Compiler unnecessary panicked due to a mistake in where clause

[Bendrien]

The compiler unexpectedly panicked where a nice error should be produced.

I tried this code:

pub fn zip<K, V, W, A, B>(a: A, b: B)
    where K: Ord,
          A: Iterator<Item=(K, V)>,
          A: Iterator<Item=(K, W)>,
{

}

I simply forgot to change the third entry in the where clause to B.

Meta

rustc --version --verbose:
rustc 1.17.0-nightly (24a70eb 2017-02-09)
binary: rustc
commit-hash: 24a70eb
commit-date: 2017-02-09
host: x86_64-pc-windows-gnu
release: 1.17.0-nightly
LLVM version: 3.9

Backtrace:
thread 'rustc' panicked at 'Box', C:\bot\slave\nightly-dist-rustc-win-gnu-64\build\src\librustc_errors\lib.rs:417
stack backtrace:
0: 0x659e854a - strncmp
1: 0x659f2fa7 - strncmp
2: 0x659f2b21 - strncmp
3: 0x659f352e - strncmp
4: 0x66242f9a - strncmp
5: 0x6625b044 - strncmp
6: 0x6f387f5e - strncmp
7: 0x6f387cb5 - strncmp
8: 0x6f3874fc - strncmp
9: 0x6f29c278 - strncmp
10: 0x6f39578b - strncmp
11: 0x6f38af55 - strncmp
12: 0x6f389971 - strncmp
13: 0x70cab10f - strncmp
14: 0x70bea5d6 - strncmp
15: 0x70c6d966 - strncmp
16: 0x70c8097a - strncmp
17: 0x70c97768 - strncmp
18: 0x70cfd984 - strncmp
19: 0x69c60b83 - strncmp
20: 0x69bbc0ab - strncmp
21: 0x69c4ace4 - strncmp
22: 0x69c343a9 - strncmp
23: 0x69c7f2b9 - strncmp
24: 0x69b8af05 - strncmp
25: 0x659f6938 - strncmp
26: 0x69ba83b4 - strncmp
27: 0x659f0d52 - strncmp
28: 0x7ffff6398363 - strncmp

[TimNN]

Backtrace from playground:

rustc 1.17.0-nightly (b1e31766d 2017-03-03)
error: internal compiler error: /checkout/src/librustc/hir/map/mod.rs:722: expected expr, found fn zip (id=4)

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 'Box<Any>', /checkout/src/librustc_errors/lib.rs:417
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: rustc_errors::Handler::bug
   1: rustc::session::opt_span_bug_fmt::{{closure}}
   2: rustc::session::opt_span_bug_fmt
   3: rustc::session::bug_fmt
   4: rustc::hir::map::Map::expect_expr
   5: rustc::traits::error_reporting::<impl rustc::infer::InferCtxt<'a, 'gcx, 'tcx>>::need_type_info
   6: rustc::traits::error_reporting::<impl rustc::infer::InferCtxt<'a, 'gcx, 'tcx>>::report_fulfillment_error
   7: rustc::traits::error_reporting::<impl rustc::infer::InferCtxt<'a, 'gcx, 'tcx>>::report_fulfillment_errors
   8: rustc_typeck::check::FnCtxt::select_all_obligations_or_error
   9: <std::thread::local::LocalKey<T>>::with
  10: rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor::check_item_well_formed
  11: rustc_typeck::check::check_wf_new
  12: rustc_typeck::check_crate
  13: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}
  14: rustc_driver::driver::phase_3_run_analysis_passes
  15: rustc_driver::driver::compile_input
  16: rustc_driver::run_compiler
  17: std::panicking::try::do_call
  18: __rust_maybe_catch_panic
             at /checkout/src/libpanic_unwind/lib.rs:98
  19: <F as alloc::boxed::FnBox<A>>::call_box
  20: std::sys::imp::thread::Thread::new::thread_start
             at /checkout/src/liballoc/boxed.rs:648
             at /checkout/src/libstd/sys_common/thread.rs:21
             at /checkout/src/libstd/sys/unix/thread.rs:84
  21: start_thread
  22: clone

[eddyb]

See #39361 (comment) for how to fix in the compiler (if anyone wants to pick this up).

[nikomatsakis]

triage: P-high

[nikomatsakis]

Good chance to make a simple, high-impact fix. =) The problem is that this line of code assumes that body_id always refers to a HIR expression (part of our AST), but that's not correct.

The right thing to do would be to match on the results of self.tcx.hir.find(cause.body_id) and check for a couple different variants:

• a NodeExpr(e) (defined in rustc::hir::map) can just return e
• for a NodeItem(i), we can match on i.node and see if it is a ItemFn, and then extract the body-id. This is kind of painful, a better way might be to extend the associated_body helper that runs on entries.

OK, not totally trivial, but I don't have time for more in-depth notes just yet. =)

[eddyb]

@nikomatsakis I don't think we need to handle anything other than NodeExpr.
That is, this is only adding some extra notes on an error, we can just skip them otherwise.

[nikomatsakis]

Never mind, the easy fix is to do something like this:

let opt_expr = match self.tcx.hir.find(cause.body_id) {
    rustc::hir::map::NodeExpr(e) => Some(e),
    _ => None,
}

then down below where we do visit_expr(e), just check if it is Some. Or move this code down there to the place where we invoke visit_expr

[nikomatsakis]

@eddyb great minds think alike...