custom derive causing ICE on beta and nightly

[sgrif]

While working on a new feature for Diesel, one of the changes caused the compiler to start ICEing on beta and nightly. The code works fine on stable. The branch is https://github.com/diesel-rs/diesel/compare/sg-rust-bug

I'm still trying to pin down what specifically caused the ICE. The crash happens before it actually gets to the custom derive at all, so it seems that just adding #[derive(ValidGrouping)] to one of those types caused it. Here's the backtrace:

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', libcore/option.rs:335:21
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
   1: std::sys_common::backtrace::_print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic
   6: std::panicking::begin_panic_fmt
   7: rust_begin_unwind
   8: core::panicking::panic_fmt
   9: core::panicking::panic
  10: rustc_resolve::build_reduced_graph::<impl rustc_resolve::Resolver<'a>>::get_module
  11: rustc_resolve::macros::<impl syntax::ext::base::Resolver for rustc_resolve::Resolver<'a>>::resolve_invoc
  12: syntax::ext::expand::MacroExpander::expand
  13: syntax::ext::expand::MacroExpander::expand_crate
  14: rustc_driver::driver::phase_2_configure_and_expand_inner::{{closure}}
  15: rustc_driver::driver::phase_2_configure_and_expand_inner
  16: rustc_driver::driver::compile_input
  17: rustc_driver::run_compiler

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 1.25.0-beta.7 (da64ca931 2018-03-03) running on x86_64-apple-darwin

[sgrif]

I'm pushing to the branch linked above as I attempt to isolate the bug. Removing all instances of #[derive(ValidGrouping)] causes the ICE to stop occurring. The body of the derive is irrelevant (I have replaced it with "".parse().unwrap()). However, just having one invocation of the derive and the empty body is not enough to reproduce it.

[sgrif]

Specifically this line (or any usage of #[derive(ValidGrouping)]) causes the ICE. If you revert that one line, the ICE no longer occurs. I'm at a bit of a loss as to what else to try deleting here

[nikomatsakis]

cc @rust-lang/release -- it would be awesome to bisect this and try to find where the problem was introduced.

@sgrif the only working end point we have is that it works on current stable, right?

[nikomatsakis]

triage: P-high

[pietroalbini]

Running a bisect.

[pietroalbini]

@nikomatsakis the bisect run points at #47181 as the cause of the regression (merge commit ca09293).

[nikomatsakis]

cc @michaelwoerister -- this seems to be due to "Use DefIndex encoding that works better with on-disk variable length integer representations." (#47181)