ICE on invalid syntax in proc macro attr

[guzt]

main.rs:

mod ice;
fn main() {
}

ice.rs:

use async_trait::async_trait;

#[async_trait]
pub trait T1 {
    async fn f1(&self) -> Result<m1::S1, ()>;
}

mod m1 {
    pub struct S1 {
        pub v1: String,
    }
}
pub enum E1 {}

#[async_trait]
impl T1 for () {
    async fn f1(&self) -> Result<m1::S1, ()> {
        if false {
            return m1::S1 {
                v1: format!(""),
            }
        } else {
            return Err(m1::E1::
        }
    }
}

(reduced sample courtesy of @ComputerDruid)

Output:

error: expected identifier, found `)`
  --> src/ice.rs:24:9
   |
24 |         }
   |         ^ expected identifier

error: incorrect close delimiter: `}`
  --> src/ice.rs:24:9
   |
22 |         } else {
   |                - close delimiter possibly meant for this
23 |             return Err(m1::E1::
   |                       - un-closed delimiter
24 |         }
   |         ^ incorrect close delimiter

thread 'rustc' panicked at 'called `Result::unwrap()` on an `Err` value: DistinctSources(DistinctSources { begin: (Real("src/main.rs"), BytePos(0)), end: (Real("src/ice.rs"), BytePos(25)) })', src/libcore/result.rs:1188:5
stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: std::io::Write::write_fmt
   3: std::panicking::default_hook::{{closure}}
   4: std::panicking::default_hook
   5: rustc_driver::report_ice
   6: <alloc::boxed::Box<F> as core::ops::function::Fn<A>>::call
             at /b/s/w/ir/k/rust/src/liballoc/boxed.rs:1036
   7: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::enter::{{closure}}::{{closure}}
             at /b/s/w/ir/k/rust/src/libproc_macro/bridge/client.rs:305
   8: std::panicking::rust_panic_with_hook
   9: rust_begin_unwind
  10: core::panicking::panic_fmt
  11: core::result::unwrap_failed
  12: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::fold
  13: <alloc::vec::Vec<T> as alloc::vec::SpecExtend<T,I>>::from_iter
  14: <rustc_errors::emitter::EmitterWriter as rustc_errors::emitter::Emitter>::emit_diagnostic
  15: <rustc_errors::json::JsonEmitter as rustc_errors::emitter::Emitter>::emit_diagnostic
  16: rustc_errors::HandlerInner::emit_diagnostic
  17: rustc_errors::diagnostic_builder::DiagnosticBuilder::emit_unless
  18: rustc_typeck::check::coercion::CoerceMany<E>::coerce_inner
  19: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_return_expr
  20: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  21: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  22: rustc_typeck::check::FnCtxt::check_block_with_expected
  23: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  24: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  25: rustc_typeck::check::_match::<impl rustc_typeck::check::FnCtxt>::check_match
  26: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  27: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  28: rustc_typeck::check::FnCtxt::check_block_with_expected
  29: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  30: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  31: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  32: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  33: rustc_typeck::check::FnCtxt::check_block_with_expected
  34: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  35: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  36: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_return_expr
  37: rustc_typeck::check::check_fn
  38: rustc_typeck::check::closure::<impl rustc_typeck::check::FnCtxt>::check_expr_closure
  39: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  40: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  41: rustc_typeck::check::FnCtxt::check_argument_types
  42: rustc_typeck::check::callee::<impl rustc_typeck::check::FnCtxt>::confirm_builtin_call
  43: rustc_typeck::check::callee::<impl rustc_typeck::check::FnCtxt>::check_call
  44: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  45: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  46: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_return_expr
  47: rustc_typeck::check::check_fn
  48: rustc::ty::context::tls::with_context::{{closure}}
  49: rustc_typeck::check::typeck_tables_of
  50: rustc::ty::query::__query_compute::typeck_tables_of
  51: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  52: rustc::dep_graph::graph::DepGraph::with_task_impl
  53: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  54: rustc::ty::query::__query_compute::typeck_tables_of
  55: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::typeck_tables_of>::compute
  56: rustc::dep_graph::graph::DepGraph::with_task_impl
  57: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  58: rustc_typeck::collect::type_of
  59: rustc::ty::query::__query_compute::type_of
  60: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::type_of>::compute
  61: rustc::dep_graph::graph::DepGraph::with_task_impl
  62: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  63: rustc::hir::intravisit::walk_expr
  64: rustc::hir::intravisit::Visitor::visit_fn
  65: rustc::hir::intravisit::walk_item
  66: <rustc_typeck::collect::CollectItemTypesVisitor as rustc::hir::intravisit::Visitor>::visit_item
  67: rustc::hir::map::Map::visit_item_likes_in_module
  68: rustc_typeck::collect::collect_mod_item_types
  69: rustc::ty::query::__query_compute::collect_mod_item_types
  70: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::collect_mod_item_types>::compute
  71: rustc::dep_graph::graph::DepGraph::with_task_impl
  72: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  73: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::ensure_query
  74: rustc_typeck::check_crate::{{closure}}::{{closure}}
  75: rustc::util::common::time
  76: rustc_typeck::check_crate
  77: rustc_interface::passes::analysis
  78: rustc::ty::query::__query_compute::analysis
  79: rustc::dep_graph::graph::DepGraph::with_task_impl
  80: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  81: rustc::ty::context::tls::enter_global
  82: rustc_interface::interface::run_compiler_in_existing_thread_pool
  83: std::thread::local::LocalKey<T>::with
  84: scoped_tls::ScopedKey<T>::set
  85: syntax::with_globals
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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.42.0-nightly (c605199e8 2019-12-18) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
#0 [typeck_tables_of] processing `<() as ice::T1>::f1::__f1`
#1 [typeck_tables_of] processing `<() as ice::T1>::f1::__f1::{{closure}}#0`
#2 [type_of] processing `<() as ice::T1>::f1::__f1::{{closure}}#0`
#3 [collect_mod_item_types] collecting item types in module `ice`
#4 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 2 previous errors

[pnkfelix]

triage: diagnostics produced prior to ICE seem like they probably help user fix problem (maybe)? So calling this P-medium. Removing nomination label.