Compilation error after Rust nightly 2021-01-10

[DimiDimit]

Compiling on any nightly version after 2021-01-10 errors out:

   Compiling async-scoped v0.6.0 (/tmp/async-scoped)
error: custom attribute panicked
   --> src/tests.rs:6:86
    |
6   |               #[cfg_attr(all(feature = "use-tokio", not( feature = "use-async-std" )), tokio::test(flavor = "multi_thread", worker_threads=1...
    |                                                                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
21  | / test_fixtures! {
22  | |     async fn test_scope() {
23  | |         let not_copy = String::from("hello world!");
24  | |         let not_copy_ref = &not_copy;
...   |
291 | |     }
292 | | }
    | |_- in this macro invocation
    |
    = help: message: internal error: entered unreachable code
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

error: could not compile `async-scoped`

(This is for use-tokio, the error for use-async-std is very similar:)

5   |               #[cfg_attr(feature = "use-async-std", async_std::test)]
    |                                                     ^^^^^^^^^^^^^^^

After some trial and error I traced the problem to commit rust-lang/rust@6184f23, which merges rust-lang/rust#80830.

I'm not sure whether this is an issue with rustc, tokio/async-std or async-scoped, I couldn't find an existing issue about it, so I decided to submit it here for the time being.

For now, I've pushed a workaround to my fork which just adds #![cfg(test)] to the tests.rs file, which means that the tests won't be compiled during normal usage, hence avoiding the error.

[rmanoka]

@DimiDimit I don't have this compiler (the most recent nightly with rls seems to be 2020-12-31, which is what I use). The macro that errors out is simply delegating to either async_std::test or tokio::test attributes to run the the tests. Could you test the compiler with a sample function and directly using one of these attributes?

Eg.

#[test]
#[tokio::test(flavor = "multi_thread", worker_threads=1)]
fn hello_world() {
  eprintln!("Hello world!");
}

[DimiDimit]

I forgot to add this, but expanding the test_fixtures! macro manually through the wonders of copy-paste works perfectly fine, but using it errors out. I could've also used that as my workaround, now that I think about it.
I found that the #[cfg_attr] makes no difference for this error.

Also, this is an unexpected compiler panic which means it's apparently a bug with rustc.

[rmanoka]

Interesting. In the interest of finding a MWE to take to the rustc team, does the following eg. compile?

#[macro_use]
macro_rules! testing {
    ($($item:item)*) => {
        $(
            #[cfg_attr(feature = "use-async-std", test)]
            $item
        )*
    }
}

testing! {
    fn hello_world() {
        eprintln!("Hello world!")
    }
}

You would test this by: cargo test --features use-async-std -- hello_world --nocapture.

[DimiDimit]

That actually compiles and runs just fine! Oddly enough, commenting out the following tests in src/tests.rs makes it compile: scope_lifetime, cancellation_soundness, backpressure and test_async_deadlock.

running 4 tests
test tests::test_scope_and_collect ... ok
test tests::test_scope_and_block ... ok
test tests::test_scope ... ok
test tests::scope_async ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.10s

[DimiDimit]

Oh, I see!! It has to do with doc comments (notice the async_std::test, just test works fine):

#[macro_use]
macro_rules! testing {
    ($($item:item)*) => {
        $(
            #[cfg_attr(feature = "use-async-std", async_std::test)]
            $item
        )*
    }
}

testing! {
    // Normal comments work just fine.
    /// Doc comment? Nope, compilation error.
    async fn hello_world() {
        eprintln!("Hello world!")
    }
}

It just so happens that those particular tests had doc comments.

[rmanoka]

Oh interesting. You mean the above testing example compiles with test, but not async_std::test or tokio::test whenever there are doc comments? Possibly something to do with the specifics of how the two proc macros are implemented.

[DimiDimit]

Yes. I also tried trace_macros!(true) and the output was the same both for the old and new nightlies, so it seems it is indeed the proc macros.

[rmanoka]

Thanks for investigating this; we should either raise this issue in async_std or rust-lang/rust. Meanwhile, I'll update this crate with your suggested fix (using #[cfg(test)] for the mod tests) so that the crate at least builds with latest nightlies.

[DimiDimit]

I compiled rustc with debug = true, then ran with RUST_BACKTRACE=1 and here's the output:

Backtrace

   Compiling async-scoped v0.6.0 (/tmp/async-scoped)
thread 'rustc' panicked at 'internal error: entered unreachable code', compiler/rustc_expand/src/proc_macro_server.rs:193:20
stack backtrace:
thread 'rustc' panicked at 'internal error: entered unreachable code', compiler/rustc_expand/src/proc_macro_server.rs:193:20
stack backtrace:
   0: rust_begin_unwind
             at /tmp/rust/library/std/src/panicking.rs:493:5
   1: core::panicking::panic_fmt
             at /tmp/rust/library/core/src/panicking.rs:92:14
   2: core::panicking::panic
             at /tmp/rust/library/core/src/panicking.rs:50:5
   3: <proc_macro::bridge::TokenTree<rustc_expand::proc_macro_server::Group,rustc_expand::proc_macro_server::Punct,rustc_expand::proc_macro_server::Ident,rustc_expand::proc_macro_server::Literal> as rustc_expand::proc_macro_server::FromInternal<((rustc_ast::tokenstream::TokenTree,rustc_ast::tokenstream::Spacing),&rustc_session::parse::ParseSess,&mut alloc::vec::Vec<proc_macro::bridge::TokenTree<rustc_expand::proc_macro_server::Group,rustc_expand::proc_macro_server::Punct,rustc_expand::proc_macro_server::Ident,rustc_expand::proc_macro_server::Literal>>)>>::from_internal
             at /tmp/rust/compiler/rustc_expand/src/proc_macro_server.rs:193:20
   4: <rustc_expand::proc_macro_server::Rustc as proc_macro::bridge::server::TokenStreamIter>::next::{{closure}}
             at /tmp/rust/compiler/rustc_expand/src/proc_macro_server.rs:449:22
   5: core::option::Option<T>::or_else
             at /tmp/rust/library/core/src/option.rs:784:21
   6: <rustc_expand::proc_macro_server::Rustc as proc_macro::bridge::server::TokenStreamIter>::next
             at /tmp/rust/compiler/rustc_expand/src/proc_macro_server.rs:447:24
   0: rust_begin_unwind
             at /tmp/rust/library/std/src/panicking.rs:493:5
   1: core::panicking::panic_fmt
             at /tmp/rust/library/core/src/panicking.rs:92:14
   2: core::panicking::panic
             at /tmp/rust/library/core/src/panicking.rs:50:5
   7: <proc_macro::bridge::server::MarkedTypes<S> as proc_macro::bridge::server::TokenStreamIter>::next
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:72:34
   8: <proc_macro::bridge::server::Dispatcher<proc_macro::bridge::server::MarkedTypes<S>> as proc_macro::bridge::server::DispatcherTrait>::dispatch::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:106:33
   9: core::ops::function::FnOnce::call_once
             at /tmp/rust/library/core/src/ops/function.rs:227:5
  10: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at /tmp/rust/library/std/src/panic.rs:322:9
   3: <proc_macro::bridge::TokenTree<rustc_expand::proc_macro_server::Group,rustc_expand::proc_macro_server::Punct,rustc_expand::proc_macro_server::Ident,rustc_expand::proc_macro_server::Literal> as rustc_expand::proc_macro_server::FromInternal<((rustc_ast::tokenstream::TokenTree,rustc_ast::tokenstream::Spacing),&rustc_session::parse::ParseSess,&mut alloc::vec::Vec<proc_macro::bridge::TokenTree<rustc_expand::proc_macro_server::Group,rustc_expand::proc_macro_server::Punct,rustc_expand::proc_macro_server::Ident,rustc_expand::proc_macro_server::Literal>>)>>::from_internal
             at /tmp/rust/compiler/rustc_expand/src/proc_macro_server.rs:193:20
  11: std::panicking::try::do_call
             at /tmp/rust/library/std/src/panicking.rs:379:40
  12: std::panicking::try
             at /tmp/rust/library/std/src/panicking.rs:343:19
   4: <rustc_expand::proc_macro_server::Rustc as proc_macro::bridge::server::TokenStreamIter>::next::{{closure}}
             at /tmp/rust/compiler/rustc_expand/src/proc_macro_server.rs:449:22
   5: core::option::Option<T>::or_else
             at /tmp/rust/library/core/src/option.rs:784:21
   6: <rustc_expand::proc_macro_server::Rustc as proc_macro::bridge::server::TokenStreamIter>::next
             at /tmp/rust/compiler/rustc_expand/src/proc_macro_server.rs:447:24
   7: <proc_macro::bridge::server::MarkedTypes<S> as proc_macro::bridge::server::TokenStreamIter>::next
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:72:34
   8: <proc_macro::bridge::server::Dispatcher<proc_macro::bridge::server::MarkedTypes<S>> as proc_macro::bridge::server::DispatcherTrait>::dispatch::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:106:33
   9: core::ops::function::FnOnce::call_once
             at /tmp/rust/library/core/src/ops/function.rs:227:5
  10: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at /tmp/rust/library/std/src/panic.rs:322:9
  11: std::panicking::try::do_call
             at /tmp/rust/library/std/src/panicking.rs:379:40
  12: std::panicking::try
             at /tmp/rust/library/std/src/panicking.rs:343:19
  13: std::panic::catch_unwind
             at /tmp/rust/library/std/src/panic.rs:396:14
  14: <proc_macro::bridge::server::Dispatcher<proc_macro::bridge::server::MarkedTypes<S>> as proc_macro::bridge::server::DispatcherTrait>::dispatch
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:115:33
  15: <proc_macro::bridge::server::SameThread as proc_macro::bridge::server::ExecutionStrategy>::run_bridge_and_client::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:153:32
  16: <proc_macro::bridge::closure::Closure<A,R> as core::convert::From<&mut F>>::from::call
             at /tmp/rust/library/proc_macro/src/bridge/closure.rs:19:13
  13: std::panic::catch_unwind
             at /tmp/rust/library/std/src/panic.rs:396:14
  14: <proc_macro::bridge::server::Dispatcher<proc_macro::bridge::server::MarkedTypes<S>> as proc_macro::bridge::server::DispatcherTrait>::dispatch
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:115:33
  15: <proc_macro::bridge::server::SameThread as proc_macro::bridge::server::ExecutionStrategy>::run_bridge_and_client::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:153:32
  16: <proc_macro::bridge::closure::Closure<A,R> as core::convert::From<&mut F>>::from::call
             at /tmp/rust/library/proc_macro/src/bridge/closure.rs:19:13
  17: proc_macro::bridge::closure::Closure<A,R>::call
             at /tmp/rust/library/proc_macro/src/bridge/closure.rs:27:18
  18: proc_macro::bridge::client::TokenStreamIter::next::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:244:25
  19: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::with::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:336:47
  20: proc_macro::bridge::client::BridgeState::with::{{closure}}::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:293:17
  21: proc_macro::bridge::scoped_cell::ScopedCell<T>::replace
             at /tmp/rust/library/proc_macro/src/bridge/scoped_cell.rs:75:9
  17: proc_macro::bridge::closure::Closure<A,R>::call
             at /tmp/rust/library/proc_macro/src/bridge/closure.rs:27:18
  22: proc_macro::bridge::client::BridgeState::with::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:291:13
  23: std::thread::local::LocalKey<T>::try_with
             at /tmp/rust/library/std/src/thread/local.rs:272:16
  24: std::thread::local::LocalKey<T>::with
             at /tmp/rust/library/std/src/thread/local.rs:248:9
  25: proc_macro::bridge::client::BridgeState::with
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:290:9
  26: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::with
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:329:9
  27: proc_macro::bridge::client::TokenStreamIter::next
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:237:17
  28: <proc_macro::token_stream::IntoIter as core::iter::traits::iterator::Iterator>::next
             at /tmp/rust/library/proc_macro/src/lib.rs:240:13
  29: <proc_macro2::imp::TokenTreeIter as core::iter::traits::iterator::Iterator>::next
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/proc-macro2-1.0.24/src/wrapper.rs:304:46
  18: proc_macro::bridge::client::TokenStreamIter::next::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:244:25
  19: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::with::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:336:47
  20: proc_macro::bridge::client::BridgeState::with::{{closure}}::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:293:17
  21: proc_macro::bridge::scoped_cell::ScopedCell<T>::replace
             at /tmp/rust/library/proc_macro/src/bridge/scoped_cell.rs:75:9
  30: <proc_macro2::token_stream::IntoIter as core::iter::traits::iterator::Iterator>::next
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/proc-macro2-1.0.24/src/lib.rs:1213:13
  22: proc_macro::bridge::client::BridgeState::with::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:291:13
  23: std::thread::local::LocalKey<T>::try_with
             at /tmp/rust/library/std/src/thread/local.rs:272:16
  24: std::thread::local::LocalKey<T>::with
             at /tmp/rust/library/std/src/thread/local.rs:248:9
  25: proc_macro::bridge::client::BridgeState::with
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:290:9
  26: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::with
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:329:9
  27: proc_macro::bridge::client::TokenStreamIter::next
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:237:17
  28: <proc_macro::token_stream::IntoIter as core::iter::traits::iterator::Iterator>::next
             at /tmp/rust/library/proc_macro/src/lib.rs:240:13
  29: <proc_macro2::imp::TokenTreeIter as core::iter::traits::iterator::Iterator>::next
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/proc-macro2-1.0.24/src/wrapper.rs:304:46
  30: <proc_macro2::token_stream::IntoIter as core::iter::traits::iterator::Iterator>::next
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/proc-macro2-1.0.24/src/lib.rs:1213:13
  31: syn::buffer::TokenBuffer::inner_new
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.58/src/buffer.rs:53:19
  32: syn::buffer::TokenBuffer::new2
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.58/src/buffer.rs:112:9
  33: <F as syn::parse::Parser>::parse2
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.58/src/parse.rs:1207:19
  34: syn::parse::Parser::parse
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.58/src/parse.rs:1161:9
  35: syn::parse_macro_input::parse
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.58/src/parse_macro_input.rs:139:5
  36: async_attributes::test
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/async-attributes-1.1.1/src/lib.rs:99:17
  37: core::ops::function::FnOnce::call_once
             at /tmp/rust/library/core/src/ops/function.rs:227:5
  31: syn::buffer::TokenBuffer::inner_new
  38: proc_macro::bridge::client::Client<fn(proc_macro::TokenStream,proc_macro::TokenStream) .> proc_macro::TokenStream>::expand2::run::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:426:17
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.58/src/buffer.rs:53:19
  32: syn::buffer::TokenBuffer::new2
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.58/src/buffer.rs:112:9
  39: proc_macro::bridge::client::run_client::{{closure}}::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:377:26
  40: proc_macro::bridge::scoped_cell::ScopedCell<T>::set::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/scoped_cell.rs:80:33
  41: proc_macro::bridge::scoped_cell::ScopedCell<T>::replace
             at /tmp/rust/library/proc_macro/src/bridge/scoped_cell.rs:75:9
  42: proc_macro::bridge::scoped_cell::ScopedCell<T>::set
             at /tmp/rust/library/proc_macro/src/bridge/scoped_cell.rs:80:9
  43: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::enter::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:325:35
  44: std::thread::local::LocalKey<T>::try_with
             at /tmp/rust/library/std/src/thread/local.rs:272:16
  45: std::thread::local::LocalKey<T>::with
             at /tmp/rust/library/std/src/thread/local.rs:248:9
  33: <F as syn::parse::Parser>::parse2
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.58/src/parse.rs:1207:19
  34: syn::parse::Parser::parse
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.58/src/parse.rs:1161:9
  35: syn::parse_macro_input::parse
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/syn-1.0.58/src/parse_macro_input.rs:139:5
  46: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::enter
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:325:9
  47: proc_macro::bridge::client::run_client::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:370:9
  48: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at /tmp/rust/library/std/src/panic.rs:322:9
  36: async_attributes::test
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/async-attributes-1.1.1/src/lib.rs:99:17
  37: core::ops::function::FnOnce::call_once
             at /tmp/rust/library/core/src/ops/function.rs:227:5
  38: proc_macro::bridge::client::Client<fn(proc_macro::TokenStream,proc_macro::TokenStream) .> proc_macro::TokenStream>::expand2::run::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:426:17
  39: proc_macro::bridge::client::run_client::{{closure}}::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:377:26
  49: std::panicking::try::do_call
             at /tmp/rust/library/std/src/panicking.rs:379:40
  50: __rust_try
  40: proc_macro::bridge::scoped_cell::ScopedCell<T>::set::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/scoped_cell.rs:80:33
  51: std::panicking::try
             at /tmp/rust/library/std/src/panicking.rs:343:19
  52: std::panic::catch_unwind
             at /tmp/rust/library/std/src/panic.rs:396:14
  41: proc_macro::bridge::scoped_cell::ScopedCell<T>::replace
             at /tmp/rust/library/proc_macro/src/bridge/scoped_cell.rs:75:9
  53: proc_macro::bridge::client::run_client
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:369:5
  42: proc_macro::bridge::scoped_cell::ScopedCell<T>::set
             at /tmp/rust/library/proc_macro/src/bridge/scoped_cell.rs:80:9
  54: proc_macro::bridge::client::Client<fn(proc_macro::TokenStream,proc_macro::TokenStream) .> proc_macro::TokenStream>::expand2::run
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:425:13
  43: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::enter::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:325:35
  55: <proc_macro::bridge::server::SameThread as proc_macro::bridge::server::ExecutionStrategy>::run_bridge_and_client
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:155:9
  56: proc_macro::bridge::server::run_server
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:291:9
  44: std::thread::local::LocalKey<T>::try_with
             at /tmp/rust/library/std/src/thread/local.rs:272:16
  45: std::thread::local::LocalKey<T>::with
             at /tmp/rust/library/std/src/thread/local.rs:248:9
  57: proc_macro::bridge::server::<impl proc_macro::bridge::client::Client<fn(proc_macro::TokenStream,proc_macro::TokenStream) .> proc_macro::TokenStream>>::run
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:334:9
  58: <rustc_expand::proc_macro::AttrProcMacro as rustc_expand::base::AttrProcMacro>::expand
             at /tmp/rust/compiler/rustc_expand/src/proc_macro.rs:53:9
  46: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::enter
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:325:9
  47: proc_macro::bridge::client::run_client::{{closure}}
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:370:9
  48: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
             at /tmp/rust/library/std/src/panic.rs:322:9
  49: std::panicking::try::do_call
             at /tmp/rust/library/std/src/panicking.rs:379:40
  50: __rust_try
  51: std::panicking::try
             at /tmp/rust/library/std/src/panicking.rs:343:19
  52: std::panic::catch_unwind
             at /tmp/rust/library/std/src/panic.rs:396:14
  59: rustc_expand::expand::MacroExpander::expand_invoc
             at /tmp/rust/compiler/rustc_expand/src/expand.rs:753:44
  60: rustc_expand::expand::MacroExpander::fully_expand_fragment
             at /tmp/rust/compiler/rustc_expand/src/expand.rs:497:53
  53: proc_macro::bridge::client::run_client
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:369:5
  54: proc_macro::bridge::client::Client<fn(proc_macro::TokenStream,proc_macro::TokenStream) .> proc_macro::TokenStream>::expand2::run
             at /tmp/rust/library/proc_macro/src/bridge/client.rs:425:13
  55: <proc_macro::bridge::server::SameThread as proc_macro::bridge::server::ExecutionStrategy>::run_bridge_and_client
  61: rustc_expand::expand::MacroExpander::expand_crate
             at /tmp/rust/compiler/rustc_expand/src/expand.rs:390:15
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:155:9
  56: proc_macro::bridge::server::run_server
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:291:9
  57: proc_macro::bridge::server::<impl proc_macro::bridge::client::Client<fn(proc_macro::TokenStream,proc_macro::TokenStream) .> proc_macro::TokenStream>>::run
             at /tmp/rust/library/proc_macro/src/bridge/server.rs:334:9
  58: <rustc_expand::proc_macro::AttrProcMacro as rustc_expand::base::AttrProcMacro>::expand
             at /tmp/rust/compiler/rustc_expand/src/proc_macro.rs:53:9
  62: rustc_interface::passes::configure_and_expand_inner::{{closure}}::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/passes.rs:302:50
  63: rustc_data_structures::profiling::VerboseTimingGuard::run
             at /tmp/rust/compiler/rustc_data_structures/src/profiling.rs:570:9
  64: rustc_session::utils::<impl rustc_session::session::Session>::time
             at /tmp/rust/compiler/rustc_session/src/utils.rs:9:9
  65: rustc_interface::passes::configure_and_expand_inner::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/passes.rs:302:21
  66: rustc_data_structures::profiling::VerboseTimingGuard::run
             at /tmp/rust/compiler/rustc_data_structures/src/profiling.rs:570:9
  67: rustc_session::utils::<impl rustc_session::session::Session>::time
             at /tmp/rust/compiler/rustc_session/src/utils.rs:9:9
  59: rustc_expand::expand::MacroExpander::expand_invoc
             at /tmp/rust/compiler/rustc_expand/src/expand.rs:753:44
  60: rustc_expand::expand::MacroExpander::fully_expand_fragment
  68: rustc_interface::passes::configure_and_expand_inner
             at /tmp/rust/compiler/rustc_expand/src/expand.rs:497:53
             at /tmp/rust/compiler/rustc_interface/src/passes.rs:254:13
  69: rustc_interface::passes::configure_and_expand::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/passes.rs:118:19
  61: rustc_expand::expand::MacroExpander::expand_crate
             at /tmp/rust/compiler/rustc_expand/src/expand.rs:390:15
  70: alloc::boxed::<impl core::ops::generator::Generator<R> for core::pin::Pin<alloc::boxed::Box<G,A>>>::resume
             at /tmp/rust/library/alloc/src/boxed.rs:1606:9
  71: rustc_data_structures::box_region::PinnedGenerator<I,A,R>::new
             at /tmp/rust/compiler/rustc_data_structures/src/box_region.rs:44:26
  72: rustc_interface::passes::BoxedResolver::new
             at /tmp/rust/compiler/rustc_data_structures/src/box_region.rs:101:41
  73: rustc_interface::passes::configure_and_expand
             at /tmp/rust/compiler/rustc_interface/src/passes.rs:114:30
  62: rustc_interface::passes::configure_and_expand_inner::{{closure}}::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/passes.rs:302:50
  63: rustc_data_structures::profiling::VerboseTimingGuard::run
             at /tmp/rust/compiler/rustc_data_structures/src/profiling.rs:570:9
  64: rustc_session::utils::<impl rustc_session::session::Session>::time
             at /tmp/rust/compiler/rustc_session/src/utils.rs:9:9
  65: rustc_interface::passes::configure_and_expand_inner::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/passes.rs:302:21
  66: rustc_data_structures::profiling::VerboseTimingGuard::run
             at /tmp/rust/compiler/rustc_data_structures/src/profiling.rs:570:9
  67: rustc_session::utils::<impl rustc_session::session::Session>::time
             at /tmp/rust/compiler/rustc_session/src/utils.rs:9:9
  74: rustc_interface::queries::Queries::expansion::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/queries.rs:180:13
  75: rustc_interface::queries::Query<T>::compute
             at /tmp/rust/compiler/rustc_interface/src/queries.rs:39:28
  76: rustc_interface::queries::Queries::expansion
             at /tmp/rust/compiler/rustc_interface/src/queries.rs:176:9
  68: rustc_interface::passes::configure_and_expand_inner
             at /tmp/rust/compiler/rustc_interface/src/passes.rs:254:13
  69: rustc_interface::passes::configure_and_expand::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/passes.rs:118:19
  70: alloc::boxed::<impl core::ops::generator::Generator<R> for core::pin::Pin<alloc::boxed::Box<G,A>>>::resume
             at /tmp/rust/library/alloc/src/boxed.rs:1606:9
  71: rustc_data_structures::box_region::PinnedGenerator<I,A,R>::new
             at /tmp/rust/compiler/rustc_data_structures/src/box_region.rs:44:26
  77: rustc_driver::run_compiler::{{closure}}::{{closure}}
             at /tmp/rust/compiler/rustc_driver/src/lib.rs:393:13
  78: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
             at /tmp/rust/compiler/rustc_interface/src/queries.rs:418:19
  72: rustc_interface::passes::BoxedResolver::new
             at /tmp/rust/compiler/rustc_data_structures/src/box_region.rs:101:41
  79: rustc_driver::run_compiler::{{closure}}
  73: rustc_interface::passes::configure_and_expand
             at /tmp/rust/compiler/rustc_driver/src/lib.rs:341:22
             at /tmp/rust/compiler/rustc_interface/src/passes.rs:114:30
  80: rustc_interface::interface::create_compiler_and_run::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/interface.rs:197:13
  81: rustc_span::with_source_map
             at /tmp/rust/compiler/rustc_span/src/lib.rs:787:5
  82: rustc_interface::interface::create_compiler_and_run
             at /tmp/rust/compiler/rustc_interface/src/interface.rs:191:5
  83: rustc_interface::interface::run_compiler::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/interface.rs:213:12
  84: rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals::{{closure}}::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/util.rs:152:13
  85: scoped_tls::ScopedKey<T>::set
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137:9
  86: rustc_span::with_session_globals
             at /tmp/rust/compiler/rustc_span/src/lib.rs:103:5
  87: rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/util.rs:150:9
  88: rustc_interface::util::scoped_thread::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/util.rs:125:24
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

  74: rustc_interface::queries::Queries::expansion::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/queries.rs:180:13
  75: rustc_interface::queries::Query<T>::compute
             at /tmp/rust/compiler/rustc_interface/src/queries.rs:39:28
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-dev running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C linker=/usr/bin/clang -C incremental -C link-arg=-fuse-ld=lld

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

query stack during panic:
  76: rustc_interface::queries::Queries::expansion
end of query stack
             at /tmp/rust/compiler/rustc_interface/src/queries.rs:176:9
  77: rustc_driver::run_compiler::{{closure}}::{{closure}}
             at /tmp/rust/compiler/rustc_driver/src/lib.rs:393:13
  78: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
             at /tmp/rust/compiler/rustc_interface/src/queries.rs:418:19
  79: rustc_driver::run_compiler::{{closure}}
             at /tmp/rust/compiler/rustc_driver/src/lib.rs:341:22
  80: rustc_interface::interface::create_compiler_and_run::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/interface.rs:197:13
  81: rustc_span::with_source_map
             at /tmp/rust/compiler/rustc_span/src/lib.rs:787:5
  82: rustc_interface::interface::create_compiler_and_run
             at /tmp/rust/compiler/rustc_interface/src/interface.rs:191:5
error: custom attribute panicked
  --> src/tests.rs:5:13
   |
5  |               #[async_std::test]
   |               ^^^^^^^^^^^^^^^^^^
...
11 | / test_fixtures! {
12 | |     /// Doc comment.
13 | |     async fn test() {
14 | |         eprintln!("Hello world!");
15 | |     }
16 | | }
   | |_- in this macro invocation
   |
   = help: message: internal error: entered unreachable code
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

  83: rustc_interface::interface::run_compiler::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/interface.rs:213:12
  84: rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals::{{closure}}::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/util.rs:152:13
  85: scoped_tls::ScopedKey<T>::set
             at /home/dimitar/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-1.0.0/src/lib.rs:137:9
  86: rustc_span::with_session_globals
             at /tmp/rust/compiler/rustc_span/src/lib.rs:103:5
  87: rustc_interface::util::setup_callbacks_and_run_in_thread_pool_with_globals::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/util.rs:150:9
  88: rustc_interface::util::scoped_thread::{{closure}}
             at /tmp/rust/compiler/rustc_interface/src/util.rs:125:24
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-dev running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C linker=/usr/bin/clang -C incremental -C link-arg=-fuse-ld=lld --crate-type lib

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

query stack during panic:
end of query stack
error: custom attribute panicked
  --> src/tests.rs:5:13
   |
5  |               #[async_std::test]
   |               ^^^^^^^^^^^^^^^^^^
...
11 | / test_fixtures! {
12 | |     /// Doc comment.
13 | |     async fn test() {
14 | |         eprintln!("Hello world!");
15 | |     }
16 | | }
   | |_- in this macro invocation
   |
   = help: message: internal error: entered unreachable code
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

error: aborting due to previous error

error: could not compile `async-scoped`

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed

In short, we end up in compiler/rustc_expand/src/proc_macro_server.rs:193:

    Eof => unreachable!(),

I also had a look at async_std::test, and it's trivial so we can rule async_std out completely. Judging by everything up until now as well as this:

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

I think we can safely say it's an issue with rustc and submit it there. I'll do the honors.

[DimiDimit]

rust-lang/rust#81007

[rmanoka]

Looks like recent nightlies have fixed the ICE (verified on 22ddcd1a1 2021-01-22).

[DimiDimit]

I can confirm that it is indeed fixed after rust-lang/rust@11b1e37, and evened out my fork with upstream.