ICE: index out of bounds: the len is 0 but the index is 0

[DutchGhost]

The following ICE's on nightly and beta:

use std::marker::PhantomData;

use std::mem::{self, MaybeUninit};

struct Bug<S> {
    A: [(); {
        let x: S = MaybeUninit::uninit();
        let b = &*(&x as *const _ as *const S);
        0
    }],
}

Backtrace:

``` thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', /rustc/da3629b05f8f1b425a738bfe9fe9aedd47c5417a/src/libcore/slice/mod.rs:2791:10 stack backtrace: 0: backtrace::backtrace::libunwind::trace at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88 1: backtrace::backtrace::trace_unsynchronized at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66 2: std::sys_common::backtrace::_print_fmt at src/libstd/sys_common/backtrace.rs:77 3: ::fmt at src/libstd/sys_common/backtrace.rs:59 4: core::fmt::write at src/libcore/fmt/mod.rs:1057 5: std::io::Write::write_fmt at src/libstd/io/mod.rs:1426 6: std::sys_common::backtrace::_print at src/libstd/sys_common/backtrace.rs:62 7: std::sys_common::backtrace::print at src/libstd/sys_common/backtrace.rs:49 8: std::panicking::default_hook::{{closure}} at src/libstd/panicking.rs:195 9: std::panicking::default_hook at src/libstd/panicking.rs:215 10: rustc_driver::report_ice 11: std::panicking::rust_panic_with_hook at src/libstd/panicking.rs:476 12: rust_begin_unwind at src/libstd/panicking.rs:376 13: core::panicking::panic_fmt at src/libcore/panicking.rs:84 14: core::panicking::panic_bounds_check at src/libcore/panicking.rs:62 15: rustc::ty::Generics::type_param 16: rustc::ty::error::::note_and_explain_type_err 17: rustc::infer::error_reporting::::note_type_err 18: rustc::infer::error_reporting::::report_and_explain_type_error 19: rustc::infer::InferCtxt::report_mismatched_types 20: rustc_typeck::check::demand::::demand_coerce_diag 21: rustc_typeck::check::demand::::demand_coerce 22: rustc_typeck::check::FnCtxt::check_decl_local 23: rustc_typeck::check::FnCtxt::check_stmt 24: rustc_typeck::check::FnCtxt::check_block_with_expected 25: rustc_typeck::check::expr::::check_expr_kind 26: rustc_typeck::check::expr::::check_expr_with_expectation_and_needs 27: rustc::ty::context::tls::with_context::{{closure}} 28: rustc_typeck::check::typeck_tables_of 29: rustc::ty::query::__query_compute::typeck_tables_of 30: rustc::ty::query::::compute 31: rustc::dep_graph::graph::DepGraph::with_task_impl 32: rustc::ty::query::plumbing::::get_query 33: rustc_mir::const_eval::eval_queries::const_eval_raw_provider 34: rustc::ty::query::__query_compute::const_eval_raw 35: rustc::ty::query::::compute 36: rustc::dep_graph::graph::DepGraph::with_task_impl 37: rustc::ty::query::plumbing::::get_query 38: rustc_mir::const_eval::eval_queries::const_eval_validated_provider 39: rustc::ty::query::__query_compute::const_eval_validated 40: rustc::ty::query::::compute 41: rustc::dep_graph::graph::DepGraph::with_task_impl 42: rustc::ty::query::plumbing::::get_query 43: rustc_mir::const_eval::eval_queries::const_eval_validated_provider 44: rustc::ty::query::__query_compute::const_eval_validated 45: rustc::ty::query::::compute 46: rustc::dep_graph::graph::DepGraph::with_task_impl 47: rustc::ty::query::plumbing::::get_query 48: rustc::mir::interpret::queries::::const_eval_instance 49: rustc::ty::sty::Const::eval::{{closure}} 50: rustc::ty::sty::Const::eval 51: rustc::ty::structural_impls::::super_fold_with 52: ::fold_ty 53: rustc::traits::project::normalize 54: rustc_typeck::check::FnCtxt::normalize_associated_types_in 55: as core::iter::traits::iterator::Iterator>::fold 56: rustc::ty::context::GlobalCtxt::enter_local 57: rustc_typeck::check::wfcheck::check_item_well_formed 58: rustc::ty::query::__query_compute::check_item_well_formed 59: rustc::ty::query::::compute 60: rustc::dep_graph::graph::DepGraph::with_task_impl 61: rustc::ty::query::plumbing::::get_query 62: rustc::ty::query::plumbing::::ensure_query 63: __rust_maybe_catch_panic at src/libpanic_unwind/lib.rs:79 64: rustc_data_structures::sync::par_for_each_in 65: __rust_maybe_catch_panic at src/libpanic_unwind/lib.rs:79 66: rustc::hir::Crate::par_visit_all_item_likes 67: rustc::util::common::time 68: rustc_typeck::check_crate 69: rustc_interface::passes::analysis 70: rustc::ty::query::__query_compute::analysis 71: rustc::dep_graph::graph::DepGraph::with_task_impl 72: rustc::ty::query::plumbing::::get_query 73: rustc::ty::context::tls::enter_global 74: rustc_interface::interface::run_compiler_in_existing_thread_pool 75: scoped_tls::ScopedKey::set 76: syntax::with_globals note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace. 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: rustc 1.42.0-nightly (da3629b05 2019-12-29) running on x86_64-unknown-linux-gnu note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type lib note: some of the compiler flags provided by cargo are hidden query stack during panic: #0 [typeck_tables_of] processing `Bug::A::{{constant}}#0` #1 [const_eval_raw] const-evaluating `Bug::A::{{constant}}#0` #2 [const_eval_validated] const-evaluating + checking `Bug::A::{{constant}}#0` #3 [const_eval_validated] const-evaluating + checking `Bug::A::{{constant}}#0` #4 [check_item_well_formed] processing `Bug` #5 [analysis] running analysis passes on this crate end of query stack error: could not compile `playground`. To learn more, run the command again with --verbose. ```

[Centril]

Reduced:

enum Bug<S> {
    Var = {
        let x: S = 0;
        0
    },
}

ICE most likely occurs in:

https://github.com/rust-lang/rust/blob/ebbb2bf37aedaaa64dfaa52ba337ca6efb6b9093/src/librustc/ty/mod.rs#L978

I cannot seem to reproduce this without an AnonConst, so cc @varkor.

[Centril]

Reduced even more:

#![feature(type_ascription)]

enum Bug<S> {
    Var = 0: S,
}

(Please include all 3 variants thus far in a regression test.)

[davidtwco]

I looked into this briefly and found that on this line:

https://github.com/rust-lang/rust/blob/ebbb2bf37aedaaa64dfaa52ba337ca6efb6b9093/src/librustc/ty/mod.rs#L978

...that self has the value:

Generics { parent: None, parent_count: 0, params: [], param_def_id_to_index: {}, has_self: false, has_late_bound_regions: None }

Digging a bit deeper, the generics appear to be empty because of the following lines:

https://github.com/rust-lang/rust/blob/ebbb2bf37aedaaa64dfaa52ba337ca6efb6b9093/src/librustc_typeck/collect.rs#L1006-L1017

I can confirm that adding #![feature(const_generics)] makes this code compile, so I suspect that this is blocked on #43408.

[estebank]

What would be the consequences of always returning the DefId?

[varkor]

What would be the consequences of always returning the DefId?

It causes query cycle errors at the moment.

[pnkfelix]

Discussed at T-compiler meeting. Due to all the known examples being compile-time errors, the ICE here does not seem so strong a hazard as to warrant weekly visits during triage.

(Certainly failing more gracefully would be great. But its not something we're going to e.g. block a release on.)

Downgrading priority to P-medium.

[nikomatsakis]

Some relevant examples. This gives an (incorrect) error on stable that S: Sized does not hold:

use std::marker::PhantomData;

use std::mem::{self, MaybeUninit};

struct Bug<S> {
    A: [(); {
        let x: Option<S> = None;
        0
    }],
}

This ICEs on stable:

use std::marker::PhantomData;

use std::mem::{self, MaybeUninit};

struct Bug<S> {
    A: [(); {
        let x: Option<Box<S>> = None;
        0
    }],
}

[BenLewis-Seequent]

Is there a reason why we can't just disallow type parameters from appearing within constants during resolve, like we do for type parameters from outer functions? Obviously const generics/lazy normization willl loosen this rule.

[nikomatsakis]

@skinny121 good point, I imagine that might work.

[BenLewis-Seequent]

Another variant on this bug:

struct Bug<S:?Sized> {
    A: [(); {
        let x: Option<Box<Self>> = None;
        0
    }],
    B: S
}

[JohnTitor]

Triage: The two examples that Centril posted (https://github.com/rust-lang/rust/issues/67945#issuecomment-571251334 and https://github.com/rust-lang/rust/issues/67945#issuecomment-571252306) are no longer ICEs in the latest nightly. Possibly related to #70825?

[nikomatsakis]

Maybe we should just add tests then?

[JohnTitor]

Maybe we should just add tests then?

Sounds good! I'll do later.

[varkor]

Another variant (though likely the same):

use std::mem::MaybeUninit;

struct Bug<S> {
    A: [(); {
        let x: S = MaybeUninit::uninit();
        let b = &*(&x as *const _ as *const S);
        0
    }],
}

This gives a different to the original, despite being essentially the same code.

[JohnTitor]

Triage: All the snippets including the original are no longer ICE with the latest nightly: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=fc90d68f3fd2b8958e7365870eee6202