[ICE] in normalize_erasing_regions with mir-opt-level=2

[leonardo-m]

use std::mem::MaybeUninit;
const N: usize = 2;

trait CollectArray<A>: Iterator<Item=A> {
    fn inner_array(&mut self) -> [A; N];
    fn collect_array(&mut self) -> [A; N] {
        let result = self.inner_array();
        assert!(self.next().is_none());
        result
    }
}

impl<A, I: ?Sized> CollectArray<A> for I where I: Iterator<Item=A>  {
    fn inner_array(&mut self) -> [A; N] {
        let mut result: [MaybeUninit<A>; N] = unsafe {
            MaybeUninit::uninit().assume_init()
        };
        for (dest, item) in result.iter_mut().zip(self) {
            *dest = MaybeUninit::new(item);
        }
        let temp_ptr: *const [MaybeUninit<A>; N] = &result;
        unsafe { std::ptr::read(temp_ptr as *const [A; N]) }
    }
}

fn main() {
     [[1, 2], [3, 4]]
     .iter()
     .map(|row| row.iter().collect_array())
     .collect_array();
}

Compiled with: rustc -Z mir-opt-level=2

Gives:

error: internal compiler error: compiler\rustc_traits\src\normalize_erasing_regions.rs:37:32: could not fully normalize `<&mut I as std::iter::IntoIterator>::IntoIter`

thread 'rustc' panicked at 'Box<Any>', compiler\rustc_errors\src\lib.rs:945:9
stack backtrace:
   0: std::panicking::begin_panic
   1: rustc_errors::HandlerInner::bug
   2: rustc_errors::Handler::bug
   3: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
   4: rustc_middle::ty::context::tls::with_opt::{{closure}}
   5: rustc_middle::ty::context::tls::with_opt
   6: rustc_middle::util::bug::opt_span_bug_fmt
   7: rustc_middle::util::bug::bug_fmt
   8: rustc_infer::infer::InferCtxtBuilder::enter
   9: rustc_traits::normalize_erasing_regions::normalize_generic_arg_after_erasing_regions
  10: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  11: rustc_data_structures::stack::ensure_sufficient_stack
  12: rustc_query_system::query::plumbing::get_query_impl
  13: rustc_middle::ty::normalize_erasing_regions::<impl rustc_middle::ty::context::TyCtxt>::normalize_erasing_regions
  14: <rustc_middle::ty::layout::LayoutCx<rustc_middle::ty::context::TyCtxt> as rustc_target::abi::LayoutOf>::layout_of
  15: rustc_mir::transform::inline::Inliner::run_pass
  16: <rustc_mir::transform::inline::Inline as rustc_mir::transform::MirPass>::run_pass
  17: rustc_mir::transform::run_passes
  18: rustc_mir::transform::run_optimization_passes
  19: rustc_mir::transform::optimized_mir
  20: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  21: rustc_data_structures::stack::ensure_sufficient_stack
  22: rustc_query_system::query::plumbing::get_query_impl
  23: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::instance_mir
  24: rustc_mir::monomorphize::collector::collect_neighbours
  25: rustc_mir::monomorphize::collector::collect_items_rec
  26: rustc_mir::monomorphize::collector::collect_items_rec
  27: rustc_mir::monomorphize::collector::collect_items_rec
  28: rustc_session::utils::<impl rustc_session::session::Session>::time
  29: rustc_mir::monomorphize::collector::collect_crate_mono_items
  30: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  31: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::collect_and_partition_mono_items>::compute
  32: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  33: rustc_data_structures::stack::ensure_sufficient_stack
  34: rustc_query_system::query::plumbing::get_query_impl
  35: rustc_codegen_ssa::base::codegen_crate
  36: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  37: rustc_interface::passes::QueryContext::enter
  38: rustc_interface::queries::Queries::ongoing_codegen
  39: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  40: rustc_span::with_source_map
  41: rustc_interface::interface::create_compiler_and_run
  42: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.49.0-nightly (beb5ae474 2020-10-04) running on x86_64-pc-windows-gnu

note: compiler flags: -Z mir-opt-level=2

query stack during panic:
#0 [normalize_generic_arg_after_erasing_regions] normalizing `<&mut I as std::iter::IntoIterator>::IntoIter`
#1 [optimized_mir] optimizing MIR for `<I as CollectArray<A>>::inner_array`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error

[RalfJung]

This is a recent regression that also affects Miri. The regression was introduced somewhere in a835b48...4ccf5f7.

[RalfJung]

Cc @rust-lang/wg-mir-opt

[RalfJung]

Cc @ecstatic-morse @lcnr I bisected this regression to #77430.

[lcnr]

looking into this