ICE using `impl ?Sized` in return position

[asquared31415]

Code

#[allow(dead_code)]
fn uwu() -> impl ?Sized {}

Playground

Meta

Error occurs on
Stable 1.61.0
Beta 1.62.0-beta.2
Nightly 2022-05-19

Error output

Backtrace

   Compiling playground v0.0.1 (/playground)
error: internal compiler error: no errors encountered even though `delay_span_bug` issued

error: internal compiler error: broken MIR in DefId(0:3 ~ playground[f439]::uwu) (NoSolution): could not prove Binder(TraitPredicate(<impl ?Sized as std::marker::Sized>, polarity:Positive), [])
 --> src/lib.rs:2:25
  |
2 | fn uwu() -> impl ?Sized {}
  |                         ^^
  |
  = note: delayed at compiler/rustc_borrowck/src/type_check/canonical.rs:149:13

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1309:13
stack backtrace:
   0:     0x7fa0b5b141cd - std::backtrace_rs::backtrace::libunwind::trace::h22893a5306c091b4
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7fa0b5b141cd - std::backtrace_rs::backtrace::trace_unsynchronized::h29c3bc6f9e91819d
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7fa0b5b141cd - std::sys_common::backtrace::_print_fmt::he497d8a0ec903793
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/sys_common/backtrace.rs:66:5
   3:     0x7fa0b5b141cd - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h9c2a9d2774d81873
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/sys_common/backtrace.rs:45:22
   4:     0x7fa0b5b6e01c - core::fmt::write::hba4337c43d992f49
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/core/src/fmt/mod.rs:1194:17
   5:     0x7fa0b5b057c1 - std::io::Write::write_fmt::heb73de6e02cfabed
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/io/mod.rs:1655:15
   6:     0x7fa0b5b172b5 - std::sys_common::backtrace::_print::h63c8b24acdd8e8ce
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/sys_common/backtrace.rs:48:5
   7:     0x7fa0b5b172b5 - std::sys_common::backtrace::print::h426700d6240cdcc2
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/sys_common/backtrace.rs:35:9
   8:     0x7fa0b5b172b5 - std::panicking::default_hook::{{closure}}::hc9a76eed0b18f82b
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:295:22
   9:     0x7fa0b5b16f69 - std::panicking::default_hook::h2e88d02087fae196
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:314:9
  10:     0x7fa0b62941d1 - rustc_driver[413d55b6344850e3]::DEFAULT_HOOK::{closure#0}::{closure#0}
  11:     0x7fa0b5b17a00 - std::panicking::rust_panic_with_hook::habfdcc2e90f9fd4c
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/panicking.rs:702:17
  12:     0x7fa0b7364141 - std[3b302b3c9093c22]::panicking::begin_panic::<rustc_errors[a7dbbf867eaca825]::ExplicitBug>::{closure#0}
  13:     0x7fa0b7363e96 - std[3b302b3c9093c22]::sys_common::backtrace::__rust_end_short_backtrace::<std[3b302b3c9093c22]::panicking::begin_panic<rustc_errors[a7dbbf867eaca825]::ExplicitBug>::{closure#0}, !>
  14:     0x7fa0b736b406 - std[3b302b3c9093c22]::panicking::begin_panic::<rustc_errors[a7dbbf867eaca825]::ExplicitBug>
  15:     0x7fa0b7377ec6 - std[3b302b3c9093c22]::panic::panic_any::<rustc_errors[a7dbbf867eaca825]::ExplicitBug>
  16:     0x7fa0b8a5613a - <rustc_errors[a7dbbf867eaca825]::HandlerInner as core[a3420257622cd381]::ops::drop::Drop>::drop
  17:     0x7fa0b8172f38 - core[a3420257622cd381]::ptr::drop_in_place::<rustc_session[8df92abe178df33c]::parse::ParseSess>
  18:     0x7fa0b81763f3 - <alloc[59b4cdf2bf51ef42]::rc::Rc<rustc_session[8df92abe178df33c]::session::Session> as core[a3420257622cd381]::ops::drop::Drop>::drop
  19:     0x7fa0b815cb9d - core[a3420257622cd381]::ptr::drop_in_place::<rustc_interface[550884b2b179878f]::interface::Compiler>
  20:     0x7fa0b815c215 - rustc_span[7238ca7210ca999e]::with_source_map::<core[a3420257622cd381]::result::Result<(), rustc_errors[a7dbbf867eaca825]::ErrorGuaranteed>, rustc_interface[550884b2b179878f]::interface::create_compiler_and_run<core[a3420257622cd381]::result::Result<(), rustc_errors[a7dbbf867eaca825]::ErrorGuaranteed>, rustc_driver[413d55b6344850e3]::run_compiler::{closure#1}>::{closure#1}>
  21:     0x7fa0b81780a4 - rustc_interface[550884b2b179878f]::interface::create_compiler_and_run::<core[a3420257622cd381]::result::Result<(), rustc_errors[a7dbbf867eaca825]::ErrorGuaranteed>, rustc_driver[413d55b6344850e3]::run_compiler::{closure#1}>
  22:     0x7fa0b815a7c2 - <scoped_tls[7ed3ce20ede545d8]::ScopedKey<rustc_span[7238ca7210ca999e]::SessionGlobals>>::set::<rustc_interface[550884b2b179878f]::interface::run_compiler<core[a3420257622cd381]::result::Result<(), rustc_errors[a7dbbf867eaca825]::ErrorGuaranteed>, rustc_driver[413d55b6344850e3]::run_compiler::{closure#1}>::{closure#0}, core[a3420257622cd381]::result::Result<(), rustc_errors[a7dbbf867eaca825]::ErrorGuaranteed>>
  23:     0x7fa0b817038f - std[3b302b3c9093c22]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[550884b2b179878f]::util::run_in_thread_pool_with_globals<rustc_interface[550884b2b179878f]::interface::run_compiler<core[a3420257622cd381]::result::Result<(), rustc_errors[a7dbbf867eaca825]::ErrorGuaranteed>, rustc_driver[413d55b6344850e3]::run_compiler::{closure#1}>::{closure#0}, core[a3420257622cd381]::result::Result<(), rustc_errors[a7dbbf867eaca825]::ErrorGuaranteed>>::{closure#0}, core[a3420257622cd381]::result::Result<(), rustc_errors[a7dbbf867eaca825]::ErrorGuaranteed>>
  24:     0x7fa0b815baf9 - <<std[3b302b3c9093c22]::thread::Builder>::spawn_unchecked_<rustc_interface[550884b2b179878f]::util::run_in_thread_pool_with_globals<rustc_interface[550884b2b179878f]::interface::run_compiler<core[a3420257622cd381]::result::Result<(), rustc_errors[a7dbbf867eaca825]::ErrorGuaranteed>, rustc_driver[413d55b6344850e3]::run_compiler::{closure#1}>::{closure#0}, core[a3420257622cd381]::result::Result<(), rustc_errors[a7dbbf867eaca825]::ErrorGuaranteed>>::{closure#0}, core[a3420257622cd381]::result::Result<(), rustc_errors[a7dbbf867eaca825]::ErrorGuaranteed>>::{closure#1} as core[a3420257622cd381]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  25:     0x7fa0b5b21bf3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::ha99802c2c52ada61
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/alloc/src/boxed.rs:1861:9
  26:     0x7fa0b5b21bf3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::ha39aea1c57e28a15
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/alloc/src/boxed.rs:1861:9
  27:     0x7fa0b5b21bf3 - std::sys::unix::thread::Thread::new::thread_start::h9f8e3d72b1f7662f
                               at /rustc/fe5b13d681f25ee6474be29d748c65adcd91f69e/library/std/src/sys/unix/thread.rs:108:17
  28:     0x7fa0b5a51609 - start_thread
  29:     0x7fa0b596a133 - clone
  30:                0x0 - <unknown>

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 1.61.0 (fe5b13d68 2022-05-18) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type lib -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

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

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

[compiler-errors]

This regressed between 1.60 -> 1.61 from an error to an ICE.

I will run a bisect in a few hours, but I encourage anyone to do it sooner. Edit: Running bisect now.

[compiler-errors]

searched nightlies: from nightly-2022-01-15 to nightly-2022-05-22
regressed nightly: nightly-2022-04-30
searched commit range: e85edd9...a707f40
regressed commit: 87937d3

bisected with cargo-bisect-rustc v0.6.2

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --start=2022-01-15 --end=2022-05-22 --regress=ice 

cc @oli-obk, this regressed in #96516.

[Alexendoo]

Fixed by #97431

[compiler-errors]

This is a regression on stable and beta, reopening until backport is settled.