ICE in `region_inference/mod.rs`

[apasel422]

Attempting to compile https://github.com/reem/rust-traverse with cargo 0.0.1-pre-nightly (453ae9f 2015-01-29 00:56:56 +0000) and rustc 1.0.0-nightly (c5961ad06 2015-01-28 21:49:38 +0000) yields the following ICE:

thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/librustc/middle/infer/region_inference/mod.rs:1015

stack backtrace:
   1:     0x7f8da355a880 - sys::backtrace::write::h471f2f26c91df096Q8t
   2:     0x7f8da357df80 - failure::on_fail::h389257f46ebc50afikB
   3:     0x7f8da34ea220 - rt::unwind::begin_unwind_inner::haa1ac937607e9179jZA
   4:     0x7f8da34ead50 - rt::unwind::begin_unwind_fmt::ha84452bff441c4ffPXA
   5:     0x7f8da357dde0 - rust_begin_unwind
   6:     0x7f8da35c6ef0 - panicking::panic_fmt::ha791a233597f33dbBRv
   7:     0x7f8da35cf5c0 - panicking::panic_bounds_check::h7964adf489703576CQv
   8:     0x7f8da16d6ce0 - middle::infer::region_inference::RegionVarBindings<'a, 'tcx>::infer_variable_values::hdb795f883c676391mJw
   9:     0x7f8da16d6970 - middle::infer::region_inference::RegionVarBindings<'a, 'tcx>::resolve_regions::h2570f98c1810a795Ymw
  10:     0x7f8da16fc020 - middle::infer::InferCtxt<'a, 'tcx>::resolve_regions_and_report_errors::ha6a986062307cc873YA
  11:     0x7f8da2cc9b70 - check::compare_method::compare_impl_method::h33ae495a649fd66cIgk
  12:     0x7f8da2cd21a0 - check::check_item::hf11061a9881d4ad88ll
  13:     0x7f8da2cd87f0 - visit::walk_item::h17525548578187088888
  14:     0x7f8da2cd87f0 - visit::walk_item::h17525548578187088888
  15:     0x7f8da2da1ee0 - check_crate::closure.32081
  16:     0x7f8da2d9c9c0 - check_crate::he229bdeb8fdc745cDNy
  17:     0x7f8da3ae5d80 - driver::phase_3_run_analysis_passes::h2272be7454e1eb80NFa
  18:     0x7f8da3acd130 - driver::compile_input::hb694f278d4428f99Bba
  19:     0x7f8da3b91940 - run_compiler::hc3e25a8f6c4a1b50n9b
  20:     0x7f8da3b90050 - thunk::F.Invoke<A, R>::invoke::h8035325489603501466
  21:     0x7f8da3b8efb0 - rt::unwind::try::try_fn::h9312387582241745294
  22:     0x7f8da35e9320 - rust_try_inner
  23:     0x7f8da35e9310 - rust_try
  24:     0x7f8da3b8f260 - thunk::F.Invoke<A, R>::invoke::h8464768559817507673
  25:     0x7f8da356a770 - sys::thread::thread_start::he52b8673a3629881W3w
  26:     0x7f8d9d767fe0 - start_thread
  27:     0x7f8da316c859 - __clone
  28:                0x0 - <unknown>

[renato-zannon]

Reduced test case:

pub trait Arg<A> { }

pub trait Traversal {
    type Item;
    fn foreach<F: Arg<Self::Item>>(F);
}

impl<'a> Traversal for i32 {
    type Item = &'a i32;
    fn foreach<F: Arg<&'a i32>>(f: F) { }
}

fn main() {}

Still ICEs with rustc 1.0.0-dev (3d6f5100a 2015-01-29 11:28:30 +0000)

The ICE goes away if the Item on the impl is changed to a bare i32 instead of a reference.

[msiemens]

Slightly different test case:

pub trait Traversal {
    type Item;

    fn foreach<F>(self, F) where F: FnMut(Self::Item);
}

impl<'a, T> Traversal for &'a [T] {
    type Item = &'a T;

    fn foreach<F>(self, mut f: F) where F: FnMut(&'a T) {
        //
    }
}

EDIT: Nevermind, it's the same but uses a closure instead of the more general Arg<A>.

[renato-zannon]

I have bisected this issue to c61d788 (from #21019)

[edwardw]

A workaround would be simply:

impl<'a> Traversal for i32 {
    ...
    fn foreach<F>(f: F) { }
}

[renato-zannon]

@edwardw This doesn't help much if you need to call methods that are on the Arg trait (which was FnMut on the original code).

[nikomatsakis]

cc me

[Gankra]

This is blocking collect-rs from compiling.

[edwardw]

This is interesting. The culprit is compare_method.rs#L251-L264: it registers obligations from implementation's methods into an hybrid parameter environment which has trait's methods in it. In the example given in this issue, when the associate type is projected out, the implementation's method acquires an extra lifetime variable. The trait's method doesn't have it so the lifetime resolution fails in an ICE later on.

I'm not sure how to fix that though.

[edwardw]

This compiles so at first glance it looks like a viable workaround:

impl<'a> Traversal for i32 {
    type Item = &'a i32;
    fn foreach<F: Arg<<Self as Traversal>::Item>>(f: F) { }
}

But if one tries to really use f as an Arg, compiler panics on an assertion failure:

pub trait Arg<A> {
    fn arg(&self);
}

pub trait Traversal {
    type Item;
    fn foreach<F: Arg<Self::Item>>(F);
}

impl<'a> Traversal for i32 {
    type Item = &'a i32;
    fn foreach<F: Arg<<Self as Traversal>::Item>>(f: F) {
        f.arg();
    }
}