Closed
Description
openedon Sep 17, 2020
The following code triggers an ICE.
Code
#![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));
}This example can no doubt be minimised further, but it should be obvious what I'm attempting to do and why. This would be, I assume, a fairly common use-case for GATs and are likely to be essential if GATs are ever to be integrated into Iterator.
Meta
rustc --version --verbose:
rustc 1.48.0-nightly (d006f5734 2020-08-28)
Error output
error: internal compiler error: src/librustc_trait_selection/traits/codegen/mod.rs:75:17: Encountered error `OutputTypeParameterMismatch(Binder(<[closure@gats.rs:41:19: 41:50] as std::ops::FnMut<(<Windows<i32> as Iter>::Item<'_>,)>>), Binder(<[closure@gats.rs:41:19: 41:50] as std::ops::FnMut<(&mut [i32],)>>), Sorts(ExpectedFound { expected: &mut [i32], found: <Windows<i32> as Iter>::Item<'_> }))` selecting `Binder(<[closure@gats.rs:41:19: 41:50] as std::ops::FnMut<(&mut [i32],)>>)` during codegen
thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:918:9
Backtrace
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_trait_selection::traits::codegen::codegen_fulfill_obligation
10: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::codegen_fulfill_obligation>::compute
11: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
12: rustc_data_structures::stack::ensure_sufficient_stack
13: rustc_query_system::query::plumbing::get_query_impl
14: rustc_ty::instance::inner_resolve_instance
15: rustc_ty::instance::resolve_instance
16: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::resolve_instance>::compute
17: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
18: rustc_data_structures::stack::ensure_sufficient_stack
19: rustc_query_system::query::plumbing::get_query_impl
20: rustc_middle::ty::instance::Instance::resolve_opt_const_arg
21: rustc_middle::ty::instance::Instance::resolve
22: <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_terminator
23: rustc_mir::monomorphize::collector::collect_neighbours
24: rustc_mir::monomorphize::collector::collect_items_rec
25: rustc_mir::monomorphize::collector::collect_items_rec
26: rustc_session::utils::<impl rustc_session::session::Session>::time
27: rustc_mir::monomorphize::collector::collect_crate_mono_items
28: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
29: 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
30: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
31: rustc_data_structures::stack::ensure_sufficient_stack
32: rustc_query_system::query::plumbing::get_query_impl
33: rustc_codegen_ssa::base::codegen_crate
34: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
35: rustc_interface::passes::QueryContext::enter
36: rustc_interface::queries::Queries::ongoing_codegen
37: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
38: rustc_span::with_source_map
39: rustc_interface::interface::create_compiler_and_run
40: 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 1.48.0-nightly (d006f5734 2020-08-28) running on x86_64-unknown-linux-gnu
query stack during panic:
#0 [codegen_fulfill_obligation] checking if `std::ops::FnMut` fulfills its obligations
#1 [resolve_instance] resolving instance `<[closure@gats.rs:41:19: 41:50] as std::ops::FnMut<(&mut [i32],)>>::call_mut`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error; 1 warning emitted
Removing the final line (.for_each(|slice| println!("{:?}", slice))) prevents the ICE, implying that this is a codegen issue (as corroborated by the stack trace).
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