impossible case reached on lifetime inference for impl Trait in return position #49926
Description
summery
when the inferred associated types of the trait contain lifetimes, the compiler produces an internal error
details
I tried this code:
fn iter() -> impl Iterator {
(1..2).map(|_| "hello")
}I expected to see this happen:
the return type is inferred to be impl Iterator<Item=&'static str>,
which, when written like that, is accepted by the compiler.
Instead, this happened:
error: internal compiler error: librustc/infer/error_reporting/mod.rs:184: impossible case reached
thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:542:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: aborting due to previous error
Meta
rustc 1.25.0 (84203cac6 2018-03-25)
binary: rustc
commit-hash: 84203cac67e65ca8640b8392348411098c856985
commit-date: 2018-03-25
host: x86_64-unknown-linux-gnu
release: 1.25.0
LLVM version: 6.0
Backtrace
0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
1: std::sys_common::backtrace::print
at libstd/sys_common/backtrace.rs:71
at libstd/sys_common/backtrace.rs:59
2: std::panicking::default_hook::{{closure}}
at libstd/panicking.rs:206
3: std::panicking::default_hook
at libstd/panicking.rs:222
4: rustc::util::common::panic_hook
5: std::panicking::rust_panic_with_hook
at libstd/panicking.rs:401
6: std::panicking::begin_panic
7: rustc_errors::Handler::bug
8: rustc::session::opt_span_bug_fmt::{{closure}}
9: rustc::ty::context::tls::with_opt::{{closure}}
10: rustc::ty::context::tls::with_context_opt
11: rustc::ty::context::tls::with_opt
12: rustc::session::opt_span_bug_fmt
13: rustc::session::bug_fmt
14: rustc::infer::error_reporting::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::msg_span_from_free_region
15: <rustc::infer::anon_types::ReverseMapper<'cx, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_region
16: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable<'tcx> for &'tcx rustc::ty::TyS<'tcx>>::super_fold_with
17: <rustc::infer::anon_types::ReverseMapper<'cx, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
18: <rustc_data_structures::accumulate_vec::AccumulateVec<A> as core::iter::traits::FromIterator<<A as rustc_data_structures::array_vec::Array>::Element>>::from_iter
19: rustc::ty::fold::TypeFoldable::fold_with
20: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable<'tcx> for &'tcx rustc::ty::TyS<'tcx>>::super_fold_with
21: <rustc::infer::anon_types::ReverseMapper<'cx, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
22: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &'a mut F>::call_once
23: <rustc_data_structures::accumulate_vec::AccumulateVec<A> as core::iter::traits::FromIterator<<A as rustc_data_structures::array_vec::Array>::Element>>::from_iter
24: <T as rustc::ty::context::InternIteratorElement<T, R>>::intern_with
25: <rustc::infer::anon_types::ReverseMapper<'cx, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
26: <rustc_data_structures::accumulate_vec::AccumulateVec<A> as core::iter::traits::FromIterator<<A as rustc_data_structures::array_vec::Array>::Element>>::from_iter
27: rustc::ty::fold::TypeFoldable::fold_with
28: rustc::ty::structural_impls::<impl rustc::ty::fold::TypeFoldable<'tcx> for &'tcx rustc::ty::TyS<'tcx>>::super_fold_with
29: <rustc::infer::anon_types::ReverseMapper<'cx, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
30: rustc::infer::anon_types::<impl rustc::infer::InferCtxt<'a, 'gcx, 'tcx>>::infer_anon_definition_from_instantiation
31: rustc_typeck::check::writeback::WritebackCx::visit_anon_types
32: rustc_typeck::check::writeback::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::resolve_type_vars_in_body
33: rustc::ty::context::tls::with_related_context
34: rustc::infer::InferCtxtBuilder::enter
35: rustc_typeck::check::typeck_tables_of
36: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::compute_result
37: rustc::dep_graph::graph::DepGraph::with_task_impl
38: rustc::ty::context::tls::with_related_context
39: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::force_with_lock
40: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::try_get
41: rustc::ty::maps::TyCtxtAt::typeck_tables_of
42: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_tables_of<'tcx>>::ensure
43: rustc::session::Session::track_errors
44: rustc_typeck::check::typeck_item_bodies
45: rustc::dep_graph::graph::DepGraph::with_task_impl
46: rustc::ty::context::tls::with_related_context
47: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_item_bodies<'tcx>>::force_with_lock
48: rustc::ty::maps::<impl rustc::ty::maps::queries::typeck_item_bodies<'tcx>>::try_get
49: rustc::ty::maps::TyCtxtAt::typeck_item_bodies
50: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::typeck_item_bodies
51: rustc_typeck::check_crate
52: rustc::ty::context::tls::enter_context
53: <std::thread::local::LocalKey<T>>::with
54: rustc::ty::context::TyCtxt::create_and_enter
55: rustc_driver::driver::compile_input
56: rustc_driver::run_compiler_impl
57: syntax::with_globals
58: rustc_driver::run
59: rustc_driver::main
60: std::rt::lang_start::{{closure}}
61: std::panicking::try::do_call
at libstd/rt.rs:59
at libstd/panicking.rs:305
62: __rust_maybe_catch_panic
at libpanic_unwind/lib.rs:101
63: std::rt::lang_start_internal
at libstd/panicking.rs:284
at libstd/panic.rs:361
at libstd/rt.rs:58
64: main
65: __libc_start_main
66: <unknown>
query stack during panic:
#0 [typeck_tables_of] processing `iter`
--> src/main.rs:1:1
|
1 | fn iter() -> impl Iterator {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^
#1 [typeck_item_bodies] type-checking all item bodies
end of query stack