d4b63366
  45:     0x7f22a41f8865 - core::ops::function::FnOnce::call_once{{vtable.shim}}::h44f63dc93d63fbac
  46:     0x7f22a1f3b9f7 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hd5b860c6542b008a
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/alloc/src/boxed.rs:1572:9
  47:     0x7f22a1f3b9f7 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hc87b98c8a3a585ed
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/alloc/src/boxed.rs:1572:9
  48:     0x7f22a1f3b9f7 - std::sys::unix::thread::Thread::new::thread_start::ha990175c8f3779cc
                               at /rustc/74ef0c3e404cc72c08b2d1e14506f90d9e877269/library/std/src/sys/unix/thread.rs:74:17
  49:     0x7f22a1e47259 - start_thread
  50:     0x7f22a1d5c5e3 - __GI___clone
  51:                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:/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.55.0-nightly (74ef0c3e4 2021-07-16) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z polymorphize=on

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `std::option::Option<<Windows<T> as Iter>::Item<'_>>`
end of query stack

[jackh726]

Yep, I suppose this is expected. This is caused by

rust/compiler/rustc_middle/src/ty/layout.rs

Line 2486 in f502bd3

ty::FnDef(def_id, substs) if tcx.sess.opts.debugging_opts.polymorphize => tcx

.

While this doesn't have the generic_associated_types feature gate, it isn't a GATs issue. It's due to how we're handling bound vars (in GATs right now, but soon everywhere). The current design of normalize_erasing_regions isn't fully correct, since it normalizes types, rather than values (where the value has a Binder, but we've lost it by the time we normalize the inner types). Turns out we only hit the case where we have both projections and bound vars (in current tests) when -Z polymorphize is on, because of the bit of code linked above.

I'm going to mark this is a polymorphization feature bug, since it is the one that requires the workaround linked above. But we more generally have a problem here with normalize_erasing_regions