rustc panic

[apt1002]

rustc asked me to file a bug report:

error: internal compiler error: /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/librustc/infer/mod.rs:626: Encountered errors [FulfillmentError(Obligation(predicate=Binder(ProjectionPredicate(ProjectionTy { trait_ref: <std::vec::Vec<f32> as array::Array<'a, usize>>, item_name: Element(610) }, _)),depth=1),MismatchedProjectionTypes(RegionsInsufficientlyPolymorphic(BrNamed(CrateNum(0):DefIndex(1030), 'a(180), WontChange), '_#0r)))] resolving bounds after type-checking

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

thread 'rustc' panicked at 'Box', /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/librustc_errors/lib.rs:382
note: Run with RUST_BACKTRACE=1 for a backtrace.

Here's a fragment of code sufficient to trigger the bug. It's not minimal yet, sorry. The code might not be correct; I was kinda relying on the compiler to tell me. ;-)

$ rustc --version --verbose
rustc 1.15.1 (021bd29 2017-02-08)
binary: rustc
commit-hash: 021bd29
commit-date: 2017-02-08
host: x86_64-unknown-linux-gnu
release: 1.15.1
LLVM version: 3.9

I hope you can make something of this.

[est31]

Fairly minimized: https://play.rust-lang.org/?gist=621b19255bd764d427cc9d05f2b1ba80&version=stable&backtrace=0

[est31]

Even smaller: https://play.rust-lang.org/?gist=55a654881aa050566fa44f9c0bdd011d&version=stable&backtrace=0

[apt1002]

https://play.rust-lang.org/?gist=9404547723645a7f5d9c4661be8364c5&version=stable&backtrace=0 <-- I've been minimising too!

[apt1002]

The following features seem to be essential:

• Type Element must be a reference.
• The bound "Element=V" must be present.

[est31]

I think I found it:

trait Array<'a> {
    type Element: 'a;
}

trait Visit {
    fn visit() {}
}

impl<'a> Array<'a> for () {
    type Element = &'a ();
}

impl Visit for () where
    //(): for<'a> Array<'a, Element=&'a ()>, // No ICE
    (): for<'a> Array<'a, Element=()>, // ICE
{}

fn main() {
    <() as Visit>::visit();
}

This gives an error message like:

error: internal compiler error: /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/librustc/infer/mod.rs:626: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(ProjectionPredicate(ProjectionTy { trait_ref: <() as Array<'a>>, item_name: Element(83) }, ())),depth=1),MismatchedProjectionTypes(Sorts(ExpectedFound { expected: &(), found: () })))]` resolving bounds after type-checking

Note the expected: &(), found: () here

[apt1002]

Good spot. Can also interchange () with &() to get an even simpler version.

[apt1002]

The <() as Visit> part seems to be essential. No ICE.

[apt1002]

Another slight simplification.

[apt1002]

I have some more evidence that I think is worth recording here. Our minimised version above crashes the compiler in a case where it should be reporting an error. I now have a version that crashes the compiler when it should be accepting the program.

I returned to my original program and specialized it to the case I care about, i.e. I instantiated all the generics. After minor fixes, it works. However, replacing &'a mut f32 with the equivalent <Vec<f32> as Array<'a, usize>>::Element crashes the compiler.

Parially minmized. Fully minimized.