Rust: Compiler panics when an array is initialized with length of a certain associated constant

[TurkeyMcMac]

I was playing around with the new associated constants trying to make type-level integers. Creating an array with a length defined by Five works, but creating one with the product of Multiply as a length causes the compiler to panic.

This is the code I tried to run:

use std::marker::PhantomData;

fn main() {
    let _arr = [1; <Multiply<Five, Five>>::VAL]; // The compiler finds this scary for some reason.
}

trait TypeVal<T> {
    const VAL: T;
}

struct Five;

impl TypeVal<usize> for Five {
    const VAL: usize = 5;
}

struct Multiply<N, M> {
    _n: PhantomData<N>,
    _m: PhantomData<M>,
}

impl<N, M> TypeVal<usize> for Multiply<N, M>
    where N: TypeVal<usize>,
          M: TypeVal<usize>,
{
    const VAL: usize = N::VAL * M::VAL;
}

Meta

rustc --version --verbose:

rustc 1.20.0 (f3d6973f4 2017-08-27)
binary: rustc
commit-hash: f3d6973f41a7d1fb83029c9c0ceaf0f5d4fd7208
commit-date: 2017-08-27
host: x86_64-unknown-linux-gnu
release: 1.20.0
LLVM version: 4.0

Backtrace:

thread 'rustc' panicked at 'internal error: entered unreachable code', /checkout/src/librustc_mir/hair/cx/mod.rs:140:8
stack backtrace:
   0:     0x7f0518299d13 - std::sys::imp::backtrace::tracing::imp::unwind_backtrace::hcdf51e4c9dc54357
                               at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1:     0x7f0518294524 - std::sys_common::backtrace::_print::h9da91fd31a37d0f1
                               at /checkout/src/libstd/sys_common/backtrace.rs:71
   2:     0x7f05182a7333 - std::panicking::default_hook::{{closure}}::h46820a72bf0cb624
                               at /checkout/src/libstd/sys_common/backtrace.rs:60
                               at /checkout/src/libstd/panicking.rs:380
   3:     0x7f05182a703a - std::panicking::default_hook::h4c1ef1cc83189c8e
                               at /checkout/src/libstd/panicking.rs:390
   4:     0x7f05182a77f7 - std::panicking::rust_panic_with_hook::h99016f44bdcb8544
                               at /checkout/src/libstd/panicking.rs:611
   5:     0x7f05160699fa - std::panicking::begin_panic_new::h20bde0fa96b5baa3
   6:     0x7f05160df42c - rustc_mir::hair::cx::Cx::fatal_const_eval_err::hf97cdba7fc5f66cd
   7:     0x7f05160e3be8 - rustc_mir::hair::cx::expr::make_mirror_unadjusted::hbe391e56243cb2db
   8:     0x7f05160dfda8 - rustc_mir::hair::cx::expr::<impl rustc_mir::hair::Mirror<'tcx> for &'tcx rustc::hir::Expr>::make_mirror::h4821ea617bc41d74
   9:     0x7f05160c6b96 - <rustc_mir::hair::ExprRef<'tcx> as rustc_mir::build::into::EvalInto<'tcx>>::eval_into::h40e42cf42cf544dd
  10:     0x7f05160cbed5 - rustc_mir::build::matches::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::expr_into_pattern::h4a9bcd58a09e3d5d
  11:     0x7f05160d1814 - rustc_mir::build::scope::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::in_scope::h81e04d3bdbceb472
  12:     0x7f05160b83d0 - rustc_mir::build::block::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::ast_block_stmts::hafef382cef3e8204
  13:     0x7f05160d1b99 - rustc_mir::build::scope::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::in_scope::hc808f0a51c457ee5
  14:     0x7f05160d170b - rustc_mir::build::scope::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::in_opt_scope::hd179717c8f13dbe1
  15:     0x7f05160c1796 - rustc_mir::build::expr::into::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::into_expr::h111f38b2b7786042
  16:     0x7f05160c6bbf - <rustc_mir::hair::ExprRef<'tcx> as rustc_mir::build::into::EvalInto<'tcx>>::eval_into::h40e42cf42cf544dd
  17:     0x7f05160c11cd - rustc_mir::build::expr::into::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::into_expr::h111f38b2b7786042
  18:     0x7f05160c6bbf - <rustc_mir::hair::ExprRef<'tcx> as rustc_mir::build::into::EvalInto<'tcx>>::eval_into::h40e42cf42cf544dd
  19:     0x7f05160c11cd - rustc_mir::build::expr::into::<impl rustc_mir::build::Builder<'a, 'gcx, 'tcx>>::into_expr::h111f38b2b7786042
  20:     0x7f0516114bdc - rustc_mir::transform::mir_const::h44528d9ec6133f0c
  21:     0x7f051534640f - rustc::ty::maps::<impl rustc::ty::maps::queries::mir_const<'tcx>>::try_get_with::{{closure}}::run_provider::hbd2ca39015d187b6
  22:     0x7f051512f1da - rustc::dep_graph::graph::DepGraph::with_task::h296e6bba1fa15a03
  23:     0x7f0515346940 - rustc::ty::maps::<impl rustc::ty::maps::queries::mir_const<'tcx>>::try_get::h11e76ae1dcdda37d
  24:     0x7f05153805ba - rustc::ty::maps::TyCtxtAt::mir_const::hdaca076bb4373d8e
  25:     0x7f051537d1c6 - rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::mir_const::h12dfe73edca72c4f
  26:     0x7f0516115aea - rustc_mir::transform::mir_validated::h4ab61ba27ff483ec
  27:     0x7f051534773f - rustc::ty::maps::<impl rustc::ty::maps::queries::mir_validated<'tcx>>::try_get_with::{{closure}}::run_provider::h4e5b76b3ab5f815f
  28:     0x7f051512f1da - rustc::dep_graph::graph::DepGraph::with_task::h296e6bba1fa15a03
  29:     0x7f0515347c70 - rustc::ty::maps::<impl rustc::ty::maps::queries::mir_validated<'tcx>>::try_get::h89ca9fc9b1475a29
  30:     0x7f05153806fa - rustc::ty::maps::TyCtxtAt::mir_validated::ha53b8d1b04be8f61
  31:     0x7f051537d1f6 - rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::mir_validated::hd499970927ef8808
  32:     0x7f0517c304ca - rustc_borrowck::borrowck::borrowck::h0d64b1bfc871d914
  33:     0x7f051512eeb4 - rustc::dep_graph::graph::DepGraph::with_task::h1ba8376c1cfaccee
  34:     0x7f05153523df - rustc::ty::maps::<impl rustc::ty::maps::queries::borrowck<'tcx>>::try_get::hfdf892ac1155f936
  35:     0x7f051537d532 - rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::borrowck::h9e740c44bdab1305
  36:     0x7f0517c30367 - rustc_borrowck::borrowck::check_crate::h0d548219b93e53b4
  37:     0x7f051863b014 - rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}::h5664029733f1f6d5
  38:     0x7f051862efb0 - rustc_driver::driver::phase_3_run_analysis_passes::hf53b3e845cb6e837
  39:     0x7f0518616d76 - rustc_driver::driver::compile_input::h26cfd26116d1d14c
  40:     0x7f0518654ce1 - rustc_driver::run_compiler::h8741b5aff2ed69fc
  41:     0x7f051857f21f - std::sys_common::backtrace::__rust_begin_short_backtrace::h4e951eb2de4f99f0
  42:     0x7f05182d6d4c - __rust_maybe_catch_panic
                               at /checkout/src/libpanic_unwind/lib.rs:98
  43:     0x7f05185b0900 - <F as alloc::boxed::FnBox<A>>::call_box::h140b04f7a2d92fc5
  44:     0x7f05182a627b - std::sys::imp::thread::Thread::new::thread_start::h10fad04495d944f7
                               at /checkout/src/liballoc/boxed.rs:661
                               at /checkout/src/libstd/sys_common/thread.rs:21
                               at /checkout/src/libstd/sys/unix/thread.rs:84
  45:     0x7f05135596b9 - start_thread
  46:     0x7f0517f4f3dc - clone
  47:                0x0 - <unknown>

I hope this is helpful.

[tirr-c]

Found something that might help:

use std::marker::PhantomData;

trait Val {
    const VAL: usize;
}

struct Five;
impl Val for Five {
    const VAL: usize = 5;
}

struct Multiply<T, U> {
    _t: PhantomData<T>,
    _u: PhantomData<U>,
}
impl<T: Val, U: Val> Val for Multiply<T, U> {
    const VAL: usize = T::VAL * U::VAL;
}

fn main() {
    let arr: [usize; 25] = [1; <Multiply<Five, Five>>::VAL];
    println!("{:?}", arr);
}

<Multiply<Five, Five>>::VAL is 25usize, so there should be no type mismatch. However, I got this:

error[E0308]: mismatched types
  --> src/main.rs:21:28
   |
21 |     let arr: [usize; 25] = [1; <Multiply<Five, Five>>::VAL];
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an array with a fixed size of 25 elements, found one with 0 elements
   |
   = note: expected type `[usize; 25]`
              found type `[usize; 0]`

[TurkeyMcMac]

I get the same backtrace when I annotate the type as [usize; 0].

[TurkeyMcMac]

The expression seems to evaulate properly to 25 in this context:

let num = <Multiply<Five, Five>>::VAL;

assert_eq!(num, 25);

[kennytm]

Note that associated constants are not supported in array type [T; n] yet (#44168).

[TurkeyMcMac]

It seems like the compiler should say that, rather than crashing. This is the stable release, after all.

[varkor]

This no longer seems to ICE.