ICE returning arrays with length specified by an associated constant

[dcrewi]

code

#![feature(associated_consts)]

trait Trait {
    const SIZE: usize;
}

pub fn cause_compiler_to_panic<T: Trait>() -> [u8; T::SIZE] {
    unimplemented!()
}

compiler output

$ RUST_BACKTRACE=1 rustc --crate-type rlib compiler_panic.rs
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
thread 'rustc' panicked at 'no path elem for NodeTyParam(TyParam { ident: T#0, id: 12, bounds: [TraitTyParamBound(PolyTraitRef { bound_lifetimes: [], trait_ref: TraitRef { path: path(Trait), ref_id: 13 }, span: compiler_panic.rs:15:35: 15:40 }, None)], default: None, span: compiler_panic.rs:15:32: 15:33 })', src/librustc/front/map/mod.rs:488

stack backtrace:
   1:     0x7f49381bbc99 - sys::backtrace::tracing::imp::write::hbc14f1d64893fa38REs
   2:     0x7f49381c2946 - panicking::on_panic::h2f9a11502875a3547ox
   3:     0x7f493818641e - rt::unwind::begin_unwind_inner::h9f15d88ec266cbe3DRw
   4:     0x7f4938187167 - rt::unwind::begin_unwind_fmt::h7bb2b8a52c6074abJQw
   5:     0x7f49360c9214 - front::map::Map<'ast>::get_path_elem::h810c6a17cb85afb1jeb
   6:     0x7f493637aee7 - front::map::Map<'ast>::with_path_next::h6183113809238491628
   7:     0x7f49362268f9 - middle::ty::ctxt<'tcx>::item_path_str::h29769059e7d6586b5Vb
   8:     0x7f49362256bb - middle::def_id::DefId.fmt..Debug::fmt::h6e39007ab82770f3Oap
   9:     0x7f4938232e65 - fmt::write::ha67db0db2f6a096ce8Q
  10:     0x7f493820f7a0 - fmt::builders::DebugTuple<'a, 'b>::field::h2ce8d9f33236cafaDwQ
  11:     0x7f49361b5115 - middle::def::Def...std..fmt..Debug::fmt::h938391f0b3e7112dsJn
  12:     0x7f4938232e65 - fmt::write::ha67db0db2f6a096ce8Q
  13:     0x7f493820f274 - fmt::builders::DebugStruct<'a, 'b>::field::haef609d3419cdbd2JrQ
  14:     0x7f49362265b1 - fmt::_&'a T.Debug::fmt::h17657018559682767623
  15:     0x7f4938232e65 - fmt::write::ha67db0db2f6a096ce8Q
  16:     0x7f49381870ea - rt::unwind::begin_unwind_fmt::h7bb2b8a52c6074abJQw
  17:     0x7f49361c70c5 - middle::const_eval::eval_const_expr_partial::hbec1acc1da716a81GTk
  18:     0x7f493754f37c - astconv::ast_ty_to_ty::h7a1c0c0cca136ce6Cjx
  19:     0x7f493759718d - astconv::convert_ty_with_lifetime_elision::h62c494c14aa9de2fthw
  20:     0x7f493759fc8e - astconv::ty_of_method_or_bare_fn::hcabe6144b8119a6eHBx
  21:     0x7f49375c1a60 - collect::type_scheme_of_item::he0f646ae1f45c0d9X9z
  22:     0x7f49375b9e29 - collect::convert_typed_item::hed81c40d50d02aa4HeA
  23:     0x7f49375a55ce - collect::convert_item::h415e2accc707fad518y
  24:     0x7f49375a2577 - collect::collect_item_types::h2934ed33155c9781xky
  25:     0x7f49375edc2b - check_crate::hfe64eeb39daabb000BE
  26:     0x7f49386f027d - driver::phase_3_run_analysis_passes::closure.21698
  27:     0x7f49386d28a3 - middle::ty::ctxt<'tcx>::create_and_enter::h17134548426382567756
  28:     0x7f49386cdd7e - driver::phase_3_run_analysis_passes::h13764670269333747744
  29:     0x7f49386ad6fd - driver::compile_input::h3c82c6370b45f42aYba
  30:     0x7f49388154fb - run_compiler::h551ac01c07dc0a95eqc
  31:     0x7f4938812e17 - boxed::F.FnBox<A>::call_box::h12386432182602546056
  32:     0x7f4938812884 - rt::unwind::try::try_fn::h15469726732675206848
  33:     0x7f49381c24e8 - __rust_try
  34:     0x7f49381ae5e2 - rt::unwind::try::inner_try::h58375fb23cadeb38wNw
  35:     0x7f4938812a18 - boxed::F.FnBox<A>::call_box::h11817215854212583186
  36:     0x7f49381c1933 - sys::thread::Thread::new::thread_start::h255e2285f2cbfbcekXv
  37:     0x7f49324c86a9 - start_thread
  38:     0x7f4937e40eec - clone
  39:                0x0 - <unknown>

verbose compiler info

$ rustc --version --verbose
rustc 1.4.0-dev (205c356ac 2015-09-05)
binary: rustc
commit-hash: 205c356ace559f511d5b7e51fd6b8c46bb4cc1ea
commit-date: 2015-09-05
host: x86_64-unknown-linux-gnu
release: 1.4.0-dev

other thoughts

I would have expected the accepted way to write the array size would be <T as Trait>::SIZE, but that fails to compile with this error message:

compiler_panic.rs:7:52: 7:70 error: array length constant evaluation error: non-constant path in constant expr [E0250]
compiler_panic.rs:7 pub fn cause_compiler_to_panic<T: Trait>() -> [u8; <T as Trait>::SIZE] {
                                                                       ^~~~~~~~~~~~~~~~~~
compiler_panic.rs:7:52: 7:70 help: run `rustc --explain E0250` to see a detailed explanation
error: aborting due to previous error

[alexispurslane]

That's strange. And the file that it talks about, src/librustc/front/map/mod.rs doesn't even exist.

[alexispurslane]

Although I can understand the second error (non-constant path in constant expr).

[jdm]

The file does exist: http://mxr.mozilla.org/rust/source/src/librustc/front/map/mod.rs

[Thiez]

It's not just with ::SIZE, using pub fn cause_compiler_to_panic<T: Trait>() -> [u8; T::FOO] { ICE's just as hard, and I never even defined FOO.

[arielb1]

I think we have to fix <DefId as Debug> globally to stop crashing like that.

[arielb1]

Anyway, paths with type parameters (e.g. T::SIZE, <T as Trait>::SIZE) are not supported in array lengths.

[arielb1]

rustc encountered an ICE while trying to display an ICE

the inner ICE is:

thread 'rustc' panicked at 'path not fully resolved: PathResolution { base_def: DefTyParam(FnSpace, 0, DefId { krate: 0, node: 12 => cause_compiler_to_panic::T }, T(66)), last_private: LastMod(AllPublic), depth: 1 }', ../src/librustc/middle/def.rs:81

This looks like a dupe of something.

[nagisa]

Dupe, dupe, dupe, semi-dupe, semi-dupe.

[alexcrichton]

Closing as a dupe of #25145