ICE: unexpected concrete region in borrowck: ReEarlyBound(0, 'a)

[matthiaskrgr]

Code

code is from ./src/test/ui/type-alias-impl-trait/associated-type-lifetime-ice.rs

// failure-status: 101
// rustc-env:RUST_BACKTRACE=0
// normalize-stderr-test "note: .*\n\n" -> ""
// normalize-stderr-test "thread 'rustc' panicked.*\n" -> ""

// compile-flags: --crate-type=rlib

// Regression test for https://github.com/rust-lang/rust/issues/78450

#![feature(min_type_alias_impl_trait)]
#![no_std]

pub trait AssociatedImpl {
    type ImplTrait;

    fn f() -> Self::ImplTrait;
}

struct S<T>(T);

trait Associated {
    type A;
}

// ICE
impl<'a, T: Associated<A = &'a ()>> AssociatedImpl for S<T> {
    type ImplTrait = impl core::fmt::Debug;

    fn f() -> Self::ImplTrait {
    //~^ ERROR unexpected concrete region in borrowck: ReEarlyBound(0, 'a)
        ()
    }
}

Meta

searched toolchains f98721f886ab52d32d622ad0a46216ad03f3e525 through 195ad4830e11a544391abe296b146450dea8411b
installing 195ad4830e11a544391abe296b146450dea8411b
cargo for x86_64-unknown-linux-gnu: 5.96 MB / 5.96 MB [=================================================================================] 100.00 % 1.50 MB/s testing...
uninstalling 195ad4830e11a544391abe296b146450dea8411b


********************************************************************************
Regression in 195ad4830e11a544391abe296b146450dea8411b
********************************************************************************

Seems to have been introduced by #82898 / 195ad48
cc @oli-obk ?

Error output

error: internal compiler error: unexpected concrete region in borrowck: ReEarlyBound(0, 'a)
  --> ./src/test/ui/type-alias-impl-trait/associated-type-lifetime-ice.rs:29:5
   |
29 | /     fn f() -> Self::ImplTrait {
30 | |     //~^ ERROR unexpected concrete region in borrowck: ReEarlyBound(0, 'a)
31 | |         ()
32 | |     }
   | |_____^
   |
   = note: delayed at compiler/rustc_mir/src/borrow_check/region_infer/opaque_types.rs:83:44

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:1012:13
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://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.52.0-nightly (195ad4830 2021-03-16) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib

query stack during panic:
end of query stack

Backtrace

error: internal compiler error: unexpected concrete region in borrowck: ReEarlyBound(0, 'a)
  --> ./src/test/ui/type-alias-impl-trait/associated-type-lifetime-ice.rs:29:5
   |
29 | /     fn f() -> Self::ImplTrait {
30 | |     //~^ ERROR unexpected concrete region in borrowck: ReEarlyBound(0, 'a)
31 | |         ()
32 | |     }
   | |_____^
   |
   = note: delayed at compiler/rustc_mir/src/borrow_check/region_infer/opaque_types.rs:83:44

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:1012:13
stack backtrace:
   0:     0x7fc856475a10 - std::backtrace_rs::backtrace::libunwind::trace::hfe3b1cace85e87d8
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/../../backtrace/src/backtrace/libunwind.rs:90:5
   1:     0x7fc856475a10 - std::backtrace_rs::backtrace::trace_unsynchronized::h542330af06479043
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7fc856475a10 - std::sys_common::backtrace::_print_fmt::h6b88726367858985
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7fc856475a10 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hcd76ce6b485adbea
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/sys_common/backtrace.rs:46:22
   4:     0x7fc8564e3f8f - core::fmt::write::h127419eb46f2ecc9
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/core/src/fmt/mod.rs:1092:17
   5:     0x7fc856469b22 - std::io::Write::write_fmt::h0facb64ef2e7f5d8
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/io/mod.rs:1567:15
   6:     0x7fc856479745 - std::sys_common::backtrace::_print::h7bf1633ea5421f7b
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/sys_common/backtrace.rs:49:5
   7:     0x7fc856479745 - std::sys_common::backtrace::print::h5cc4d09049928ba5
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/sys_common/backtrace.rs:36:9
   8:     0x7fc856479745 - std::panicking::default_hook::{{closure}}::h9e84dc005bfc9fc7
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/panicking.rs:208:50
   9:     0x7fc8564791f3 - std::panicking::default_hook::h123278a03b1f796b
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/panicking.rs:225:9
  10:     0x7fc856c4c9fb - rustc_driver::report_ice::h0dd0a80bb463fc7e
  11:     0x7fc856479eb0 - std::panicking::rust_panic_with_hook::h4040631aa6c7bc27
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/panicking.rs:595:17
  12:     0x7fc856479a27 - std::panicking::begin_panic_handler::{{closure}}::h02a17b22ac092d08
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/panicking.rs:497:13
  13:     0x7fc856475ecc - std::sys_common::backtrace::__rust_end_short_backtrace::h6063b024443b5852
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/sys_common/backtrace.rs:141:18
  14:     0x7fc856479989 - rust_begin_unwind
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/panicking.rs:493:5
  15:     0x7fc85643e14b - std::panicking::begin_panic_fmt::h10b51f96d6207ff1
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/panicking.rs:435:5
  16:     0x7fc85936c92e - rustc_errors::HandlerInner::flush_delayed::h7cdd6b7640e6949e
  17:     0x7fc85936b23b - <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop::h1ea17cd328582115
  18:     0x7fc858998d56 - core::ptr::drop_in_place<rustc_session::parse::ParseSess>::hc4ec048ba9ccb468
  19:     0x7fc85899b320 - <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop::h38e1b78997ab5d18
  20:     0x7fc858997f3d - core::ptr::drop_in_place<rustc_interface::interface::Compiler>::ha1358694e6ae6d6a
  21:     0x7fc8589977aa - rustc_span::with_source_map::h19c02e2b8cd97d79
  22:     0x7fc85899c856 - scoped_tls::ScopedKey<T>::set::ha910f4dd75d3f2b6
  23:     0x7fc85899e084 - std::sys_common::backtrace::__rust_begin_short_backtrace::h9028e0cefdffcecf
  24:     0x7fc8589b91e5 - core::ops::function::FnOnce::call_once{{vtable.shim}}::h88ad06dc90d5e88d
  25:     0x7fc856489728 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hd8ea7ada17dfa868
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/alloc/src/boxed.rs:1546:9
  26:     0x7fc856489728 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h91ce3a636c58b978
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/alloc/src/boxed.rs:1546:9
  27:     0x7fc856489728 - std::sys::unix::thread::Thread::new::thread_start::h2e193c2e23720fdf
                               at /rustc/195ad4830e11a544391abe296b146450dea8411b/library/std/src/sys/unix/thread.rs:71:17
  28:     0x7fc856392299 - start_thread
  29:     0x7fc8562a7053 - clone
  30:                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 1.52.0-nightly (195ad4830 2021-03-16) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib

query stack during panic:
end of query stack

[oli-obk]

Yes, this is a newly added test that I wanted to add as a test that will pass when that issue is fixed.

[oli-obk]

cc @matthewjasper

you added this delay_span_bug, but from the PR and the documentation on the function I can't tell what the correct way forward is. I presume this was built without type-alias-impl-trait in mind, as now lifetimes from the surrounding impl block can end up in the opaque type. The specific code emitting the delay span bug is

https://github.com/rust-lang/rust/blob/master/compiler/rustc_mir/src/borrow_check/region_infer/opaque_types.rs#L82-L87

I don't know enough about lifetime inference to judge how to proceed (removing the delay span bug makes the test pass, but... that doesn't sound like a good approach for something this subtle)

[spastorino]

After a discussion with Niko about this and assigning to myself we saw that there's already a PR. Unassigning myself then :).