“Let NullaryConstructor = something;” causes internal compiler error: “tried to overwrite interned AdtDef”

[Rufflewind]

I was trying an if let pattern match for a nullary constructor like this:

if let Some(::router::NoRoute) = err.error.downcast() {
    …
}

but the compiler choked on the pattern match in an unexpected way. The fact that it's qualified does not seem to matter. See minimal repro below.

---

I tried this code:

extern crate router;
fn test() {
    let router::NoRoute = (); // NoRoute is an empty struct in “router”
}

I expected to see this happen:

An plain, ordinary, not-scary type error.

Instead, this happened:

error: internal compiler error: src/librustc/ty/context.rs:604: Tried to overwrite interned AdtDef: router::NoRoute

note: the compiler unexpectedly panicked. this is a bug.

Meta

rustc --version --verbose:

rustc 1.12.0
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.12.0

Backtrace:

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:634
stack backtrace:
   1:     0x7f3f96d52cd9 - std::sys::backtrace::tracing::imp::write::h00e948915d1e4c72
   2:     0x7f3f96d6351c - std::panicking::default_hook::_{{closure}}::h7b8a142818383fb8
   3:     0x7f3f96d61914 - std::panicking::default_hook::h41cf296f654245d7
   4:     0x7f3f96d62078 - std::panicking::rust_panic_with_hook::h4cbd7ca63ce1aee9
   5:     0x7f3f906c7747 - std::panicking::begin_panic::hd3e81d866d6c5b0a
   6:     0x7f3f906d8325 - rustc_errors::Handler::bug::h60f29cdeff67c1a3
   7:     0x7f3f93fe74e4 - rustc::session::opt_span_bug_fmt::_{{closure}}::ha1319a72e07cd73b
   8:     0x7f3f93f289d5 - rustc::session::opt_span_bug_fmt::h4badbc71bfeea6f9
   9:     0x7f3f93f28812 - rustc::session::bug_fmt::he780dcb9e1e8a7d5
  10:     0x7f3f93f9eabc - rustc::ty::context::TyCtxt::insert_adt_def::hf29e70e1fbb3e0ab
  11:     0x7f3f9464b4f5 - rustc_metadata::decoder::get_adt_def::h78248c8ed76d9698
  12:     0x7f3f9466a294 - rustc_metadata::csearch::_<impl rustc..middle..cstore..CrateStore<'tcx> for rustc_metadata..cstore..CStore>::adt_def::h9010a99ee3288f01
  13:     0x7f3f93fc9477 - rustc::ty::_<impl rustc..ty..context..TyCtxt<'a, 'gcx, 'tcx>>::lookup_adt_def_master::h9f53a256095087eb
  14:     0x7f3f93fc9985 - rustc::ty::_<impl rustc..ty..context..TyCtxt<'a, 'gcx, 'tcx>>::lookup_adt_def::h4a8f88c8263367bd
  15:     0x7f3f9462afd9 - rustc_metadata::tydecode::TyDecoder::parse_ty::h2699ee85579ac86e
  16:     0x7f3f9462a8a0 - rustc_metadata::tydecode::TyDecoder::parse_ty::h2699ee85579ac86e
  17:     0x7f3f9464c48d - rustc_metadata::decoder::get_type::h6f6b706ba668663d
  18:     0x7f3f94669f3f - rustc_metadata::csearch::_<impl rustc..middle..cstore..CrateStore<'tcx> for rustc_metadata..cstore..CStore>::item_type::h503a61f838833975
  19:     0x7f3f93fc80fd - rustc::ty::_<impl rustc..ty..context..TyCtxt<'a, 'gcx, 'tcx>>::lookup_item_type::h32494593dee57ed3
  20:     0x7f3f9535f985 - rustc_typeck::check::_match::_<impl rustc_typeck..check..FnCtxt<'a, 'gcx, 'tcx>>::check_pat::hb595ae90c56692e6
  21:     0x7f3f953feb43 - rustc_typeck::check::FnCtxt::check_decl_local::h3566652c6c4664a1
  22:     0x7f3f953fec7e - rustc_typeck::check::FnCtxt::check_stmt::h012039db8f7fe2ee
  23:     0x7f3f953fefe7 - rustc_typeck::check::FnCtxt::check_block_with_expected::h6457d67c748c48f5
  24:     0x7f3f953cc1a1 - rustc_typeck::check::check_fn::h116c7bc1c461e3a6
  25:     0x7f3f953ca5d5 - rustc_typeck::check::check_bare_fn::haaae113cd154037a
  26:     0x7f3f953ce741 - rustc_typeck::check::check_item_body::h169c2400d0395b3e
  27:     0x7f3f953c7308 - rustc_typeck::check::check_item_bodies::h3f9ccb4bec3365b2
  28:     0x7f3f95448293 - rustc_typeck::check_crate::h5c76faa7ac951c22
  29:     0x7f3f9713da33 - rustc_driver::driver::phase_3_run_analysis_passes::_{{closure}}::he5bb0058670d90b6
  30:     0x7f3f9708e9f8 - rustc::ty::context::TyCtxt::create_and_enter::h1ad8faffb62f0367
  31:     0x7f3f970f70bf - rustc_driver::driver::compile_input::h4d6bc655b7baad10
  32:     0x7f3f97124016 - rustc_driver::run_compiler::h9165e61fc2dd486f
  33:     0x7f3f9706af82 - std::panicking::try::do_call::hc52dd3bdae996640
  34:     0x7f3f96d6f936 - __rust_maybe_catch_panic
  35:     0x7f3f9708461a - _<F as alloc..boxed..FnBox<A>>::call_box::h3aad53cfb9d2713a
  36:     0x7f3f96d5fc72 - std::sys::thread::Thread::new::thread_start::h4c0ad33b336bc6ea
  37:     0x7f3f8fe6d453 - start_thread
  38:     0x7f3f969a97de - __GI___clone
  39:                0x0 - <unknown>

[TimNN]

Works fine on 1.11.

[TimNN]

Affects beta as well.

Requires the type on the lhs to come from an extern crate.

This is fixed on nightly-2016-10-06 (nightly-2016-10-04 still fails, Changes)

I suspect this was fixed by #36814 (although I may be wrong).

[TimNN]

Originally introduced between nightly-2016-07-09 and nightly-2016-07-10 (Changes)

The most likely cause is #34365, I think.

[jseyfried]

cc @petrochenkov

[petrochenkov]

This happens when decoder::get_adt_def gets struct constructor DefId as an argument (as opposed to DefId of the struct itself) + it's the first time this struct is decoded.
#36814 doesn't fix this issue, only masks it, it still can be reproduced with tuple struct patterns.
How to fix this: add something like this at the beginning of decoder::get_adt_def:

if is_ctor_id(item_id) {
    item_id = get_struct_id(item_id)
}

I have no idea how is_ctor_id and get_struct_id can be implemented, need to investigate how new metadata works.

[petrochenkov]

Fix for beta is in #37078

[nikomatsakis]

Nice job @petrochenkov