Miri ICE: InvalidProgram(ConstPropNonsense): uninhabited variant in project_downcast

[saethlin]

Reduced example: #115145 (comment)

https://github.com/axodotdev/axoasset at 10e2f44ee1bccaa5ae0dca28ee5061af04c0ae8d

MIRIFLAGS=-Zmiri-disable-isolation cargo miri test it_copies_remote_assets

Error output

error: internal compiler error: src/tools/miri/src/diagnostics.rs:294:17: This error should be impossible in Miri: InvalidProgram(ConstPropNonsense)

Backtrace

test it_copies_remote_assets ... error: internal compiler error: src/tools/miri/src/diagnostics.rs:294:17: This error should be impossible in Miri: InvalidProgram(ConstPropNonsense)

thread 'rustc' panicked at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/compiler/rustc_errors/src/lib.rs:1635:9:
Box<dyn Any>
stack backtrace:
   0:     0x7f8f3a0ee0fc - std::backtrace_rs::backtrace::libunwind::trace::h9e6789d755ef2e35
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7f8f3a0ee0fc - std::backtrace_rs::backtrace::trace_unsynchronized::hb3533227af2eed61
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f8f3a0ee0fc - std::sys_common::backtrace::_print_fmt::hc7dbc1325a62fad4
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/std/src/sys_common/backtrace.rs:67:5
   3:     0x7f8f3a0ee0fc - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::he15329baacfb890f
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f8f3a1543bc - core::fmt::rt::Argument::fmt::he237d6d4e685b998
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/core/src/fmt/rt.rs:138:9
   5:     0x7f8f3a1543bc - core::fmt::write::h70fc50f283d0f28f
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/core/src/fmt/mod.rs:1094:21
   6:     0x7f8f3a0e0c1e - std::io::Write::write_fmt::hddcd4db09551f1e4
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/std/src/io/mod.rs:1714:15
   7:     0x7f8f3a0edee4 - std::sys_common::backtrace::_print::h79b5981c192eb87e
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f8f3a0edee4 - std::sys_common::backtrace::print::h4e1fc678281db5ea
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f8f3a0f0fda - std::panicking::panic_hook_with_disk_dump::{{closure}}::h2f6637765d241d6e
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/std/src/panicking.rs:278:22
  10:     0x7f8f3a0f0cc7 - std::panicking::panic_hook_with_disk_dump::hdf84cf3f221731ff
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/std/src/panicking.rs:312:9
  11:     0x7f8f38f0e0b9 - <rustc_driver_impl[f90d201525556a1f]::install_ice_hook::{closure#0} as core[e05ed4681f3f1168]::ops::function::FnOnce<(&core[e05ed4681f3f1168]::panic::panic_info::PanicInfo,)>>::call_once::{shim:vtable#0}
  12:     0x7f8f3a0f1880 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h18e5faf377bd4349
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/alloc/src/boxed.rs:2021:9
  13:     0x7f8f3a0f1880 - std::panicking::rust_panic_with_hook::hdde37fc27effc37e
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/std/src/panicking.rs:733:13
  14:     0x7f8f394367d1 - std[5f485339d878b58d]::panicking::begin_panic::<rustc_errors[aa284a60f2852f9]::ExplicitBug>::{closure#0}
  15:     0x7f8f39434a96 - std[5f485339d878b58d]::sys_common::backtrace::__rust_end_short_backtrace::<std[5f485339d878b58d]::panicking::begin_panic<rustc_errors[aa284a60f2852f9]::ExplicitBug>::{closure#0}, !>
  16:     0x7f8f393edfa6 - std[5f485339d878b58d]::panicking::begin_panic::<rustc_errors[aa284a60f2852f9]::ExplicitBug>
  17:     0x7f8f39487d24 - <rustc_errors[aa284a60f2852f9]::HandlerInner>::bug::<alloc[1fa734c4d2bb695]::string::String>
  18:     0x7f8f39487bb6 - <rustc_errors[aa284a60f2852f9]::Handler>::bug::<alloc[1fa734c4d2bb695]::string::String>
  19:     0x7f8f3946d55c - rustc_middle[767f7e5b95c87cd]::util::bug::opt_span_bug_fmt::<rustc_span[9b5bef724011fe8f]::span_encoding::Span>::{closure#0}
  20:     0x7f8f3946bc0a - rustc_middle[767f7e5b95c87cd]::ty::context::tls::with_opt::<rustc_middle[767f7e5b95c87cd]::util::bug::opt_span_bug_fmt<rustc_span[9b5bef724011fe8f]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  21:     0x7f8f3946bbd8 - rustc_middle[767f7e5b95c87cd]::ty::context::tls::with_context_opt::<rustc_middle[767f7e5b95c87cd]::ty::context::tls::with_opt<rustc_middle[767f7e5b95c87cd]::util::bug::opt_span_bug_fmt<rustc_span[9b5bef724011fe8f]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  22:     0x7f8f376b7ae0 - rustc_middle[767f7e5b95c87cd]::util::bug::bug_fmt
  23:     0x55fa0204ed3a - miri[f8f5eaf6d04f8c74]::diagnostics::report_error
  24:     0x55fa02053f32 - miri[f8f5eaf6d04f8c74]::eval::eval_entry
  25:     0x55fa01fa99f7 - <rustc_middle[767f7e5b95c87cd]::ty::context::GlobalCtxt>::enter::<<miri[51939fbf34570118]::MiriCompilerCalls as rustc_driver_impl[f90d201525556a1f]::Callbacks>::after_analysis::{closure#0}, ()>
  26:     0x55fa01fa1914 - <miri[51939fbf34570118]::MiriCompilerCalls as rustc_driver_impl[f90d201525556a1f]::Callbacks>::after_analysis
  27:     0x7f8f381d5c0a - <rustc_interface[27d2f6b8fe428ec0]::interface::Compiler>::enter::<rustc_driver_impl[f90d201525556a1f]::run_compiler::{closure#1}::{closure#2}, core[e05ed4681f3f1168]::result::Result<core[e05ed4681f3f1168]::option::Option<rustc_interface[27d2f6b8fe428ec0]::queries::Linker>, rustc_span[9b5bef724011fe8f]::ErrorGuaranteed>>
  28:     0x7f8f381d2f18 - std[5f485339d878b58d]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[27d2f6b8fe428ec0]::util::run_in_thread_pool_with_globals<rustc_interface[27d2f6b8fe428ec0]::interface::run_compiler<core[e05ed4681f3f1168]::result::Result<(), rustc_span[9b5bef724011fe8f]::ErrorGuaranteed>, rustc_driver_impl[f90d201525556a1f]::run_compiler::{closure#1}>::{closure#0}, core[e05ed4681f3f1168]::result::Result<(), rustc_span[9b5bef724011fe8f]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[e05ed4681f3f1168]::result::Result<(), rustc_span[9b5bef724011fe8f]::ErrorGuaranteed>>
  29:     0x7f8f381d26a5 - <<std[5f485339d878b58d]::thread::Builder>::spawn_unchecked_<rustc_interface[27d2f6b8fe428ec0]::util::run_in_thread_pool_with_globals<rustc_interface[27d2f6b8fe428ec0]::interface::run_compiler<core[e05ed4681f3f1168]::result::Result<(), rustc_span[9b5bef724011fe8f]::ErrorGuaranteed>, rustc_driver_impl[f90d201525556a1f]::run_compiler::{closure#1}>::{closure#0}, core[e05ed4681f3f1168]::result::Result<(), rustc_span[9b5bef724011fe8f]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[e05ed4681f3f1168]::result::Result<(), rustc_span[9b5bef724011fe8f]::ErrorGuaranteed>>::{closure#1} as core[e05ed4681f3f1168]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  30:     0x7f8f3a0fc1e5 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h6086e9a313a3964c
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/alloc/src/boxed.rs:2007:9
  31:     0x7f8f3a0fc1e5 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h930455f80652281f
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/alloc/src/boxed.rs:2007:9
  32:     0x7f8f3a0fc1e5 - std::sys::unix::thread::Thread::new::thread_start::hbdbfdb70d010bf36
                               at /rustc/ef85656a10657ba5e4f7fe2931a4ca6293138d51/library/std/src/sys/unix/thread.rs:108:17
  33:     0x7f8f35a8c9eb - <unknown>
  34:     0x7f8f35b10dfc - <unknown>
  35:                0x0 - <unknown>

note: we would appreciate a bug report: https://github.com/rust-lang/miri/issues/new

note: please attach the file at `/tmp/axoasset/rustc-ice-2023-08-23T18:31:20.771630796Z-2899178.txt` to your bug report

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental=[REDACTED] -Z miri-disable-isolation

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

query stack during panic:
end of query stack
error: aborting due to previous error

error: test failed, to rerun pass `--test remote_copy`

searched nightlies: from nightly-2023-01-01 to nightly-2023-08-14
regressed nightly: nightly-2023-07-26
searched commit range: 31395ec...864bdf7
regressed commit: 4fc6b33

bisected with cargo-bisect-rustc v0.6.6

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

cargo bisect-rustc --start 2023-01-01 --end 2023-08-14 --script script -c miri --with-src 

[RalfJung]

Can you run this with MIRI_BACKTRACE=1 so we learn where in the code the error is triggered?

Also, Miri should print an interpreter backtrace of where in the crate it was when the error happened; please show that as well.

[saethlin]

I set that environment variable and I do not get any additional output. There is also no interpreter backtrace.

[RalfJung]

Uh, that is strange. Does MIRI_BACKTRACE=immediate show anything? If not, what about RUSTC_CTFE_BACKTRACE=immediate?

[saethlin]

Both of those flags print the same backtrace before the ICE:

An error occurred in the MIR interpreter:
   0: <rustc_middle::mir::interpret::error::InterpErrorInfo as core::convert::From<rustc_middle::mir::interpret::error::InterpError>>::from
   1: <rustc_const_eval::interpret::eval_context::InterpCx<miri::machine::MiriMachine>>::project_downcast::<rustc_const_eval::interpret::place::PlaceTy<miri::machine::Provenance>>
   2: <rustc_const_eval::interpret::eval_context::InterpCx<miri::machine::MiriMachine>>::eval_place
   3: <rustc_const_eval::interpret::eval_context::InterpCx<miri::machine::MiriMachine>>::eval_rvalue_into_place
   4: <rustc_const_eval::interpret::eval_context::InterpCx<miri::machine::MiriMachine> as miri::concurrency::thread::EvalContextExt>::run_threads
   5: miri::eval::eval_entry
   6: <rustc_middle::ty::context::GlobalCtxt>::enter::<<miri::MiriCompilerCalls as rustc_driver_impl::Callbacks>::after_analysis::{closure#0}, ()>
   7: <miri::MiriCompilerCalls as rustc_driver_impl::Callbacks>::after_analysis
   8: <rustc_interface::interface::Compiler>::enter::<rustc_driver_impl::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
   9: std::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>
  10: <<std::thread::Builder>::spawn_unchecked_<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#0}::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>::{closure#1} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  11: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
             at /rustc/69e97df5ce571a777acd654ec3697ae8d25962ea/library/alloc/src/boxed.rs:2007:9
  12: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
             at /rustc/69e97df5ce571a777acd654ec3697ae8d25962ea/library/alloc/src/boxed.rs:2007:9
  13: std::sys::unix::thread::Thread::new::thread_start
             at /rustc/69e97df5ce571a777acd654ec3697ae8d25962ea/library/std/src/sys/unix/thread.rs:108:17
  14: <unknown>
  15: <unknown>

[RalfJung]

Oh interesting, it seems to be triggering this:

rust/compiler/rustc_const_eval/src/interpret/projection.rs

Lines 141 to 145 in 4354192

	if layout.abi.is_uninhabited() {
	// `read_discriminant` should have excluded uninhabited variants... but ConstProp calls
	// us on dead code.
	throw_inval!(ConstPropNonsense)
	}

So there's something funny going on with an enum. Hopefully #115272 will let us capture an interpreter backtrace.

[saethlin]

Yep, that PR seems to work:

Miri caused an ICE during evaluation. Here's the interpreter backtrace at the time of the panic:
note: the place in the program where the ICE was triggered
   --> /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/deadpool-0.9.5/src/managed/mod.rs:663:5
    |
663 |     timeout_type: TimeoutType,
    |     ^^^^^^^^^^^^
    |
    = note: inside closure at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/deadpool-0.9.5/src/managed/mod.rs:663:5: 663:17
    = note: inside closure at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/deadpool-0.9.5/src/managed/mod.rs:384:14: 384:19
    = note: inside closure at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/deadpool-0.9.5/src/managed/mod.rs:327:44: 327:49
    = note: inside closure at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/wiremock-0.5.19/src/mock_server/pool.rs:43:10: 43:15
    = note: inside closure at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/wiremock-0.5.19/src/mock_server/exposed_server.rs:112:59: 112:64
note: inside closure
   --> tests/remote_copy.rs:11:43
    |
11  |     let mock_server = MockServer::start().await;
    |                                           ^^^^^
    = note: inside `<std::pin::Pin<&mut dyn std::future::Future<Output = ()>> as std::future::Future>::poll` at /home/ben/rust/library/core/src/future/future.rs:125:9: 125:61
    = note: inside `<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>> as std::future::Future>::poll` at /home/ben/rust/library/core/src/future/future.rs:125:9: 125:61
    = note: inside closure at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/scheduler/current_thread/mod.rs:665:57: 665:86
    = note: inside `tokio::runtime::coop::with_budget::<std::task::Poll<()>, [closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}::{closure#0}::{closure#0}]>` at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/coop.rs:107:5: 107:8
    = note: inside `tokio::runtime::coop::budget::<std::task::Poll<()>, [closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}::{closure#0}::{closure#0}]>` at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/coop.rs:73:5: 73:38
    = note: inside closure at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/scheduler/current_thread/mod.rs:665:25: 665:87
    = note: inside `tokio::runtime::scheduler::current_thread::Context::enter::<std::task::Poll<()>, [closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}::{closure#0}]>` at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/scheduler/current_thread/mod.rs:410:19: 410:22
    = note: inside closure at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/scheduler/current_thread/mod.rs:664:36: 666:23
    = note: inside closure at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/scheduler/current_thread/mod.rs:743:68: 743:84
    = note: inside `tokio::runtime::context::scoped::Scoped::<tokio::runtime::scheduler::Context>::set::<[closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::enter<[closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}], std::option::Option<()>>::{closure#0}], (std::boxed::Box<tokio::runtime::scheduler::current_thread::Core>, std::option::Option<()>)>` at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/context/scoped.rs:40:9: 40:12
    = note: inside closure at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/context.rs:176:26: 176:47
    = note: inside `std::thread::LocalKey::<tokio::runtime::context::Context>::try_with::<[closure@tokio::runtime::context::set_scheduler<(std::boxed::Box<tokio::runtime::scheduler::current_thread::Core>, std::option::Option<()>), [closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::enter<[closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}], std::option::Option<()>>::{closure#0}]>::{closure#0}], (std::boxed::Box<tokio::runtime::scheduler::current_thread::Core>, std::option::Option<()>)>` at /home/ben/rust/library/std/src/thread/local.rs:270:16: 270:31
    = note: inside `std::thread::LocalKey::<tokio::runtime::context::Context>::with::<[closure@tokio::runtime::context::set_scheduler<(std::boxed::Box<tokio::runtime::scheduler::current_thread::Core>, std::option::Option<()>), [closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::enter<[closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}], std::option::Option<()>>::{closure#0}]>::{closure#0}], (std::boxed::Box<tokio::runtime::scheduler::current_thread::Core>, std::option::Option<()>)>` at /home/ben/rust/library/std/src/thread/local.rs:246:9: 246:25
    = note: inside `tokio::runtime::context::set_scheduler::<(std::boxed::Box<tokio::runtime::scheduler::current_thread::Core>, std::option::Option<()>), [closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::enter<[closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}], std::option::Option<()>>::{closure#0}]>` at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/context.rs:176:9: 176:48
    = note: inside `tokio::runtime::scheduler::current_thread::CoreGuard::<'_>::enter::<[closure@tokio::runtime::scheduler::current_thread::CoreGuard<'_>::block_on<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>::{closure#0}], std::option::Option<()>>` at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/scheduler/current_thread/mod.rs:743:27: 743:85
    = note: inside `tokio::runtime::scheduler::current_thread::CoreGuard::<'_>::block_on::<std::pin::Pin<&mut std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>>` at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/scheduler/current_thread/mod.rs:652:19: 720:11
    = note: inside closure at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/scheduler/current_thread/mod.rs:175:28: 175:49
    = note: inside `tokio::runtime::context::runtime::enter_runtime::<[closure@tokio::runtime::scheduler::current_thread::CurrentThread::block_on<std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>::{closure#0}], ()>` at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/context/runtime.rs:65:16: 65:38
    = note: inside `tokio::runtime::scheduler::current_thread::CurrentThread::block_on::<std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>` at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/scheduler/current_thread/mod.rs:167:9: 198:11
    = note: inside `tokio::runtime::Runtime::block_on::<std::pin::Pin<&mut dyn std::future::Future<Output = ()>>>` at /home/ben/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tokio-1.32.0/src/runtime/runtime.rs:347:47: 347:88
note: inside `it_copies_remote_assets`
   --> tests/remote_copy.rs:20:5
    |
20  | /     for route in routes {
21  | |         let resp_string = if route.to_uppercase().contains("README") {
22  | |             &readme_string
23  | |         } else {
...   |
38  | |         assert!(copied_file.exists());
39  | |     }
    | |_____^
note: inside closure
   --> tests/remote_copy.rs:10:36
    |
9   | #[tokio::test]
    | -------------- in this procedural macro expansion
10  | async fn it_copies_remote_assets() {
    |                                    ^
    = note: inside `<[closure@tests/remote_copy.rs:10:7: 40:2] as std::ops::FnOnce<()>>::call_once - shim` at /home/ben/rust/library/core/src/ops/function.rs:250:5: 250:71
    = note: inside `<fn() -> std::result::Result<(), std::string::String> as std::ops::FnOnce<()>>::call_once - shim(fn() -> std::result::Result<(), std::string::String>)` at /home/ben/rust/library/core/src/ops/function.rs:250:5: 250:71
    = note: inside `test::__rust_begin_short_backtrace::<std::result::Result<(), std::string::String>, fn() -> std::result::Result<(), std::string::String>>` at /home/ben/rust/library/test/src/lib.rs:626:18: 626:21
    = note: inside `test::types::RunnableTest::run` at /home/ben/rust/library/test/src/types.rs:146:40: 146:71
    = note: inside closure at /home/ben/rust/library/test/src/lib.rs:649:60: 649:79
    = note: inside `<std::panic::AssertUnwindSafe<[closure@test::run_test_in_process::{closure#0}]> as std::ops::FnOnce<()>>::call_once` at /home/ben/rust/library/core/src/panic/unwind_safe.rs:271:9: 271:19
    = note: inside `std::panicking::r#try::do_call::<std::panic::AssertUnwindSafe<[closure@test::run_test_in_process::{closure#0}]>, std::result::Result<(), std::string::String>>` at /home/ben/rust/library/std/src/panicking.rs:524:40: 524:43
    = note: inside `std::panicking::r#try::<std::result::Result<(), std::string::String>, std::panic::AssertUnwindSafe<[closure@test::run_test_in_process::{closure#0}]>>` at /home/ben/rust/library/std/src/panicking.rs:488:19: 488:81
    = note: inside `std::panic::catch_unwind::<std::panic::AssertUnwindSafe<[closure@test::run_test_in_process::{closure#0}]>, std::result::Result<(), std::string::String>>` at /home/ben/rust/library/std/src/panic.rs:142:14: 142:33
    = note: inside `test::run_test_in_process` at /home/ben/rust/library/test/src/lib.rs:649:27: 649:81
    = note: inside closure at /home/ben/rust/library/test/src/lib.rs:572:43: 580:18
    = note: inside closure at /home/ben/rust/library/test/src/lib.rs:600:41: 600:83
    = note: inside `std::sys_common::backtrace::__rust_begin_short_backtrace::<[closure@test::run_test::{closure#1}], ()>` at /home/ben/rust/library/std/src/sys_common/backtrace.rs:154:18: 154:21
    = note: inside closure at /home/ben/rust/library/std/src/thread/mod.rs:529:17: 529:78
    = note: inside `<std::panic::AssertUnwindSafe<[closure@std::thread::Builder::spawn_unchecked_<'_, '_, [closure@test::run_test::{closure#1}], ()>::{closure#1}::{closure#0}]> as std::ops::FnOnce<()>>::call_once` at /home/ben/rust/library/core/src/panic/unwind_safe.rs:271:9: 271:19
    = note: inside `std::panicking::r#try::do_call::<std::panic::AssertUnwindSafe<[closure@std::thread::Builder::spawn_unchecked_<'_, '_, [closure@test::run_test::{closure#1}], ()>::{closure#1}::{closure#0}]>, ()>` at /home/ben/rust/library/std/src/panicking.rs:524:40: 524:43
    = note: inside `std::panicking::r#try::<(), std::panic::AssertUnwindSafe<[closure@std::thread::Builder::spawn_unchecked_<'_, '_, [closure@test::run_test::{closure#1}], ()>::{closure#1}::{closure#0}]>>` at /home/ben/rust/library/std/src/panicking.rs:488:19: 488:81
    = note: inside `std::panic::catch_unwind::<std::panic::AssertUnwindSafe<[closure@std::thread::Builder::spawn_unchecked_<'_, '_, [closure@test::run_test::{closure#1}], ()>::{closure#1}::{closure#0}]>, ()>` at /home/ben/rust/library/std/src/panic.rs:142:14: 142:33
    = note: inside closure at /home/ben/rust/library/std/src/thread/mod.rs:528:30: 530:16
    = note: inside `<[closure@std::thread::Builder::spawn_unchecked_<'_, '_, [closure@test::run_test::{closure#1}], ()>::{closure#1}] as std::ops::FnOnce<()>>::call_once - shim(vtable)` at /home/ben/rust/library/core/src/ops/function.rs:250:5: 250:71
    = note: inside `<std::boxed::Box<dyn std::ops::FnOnce()> as std::ops::FnOnce<()>>::call_once` at /home/ben/rust/library/alloc/src/boxed.rs:2007:9: 2007:52
    = note: inside `<std::boxed::Box<std::boxed::Box<dyn std::ops::FnOnce()>> as std::ops::FnOnce<()>>::call_once` at /home/ben/rust/library/alloc/src/boxed.rs:2007:9: 2007:52
    = note: inside `std::sys::unix::thread::Thread::new::thread_start` at /home/ben/rust/library/std/src/sys/unix/thread.rs:108:17: 108:64
    = note: this note originates in the attribute macro `::core::prelude::v1::test` which comes from the expansion of the attribute macro `tokio::test` (in Nightly builds, run with -Z macro-backtrace for more info)

[RalfJung]

Oh, this may be a downcast on a generator. I had not considered those. Hm...

I think currently we guarantee that even uninhabited variants have sufficient space allocated to them, so maybe we can just remove that check? Still would be nice to have a small example.