ICE with a simple associated type, causing "index out of bounds" on a vector

[kvark]

Broken on both stable and the nightly. Affects a real use case with present() in gfx-rs.

trait Device {
    type Resources;
}
struct Foo<R>(R);

impl<D: Device> Foo<D::Resources> {
    fn present(&self, _dev: &D) {}
}

fn main() {
    struct Res;
    struct Dev;
    impl Device for Dev {
        type Resources = Res;
    }
    struct Win;
    let foo = Foo(Res);
    let dev = Dev;
    foo.present(&dev);
}

Gives me:

<anon>:19:9: 19:22 error: type `Foo<main::Res>` does not implement any method in scope named `present`
<anon>:19     foo.present(&dev);
                  ^~~~~~~~~~~~~
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 'index out of bounds: the len is 5 but the index is 5', /home/rustbuild/src/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libcollections/vec.rs:1322


playpen: application terminated with error code 101

[ebfull]

btw I'm not sure this should be possible, it seems like the impl would be unconstrained?

[kvark]

@ebfull I believe it depends on a method. If there was an fn foo(&self) {} there, and you'd try to call it, Rust wouldn't understand what implementation it belongs to. But with present() the D type is constrained, thus leaving no questions for the impl block.

[ebfull]

I believe a type is unconstrained if you cannot determine the impl from the Self. In this case, given a generic Foo<T>, you couldn't figure out what D is. It could be anything that impl's Device with that associated type. It would make more sense as a type parameter for the function:

impl<T> Foo<T> {
    fn present<D: Device<Resources=T>>(&self, _dev: &D) { } 
}

[kvark]

@ebfull huh, I stand corrected. I was thinking about how trait implementation gets chosen in a similar manner, and thought the same works for the pure impl blocks:

struct Foo;
trait Bar<T> {
    fn forget(&self, _: T) {}
}
impl<T> Bar<T> for Foo {}
fn main() {
    let foo = Foo;
    foo.forget(0i8);
}

I guess the issue is not a blocker for us then, I'll rewrite present() in gfx with D being a generic parameter on the method, not the implementation.

[ebfull]

It sounds like you might also want to consider just impl<T: Device> Foo<T> and fn present(&self, _dev: &T::Resources) if you want present to be called with only one particular T: Device type per impl. (But I don't know your API requirements.)

[arielb1]

Backtrace:

stack backtrace:
   1:     0x7fef2dd7a1d9 - sys::backtrace::write::ha86cd627a22f455205r
                        at /home/ariel/Rust/source.master/src/libstd/sys/unix/backtrace.rs:158
   2:     0x7fef2dd836cd - panicking::on_panic::h662858ec46f876968Lw
                        at /home/ariel/Rust/source.master/src/libstd/panicking.rs:47
   3:     0x7fef2dd417eb - rt::unwind::begin_unwind_inner::h496e2ee4be82b2feirw
                        at /home/ariel/Rust/source.master/src/libstd/rt/unwind/mod.rs:273
   4:     0x7fef2dd428ad - rt::unwind::begin_unwind_fmt::hb66a627b5352ee99oqw
                        at /home/ariel/Rust/source.master/src/libstd/rt/unwind/mod.rs:212
   5:     0x7fef2dd83237 - rust_begin_unwind
   6:     0x7fef2dde34aa - panicking::panic_fmt::h130c36cdf6ed19des0B
                        at /home/ariel/Rust/source.master/src/libcore/panicking.rs:64
   7:     0x7fef2dddde34 - panicking::panic_bounds_check::h97c07c7c114b10a5yZB
                        at /home/ariel/Rust/source.master/src/libcore/panicking.rs:52
   8:     0x7fef2be0e57c - middle::infer::InferCtxt<'a, 'tcx>::shallow_resolve::h56e9f53812e21da7nCz
                        at /home/ariel/Rust/source.master/src/libcollections/vec.rs:1357
                        at /home/ariel/Rust/source.master/src/librustc_data_structures/snapshot_vec.rs:95
                        at /home/ariel/Rust/source.master/src/librustc/middle/infer/type_variable.rs:126
                        at /home/ariel/Rust/source.master/src/librustc/middle/infer/mod.rs:892
   9:     0x7fef2bdd3bce - middle::infer::resolve::OpportunisticTypeResolver<'a, 'tcx>.ty_fold..TypeFolder<'tcx>::fold_ty::h2c428c2906d7983fyBx
                        at /home/ariel/Rust/source.master/src/librustc/middle/infer/resolve.rs:43
  10:     0x7fef2bdd4699 - middle::ty_fold::TypeFolder::fold_substs::h1287471176495252976
                        at /home/ariel/Rust/source.master/src/librustc/middle/ty_fold.rs:236
                        at /home/ariel/Rust/source.master/src/librustc/middle/ty_fold.rs:224
                        at /home/ariel/Rust/source.master/src/libcore/iter.rs:1693
                        at /home/ariel/Rust/source.master/src/libcore/option.rs:420
                        at /home/ariel/Rust/source.master/src/libcore/iter.rs:1693
                        at /home/ariel/Rust/source.master/src/libcollections/vec.rs:1501
                        at /home/ariel/Rust/source.master/src/libcore/iter.rs:567
                        at /home/ariel/Rust/source.master/src/librustc/middle/subst.rs:445
                        at /home/ariel/Rust/source.master/src/librustc/middle/ty_fold.rs:219
                        at /home/ariel/Rust/source.master/src/librustc/middle/ty_fold.rs:677
                        at /home/ariel/Rust/source.master/src/librustc/middle/ty_fold.rs:97
  11:     0x7fef2bea32dc - middle::traits::project::opt_normalize_projection_type::hb8a09e9b2e53377dfWP
                        at /home/ariel/Rust/source.master/src/librustc/middle/ty_fold.rs:293
                        at /home/ariel/Rust/source.master/src/librustc/middle/ty_fold.rs:722
                        at /home/ariel/Rust/source.master/src/librustc/middle/ty_fold.rs:91
                        at /home/ariel/Rust/source.master/src/librustc/middle/ty_fold.rs:272
                        at /home/ariel/Rust/source.master/src/librustc/middle/infer/mod.rs:931
                        at /home/ariel/Rust/source.master/src/librustc/middle/traits/project.rs:452
                        at /home/ariel/Rust/source.master/src/librustc/middle/traits/project.rs:357
  12:     0x7fef2be9dc75 - middle::traits::project::poly_project_and_unify_type::closure.93245
                        at /home/ariel/Rust/source.master/src/librustc/middle/traits/project.rs:116
                        at /home/ariel/Rust/source.master/src/librustc/middle/traits/project.rs:88
  13:     0x7fef2be9c398 - middle::traits::project::poly_project_and_unify_type::h42e4cd0d7749283coCP
                        at /home/ariel/Rust/source.master/src/librustc/middle/infer/mod.rs:575
                        at /home/ariel/Rust/source.master/src/librustc/middle/traits/project.rs:83
  14:     0x7fef2be937e6 - middle::traits::fulfill::FulfillmentContext<'tcx>::select::h541a7c10f723947dIeP
                        at /home/ariel/Rust/source.master/src/librustc/middle/traits/fulfill.rs:396
                        at /home/ariel/Rust/source.master/src/librustc/middle/traits/fulfill.rs:273
                        at /home/ariel/Rust/source.master/src/libcollections/vec.rs:601
                        at /home/ariel/Rust/source.master/src/librustc/middle/traits/fulfill.rs:268
  15:     0x7fef2be93283 - middle::traits::fulfill::FulfillmentContext<'tcx>::select_new_obligations::h7a5ae48603f1abd2ndP
                        at /home/ariel/Rust/source.master/src/librustc/middle/traits/fulfill.rs:218
  16:     0x7fef2d520546 - check::FnCtxt<'a, 'tcx>::select_new_obligations::h8b913ca888d49e450Mp
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:1854
  17:     0x7fef2d54ae66 - check::check_argument_types::h10dce567107b606bwgq
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:2308
  18:     0x7fef2d54c8b7 - check::check_method_argument_types::hade9045e5e301411Vdq
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:2176
  19:     0x7fef2d577317 - check::check_expr_with_unifier::check_method_call::h0a9b0a37195319d12Pq
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:2646
  20:     0x7fef2d59dd79 - check::check_expr_with_unifier::h12507636429091130913
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:3390
  21:     0x7fef2d5ad57d - check::check_stmt::h3f64930c48de8110aos
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:2494
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:3946
  22:     0x7fef2d5635fa - check::check_block_with_expected::hbc58b92bc05f127flss
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:3988
  23:     0x7fef2d5440e6 - check::check_fn::h478e5dc8b923df63W4n
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:705
  24:     0x7fef2d55d4d9 - check::check_bare_fn::h9b9c796f0d1e44a6wUn
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:524
  25:     0x7fef2d55b48f - check::check_item_body::h26d9292503efa05amlo
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:801
  26:     0x7fef2d55d049 - check::check_item_types::h141edac9d460c3eb3Rn
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:463
                        at /home/ariel/Rust/source.master/src/libsyntax/visit.rs:160
                        at /home/ariel/Rust/source.master/src/libsyntax/visit.rs:64
                        at /home/ariel/Rust/source.master/src/libsyntax/visit.rs:152
                        at /home/ariel/Rust/source.master/src/librustc_typeck/check/mod.rs:483
  27:     0x7fef2d6143e9 - check_crate::hbe43b6c5b585bffdp7C
                        at /home/ariel/Rust/source.master/src/librustc_typeck/lib.rs:341
                        at /home/ariel/Rust/source.master/src/librustc/util/common.rs:39
                        at /home/ariel/Rust/source.master/src/librustc_typeck/lib.rs:340
  28:     0x7fef2e2f4a7b - driver::phase_3_run_analysis_passes::h89f35bf8a39eb6d0tGa
                        at /home/ariel/Rust/source.master/src/librustc_driver/driver.rs:643
  29:     0x7fef2e2df119 - driver::compile_input::h3827478501655be2Qba
                        at /home/ariel/Rust/source.master/src/librustc_driver/driver.rs:122
  30:     0x7fef2e37d475 - run_compiler::h65b602076525bee675b
                        at /home/ariel/Rust/source.master/src/librustc_driver/lib.rs:156
  31:     0x7fef2e37b03c - boxed::F.FnBox<A>::call_box::h10880508204101052423
                        at /home/ariel/Rust/source.master/src/librustc_driver/lib.rs:99
                        at /home/ariel/Rust/source.master/src/librustc_driver/lib.rs:813
                        at /home/ariel/Rust/source.master/src/liballoc/boxed.rs:379
  32:     0x7fef2e37a87e - rt::unwind::try::try_fn::h8650079312810836346
  33:     0x7fef2de1f818 - rust_try_inner
  34:     0x7fef2de1f805 - rust_try
  35:     0x7fef2dd6e0bc - rt::unwind::try::inner_try::he54be6185e919225bnw
                        at /home/ariel/Rust/source.master/src/libstd/rt/unwind/mod.rs:147
  36:     0x7fef2e37aac0 - boxed::F.FnBox<A>::call_box::h10028273920383414105
                        at /home/ariel/Rust/source.master/src/libstd/rt/unwind/mod.rs:130
                        at /home/ariel/Rust/source.master/src/libstd/thread/mod.rs:346
                        at /home/ariel/Rust/source.master/src/liballoc/boxed.rs:379
  37:     0x7fef2dd8244e - sys::thread::Thread::new::thread_start::hd9103ec8865158f2rxv
  38:     0x7fef2882b0a3 - start_thread
  39:     0x7fef2d9be04c - clone
  40:                0x0 - <unknown>

It seems like there is something stupid going on with inference variables.