Type inference ICE: `assertion failed: !self.obligations_in_snapshot.get()`

[Stebalien]

Type inference is barfing on:

use std::iter::once;
fn main() {
    once::<&str>("str").fuse().filter(|a: &str| true).count();
}

This only happens to iterators over &strs types with lifetime parameters and only happens when fused, mapped or filtered, than "operated" on (e.g., counted).

The following works:

use std::iter::once;
use std::marker::PhantomData;

struct Test(&'static str);

fn main() {
    once(Test("asdf")).fuse().filter(|_| true).count();
}

Trace:

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 'assertion failed: !self.obligations_in_snapshot.get()', ../src/librustc/infer/mod.rs:892
stack backtrace:
   1:      0x385616e5833 - std::sys::backtrace::tracing::imp::write::h482d45d91246faa2
   2:      0x385616f611d - std::panicking::default_hook::_{{closure}}::h89158f66286b674e
   3:      0x385616f466d - std::panicking::default_hook::h9e30d428ee3b0c43
   4:      0x385616f4d68 - std::panicking::rust_panic_with_hook::h2224f33fb7bf2f4c
   5:      0x3855e7daf2f - std::panicking::begin_panic::h6da27a7ee15843ce
   6:      0x3855e9365e2 - rustc::infer::InferCtxt::rollback_to::ha2f21524af8937ee
   7:      0x3855fea7ca3 - rustc_typeck::check::method::probe::ProbeContext::consider_candidates::_{{closure}}::hd73cab1de5c62a79
   8:      0x3855fdebe5c - rustc_typeck::check::method::probe::ProbeContext::consider_candidates::hc4fd590b3d2a253e
   9:      0x3855fdebb77 - rustc_typeck::check::method::probe::ProbeContext::pick_method::ha1525cf018083b3f
  10:      0x3855fdeb0ca - rustc_typeck::check::method::probe::ProbeContext::pick_core::h8fe845d303e00275
  11:      0x3855fea4ecc - rustc_typeck::check::method::probe::_<impl rustc_typeck..check..FnCtxt<'a, 'gcx, 'tcx>>::probe_method::_{{closure}}::h4cc364b0888ee90b
  12:      0x3855fde7fda - rustc_typeck::check::method::probe::_<impl rustc_typeck..check..FnCtxt<'a, 'gcx, 'tcx>>::probe_method::h5bcb89eaeb8b15c2
  13:      0x3855fdf278c - rustc_typeck::check::method::_<impl rustc_typeck..check..FnCtxt<'a, 'gcx, 'tcx>>::lookup_method::ha4b60fa1e48e18d1
  14:      0x3855fe4c2df - rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_lvalue_pref::h057aec20877fc49f
  15:      0x3855fe4ed90 - rustc_typeck::check::FnCtxt::check_stmt::h0a693585a954d704
  16:      0x3855fe4f177 - rustc_typeck::check::FnCtxt::check_block_with_expected::he043b17dca8afd62
  17:      0x3855fe2245f - rustc_typeck::check::check_fn::hd55e09cd766201af
  18:      0x3855fe20c1d - rustc_typeck::check::check_bare_fn::h56d8f43b26a6d362
  19:      0x3855fe240a9 - rustc_typeck::check::check_item_body::h7d7e664619f18e7f
  20:      0x3855fe1e088 - rustc_typeck::check::check_item_bodies::h7d884d53a15202d6
  21:      0x3855fe89f53 - rustc_typeck::check_crate::h8ec8b2f490f5dc28
  22:      0x38561ad739e - rustc_driver::driver::phase_3_run_analysis_passes::_{{closure}}::h45e03cee16dcf298
  23:      0x38561a2ca9f - rustc::ty::context::TyCtxt::create_and_enter::h0021bc4caf3a2e06
  24:      0x38561a9dfb4 - rustc_driver::driver::compile_input::hd9ecc57abd3cba85
  25:      0x38561ac40ad - rustc_driver::run_compiler::h184264500271cc39
  26:      0x38561a05b63 - std::panicking::try::do_call::h17a7a17ad7240c5c
  27:      0x38561704336 - __rust_maybe_catch_panic
  28:      0x38561a22f59 - _<F as alloc..boxed..FnBox<A>>::call_box::h95ef76c8d48c6f60
  29:      0x385616f2ba0 - std::sys::thread::Thread::new::thread_start::he0bf102845911132
  30:      0x38559b60453 - start_thread
  31:      0x385613437de - __GI___clone
  32:                0x0 - <unknown>

[jonas-schievink]

What Rust version is that? Can't reproduce on the Playpen

[Stebalien]

linux, x86_64, rustc 1.13.0-nightly (e9bc1ba 2016-08-24)

[Stebalien]

Same on 1.13.0-nightly (1987131 2016-08-26)

[Stebalien]

Also happens on http://play.integer32.com/

[jonas-schievink]

Ah I see. Does not reproduce on rustc 1.13.0-nightly (f883b0b 2016-08-19)

EDIT: Nor on rustc 1.13.0-nightly (3c5a0fa 2016-08-22)

[Stebalien]

Note: This was is probably related to specialization. It's definitely not at all related to #35656, I don't even know why you would suggest that. Any suggestion that uncommenting the commented line below is at all related to this problem will be met with derision:

#![feature(fused)]
use std::iter::FusedIterator;

struct Thing<'a>(&'a str);
impl<'a> Iterator for Thing<'a> {
    type Item = &'a str;
    fn next(&mut self) -> Option<&'a str> {
        None
    }
}

//impl<'a> FusedIterator for Thing<'a> {}

fn main() {
    Thing("test").fuse().filter(|_| true).count();
}

/cc @alexcrichton @aturon

[nikomatsakis]

triage: P-high

Regression, seems bad. @arielb1 says he'll take a look at it.

[arielb1]

The problem is that rustc::traits::specialize::translate_substs, which is called during normalization, is registering obligations into the global fulfillcx creating its own internal fulfillcx, and accidentally setting a flag during method probing.

[nikomatsakis]

I'll try to gin up a patch, since I think @arielb1 is busy.

[nikomatsakis]

@arielb1 do you agree that this is basically a "false positive" of the problem that this check was designed to prevent?

[nikomatsakis]

(and hence an ok fix is just to save/restore the flag?)

[nikomatsakis]

maybe a better fix is to have fulfillment cx be parameterized by whether to set the flag