rustc panic

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.

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

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

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

The following features seem to be essential:

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

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

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

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

Another slight simplification.

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.

use the evaluation cache instead of the global fulfillment cache

Auto merge of #42840 - arielb1:poison-smoke-and-mirrors, r=nikomatsakis

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rustc panic #39970

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est31 commented on Feb 20, 2017

est31 commented on Feb 20, 2017

apt1002 commented on Feb 20, 2017

apt1002 commented on Feb 20, 2017

est31 commented on Feb 20, 2017

apt1002 commented on Feb 20, 2017

apt1002 commented on Feb 20, 2017

apt1002 commented on Feb 20, 2017

apt1002 commented on Feb 22, 2017

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