Certain kinds of errors inside before_each cause the compiler to panic

[geomaster]

Stainless version: 0.1.11
rustc version: 1.19.0-nightly (75b056812 2017-05-15)

A minimal test case follows. I put the code in src/test.rs and src/main.rs only contains configuration needed for stainless, and a mod test; declaration.

describe! my_module {
    before_each {
        let a = (0..2).map(|x| error_token)
    }
}

(Note the missing semicolon; without it, everything works fine.)

After running cargo test, I'm getting the following output:

error: internal compiler error: Error constructed but not emitted

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: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'explicit panic', /checkout/src/librustc_errors/diagnostic_builder.rs:204
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:60
             at /checkout/src/libstd/panicking.rs:355
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:365
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:549
   5: std::panicking::begin_panic
   6: <rustc_errors::diagnostic_builder::DiagnosticBuilder<'a> as core::ops::Drop>::drop
   7: core::ptr::drop_in_place
             at /checkout/src/libcore/ptr.rs:60
   8: core::ptr::drop_in_place
             at /checkout/src/libcore/ptr.rs:60
   9: <core::result::Result<T, E>>::ok
             at /checkout/src/libcore/result.rs:341
  10: <stainless::describe::DescribeState as stainless::parse::Parse<(syntax_pos::Span, &'a mut syntax::ext::base::ExtCtxt<'b>, core::option::Option<syntax_pos::symbol::Ident>)>>::parse
             at /home/geomaster/.cargo/registry/src/github.com-1ecc6299db9ec823/stainless-0.1.11/src/parse.rs:143
  11: stainless::describe::describe
             at /home/geomaster/.cargo/registry/src/github.com-1ecc6299db9ec823/stainless-0.1.11/src/describe.rs:68
  12: core::ops::Fn::call
             at /checkout/src/libcore/ops.rs:2567
  13: <F as syntax::ext::base::IdentMacroExpander>::expand
             at /checkout/src/libsyntax/ext/base.rs:260
  14: syntax::ext::expand::MacroExpander::expand_invoc
  15: syntax::ext::expand::MacroExpander::expand
  16: syntax::ext::expand::MacroExpander::expand_crate
  17: rustc_driver::driver::phase_2_configure_and_expand::{{closure}}
  18: rustc_driver::driver::phase_2_configure_and_expand
  19: rustc_driver::driver::compile_input
  20: rustc_driver::run_compiler

I'll be happy to provide any further information, if needed.