"Error constructed but not emitted" ICE on nightly parsing wonky impl

[ben0x539]

rustc 1.18.0-nightly (53f4bc3 2017-04-07)
stable/beta/nightly from a week ago seem unaffected.

$ cat > bug.rs
impl X{Y}
$ RUST_BACKTRACE=full rustc bug.rs 
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', src/librustc_errors/diagnostic_builder.rs:192
stack backtrace:
   0:        0x10fd12e83 - std::sys::imp::backtrace::tracing::imp::unwind_backtrace::hcc709dd93d82a051
   1:        0x10fd1fafd - std::panicking::default_hook::{{closure}}::he392668e67778d9e 
   2:        0x10fd1f644 - std::panicking::default_hook::h2ade4aa81d4f8204
   3:        0x10fd22277 - std::panicking::rust_panic_with_hook::h4573918fc9f80a34
   4:        0x10d2aa534 - std::panicking::begin_panic::h2a5fc248b95a3db2
   5:        0x10d2b143b - <rustc_errors::diagnostic_builder::DiagnosticBuilder<'a> as core::ops::Drop>::drop::h12de2c56938d20bb
   6:        0x10cdd289b - syntax::parse::parser::Parser::parse_impl_item::h3ddc1b71a9135797
   7:        0x10cdd50c1 - syntax::parse::parser::Parser::parse_item_impl::h284e44c48881ab63
   8:        0x10cde1356 - syntax::parse::parser::Parser::parse_item_::h44415882723eb725
   9:        0x10cdeabd8 - syntax::parse::parser::Parser::parse_item::h923de9a5bd86a0a8
  10:        0x10cdd8d5e - syntax::parse::parser::Parser::parse_mod_items::hf3c521b150993a7f
  11:        0x10cdeb7dd - syntax::parse::parser::Parser::parse_crate_mod::h19282d20641c273e
  12:        0x10cdfb19e - syntax::parse::parse_crate_from_file::hdab1619deb4b8e47
  13:        0x10b02655c - rustc_driver::driver::phase_1_parse_input::{{closure}}::hadf9f24a52d977ac
  14:        0x10b0255c5 - rustc_driver::driver::phase_1_parse_input::h4228cea37031992a
  15:        0x10b018554 - rustc_driver::driver::compile_input::h163f6d3afe612689
  16:        0x10b076046 - rustc_driver::run_compiler::h870480169c099057
  17:        0x10af49fa8 - std::panicking::try::do_call::h122fb7737645584e
  18:        0x10fd2508a - __rust_maybe_catch_panic
  19:        0x10af907ef - <F as alloc::boxed::FnBox<A>>::call_box::hdbb485c2e5737f1e
  20:        0x10fd1eae4 - std::sys::imp::thread::Thread::new::thread_start::hba7593f2e1f1beb1
  21:     0x7fff884f899c - _pthread_body
  22:     0x7fff884f8919 - _pthread_start

Reduced from impl Iterator for Foo { Item = Bar; ... } where I forgot the type.

[mgattozzi]

I had the same issue. I've pushed a separate branch of my library as a test case if needed to check if this gets fixed.

[nikomatsakis]

triage: P-medium