Broken MIR (and diagnostics) using Higher-Rank Trait Bounds and associated types

[SkiFire13]

Code

trait Bar {
    type Type;
}
struct Foo<'a>(&'a ());
impl<'a> Bar for Foo<'a> {
    type Type = ();
}

fn func<'a>(_: <Foo<'a> as Bar>::Type) {}
fn assert_is_func<A>(_: fn(A)) {}

fn test() where for<'a> <Foo<'a> as Bar>::Type: Sized {
    assert_is_func(func);
}

Playground link

Meta

rustc --version --verbose:

rustc 1.49.0 (e1884a8e3 2020-12-29)
binary: rustc
commit-hash: e1884a8e3c3e813aada8254edfa120e85bf5ffca
commit-date: 2020-12-29
host: x86_64-pc-windows-msvc
release: 1.49.0

Error output

warning: Error finalizing incremental compilation session directory `\\?\G:\WorkSpace\rust-playground\target\debug\incremental\rust_playground-13rqwjq2lprvw\s-fuvapvoonl-1e5eb07-working`: Impossibile trovare il file specificato. (os error 2)

warning: 4 warnings emitted

error: internal compiler error: broken MIR in DefId(0:16 ~ rust_playground[48a5]::test) (Terminator { source_info: SourceInfo { span: src\lib.rs:13:5: 13:25 (#0), scope: scope[0] }, kind: _1 = assert_is_func::<<Foo<'_> as 
Bar>::Type>(move _2) -> [return: bb1, unwind: bb2] }): bad arg #0 (fn(()) <- for<'r> fn(<Foo<'r> as Bar>::Type)): NoSolution
  --> src\lib.rs:13:20
   |
13 |     assert_is_func(func);
   |                    ^^^^
   |
   = note: delayed at compiler\rustc_mir\src\borrow_check\type_check\mod.rs:253:27

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler\rustc_errors\src\lib.rs:974:13
stack backtrace:
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose 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 1.49.0 (e1884a8e3 2020-12-29) running on x86_64-pc-windows-msvc

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type lib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
error: could not compile `rust-playground`

To learn more, run the command again with --verbose.

Strangely the backtrace is empty (I got the error output section with RUST_BACKTRACE=1). Should I report that bug separately?

[jyn514]

This also crashes on nightly, but it has a backtrace there (not that it's very helpful):

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:974:13
stack backtrace:
   0: rust_begin_unwind
             at /rustc/c5eae562935922f712edec56a45591bc2f8ded1c/library/std/src/panicking.rs:493:5
   1: std::panicking::begin_panic_fmt
             at /rustc/c5eae562935922f712edec56a45591bc2f8ded1c/library/std/src/panicking.rs:435:5
   2: rustc_errors::HandlerInner::flush_delayed
   3: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
   4: core::ptr::drop_in_place<rustc_session::parse::ParseSess>
   5: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
   6: core::ptr::drop_in_place<rustc_interface::interface::Compiler>
   7: rustc_span::with_source_map
   8: rustc_interface::interface::create_compiler_and_run
   9: rustc_span::with_session_globals

@SkiFire13 can you see if using nightly gets you a backtrace?

[SkiFire13]

I can't get a normal backtrace even on nightly, however I can get one with RUST_BACKTRACE=full (on both stable and nightly), but I don't think it will be very helpful:

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler\rustc_errors\src\lib.rs:974:13
stack backtrace:
   0:     0x7ffe7e17a24e - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h9b227e9f535b30ad
   1:     0x7ffe7e1a841b - core::fmt::write::h61d774f0fccca269
   2:     0x7ffe7e16bc68 - <std::io::IoSliceMut as core::fmt::Debug>::fmt::hd02efffdeb825e77
   3:     0x7ffe7e17f86d - std::panicking::take_hook::h071d302a205c6652
   4:     0x7ffe7e17f448 - std::panicking::take_hook::h071d302a205c6652
   5:     0x7ffe26f58c27 - rustc_driver::report_ice::h5fb666ac0b140095
   6:     0x7ffe7e180380 - std::panicking::rust_panic_with_hook::h3a6bf3c946dc3fd3
   7:     0x7ffe7e17fe51 - rust_begin_unwind
   8:     0x7ffe7e17ab8f - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h9b227e9f535b30ad
   9:     0x7ffe7e17fda9 - rust_begin_unwind
  10:     0x7ffe7e17fd5c - std::panicking::begin_panic_fmt::h4544c6b6ce1d7abc
  11:     0x7ffe2b670484 - rustc_errors::HandlerInner::delay_as_bug::h14efc84023f1ac5a
  12:     0x7ffe2b66ba14 - <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop::he0453085c4ee1c80
  13:     0x7ffe26f40194 - <rustc_driver::args::Error as core::fmt::Debug>::fmt::h9bf3232a4fe8dccd
  14:     0x7ffe26f45a90 - <rustc_driver::args::Error as core::fmt::Debug>::fmt::h9bf3232a4fe8dccd
  15:     0x7ffe26f4ad1a - <rustc_driver::args::Error as core::fmt::Debug>::fmt::h9bf3232a4fe8dccd
  16:     0x7ffe26f62acc - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h33be7dbc37f75bef
  17:     0x7ffe26f5b2b4 - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h33be7dbc37f75bef
  18:     0x7ffe26f6aa08 - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h33be7dbc37f75bef
  19:     0x7ffe26f5d09e - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h33be7dbc37f75bef
  20:     0x7ffe26f6f883 - <rustc_driver::Compilation as core::fmt::Debug>::fmt::h33be7dbc37f75bef
  21:     0x7ffe26edbe8d - <sha1::Sha1 as std::io::Write>::flush::hc1a90b50a4949fda
  22:     0x7ffe7e190537 - std::sys::windows::thread::Thread::new::h010378fec97bafdf
  23:     0x7ffeb4867034 - BaseThreadInitThunk
  24:     0x7ffeb4b7d0d1 - RtlUserThreadStart

[SNCPlay42]

For delay_span_bugs you can get the backtrace for where they were originally issued with -Ztreat-err-as-bug:

% RUST_BACKTRACE=1 rustc +nightly --crate-type=lib -Ztreat-err-as-bug=1 issue.rs
error: internal compiler error: broken MIR in DefId(0:16 ~ issue[8787]::test) (Terminator { source_info: SourceInfo { span: issue.rs:13:5: 13:25 (#0), scope: scope[0] }, kind: _1 = assert_is_func::<<Foo<'_> as Bar>::Type>(move _2) -> [return: bb1, unwind: bb2] }): bad arg #0 (fn(()) <- for<'r> fn(<Foo<'r> as Bar>::Type)): NoSolution
  --> issue.rs:13:20
   |
13 |     assert_is_func(func);
   |                    ^^^^

thread 'rustc' panicked at 'aborting due to `-Z treat-err-as-bug=1`', compiler/rustc_errors/src/lib.rs:990:27
stack backtrace:
   0: std::panicking::begin_panic
   1: rustc_errors::HandlerInner::emit_diagnostic
   2: rustc_errors::HandlerInner::emit_diag_at_span
   3: rustc_errors::HandlerInner::span_bug
   4: rustc_errors::Handler::delay_span_bug
   5: rustc_mir::borrow_check::type_check::TypeChecker::typeck_mir
   6: rustc_mir::borrow_check::type_check::type_check
   7: rustc_mir::borrow_check::nll::compute_regions
   8: rustc_mir::borrow_check::do_mir_borrowck
   9: rustc_infer::infer::InferCtxtBuilder::enter
  10: rustc_mir::borrow_check::mir_borrowck
  11: core::ops::function::FnOnce::call_once
  12: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_borrowck>::compute
  13: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  14: rustc_data_structures::stack::ensure_sufficient_stack
  15: rustc_query_system::query::plumbing::get_query_impl
  16: rustc_query_system::query::plumbing::ensure_query_impl
  17: rustc_session::utils::<impl rustc_session::session::Session>::time
  18: rustc_interface::passes::analysis
  19: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  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_interface::passes::QueryContext::enter
  24: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  25: rustc_span::with_source_map
  26: rustc_interface::interface::create_compiler_and_run
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose 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 1.51.0-nightly (158f8d034 2020-12-29) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z treat-err-as-bug=1 --crate-type lib

query stack during panic:
#0 [mir_borrowck] borrow-checking `test`
#1 [analysis] running analysis passes on this crate
end of query stack

(Should we automatically capture backtraces of delay_span_bug when RUST_BACKTRACE is set?)

[jackh726]

I can see how this would be fixed by #85499, but we don't have a test like this. Marking as needs-test.