&Vec to function expecting a &[u8; 32]

[mboecker]

I tried passing an empty vector to a function and the compiler crashed.
Here is a working example:
http://is.gd/EOfLkC

I think it has to do with the compiler trying to deref the vec.

Meta

rustc 1.7.0
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.7.0

But the bug is in beta and nightly, too.

I don't have a backtrace. Should be easy for you to get one though.

[alexcrichton]

Inlining the test for future reference:

pub fn test(a: &[u8; 32]) {}

fn main() {
    test(&vec![]);
}

[strega-nil]

ping @nikomatsakis, this is the same issue (I think) as #32320

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
thread 'rustc' panicked at 'index out of bounds: the len is 5 but the index is 7', ../src/libcollections/vec.rs:1166
stack backtrace:
   1:        0x1117c7f38 - sys::backtrace::tracing::imp::write::h83bc1946ac132a7d0Gu
   2:        0x1117d1475 - panicking::default_hook::_$u7b$$u7b$closure$u7d$$u7d$::closure.43767
   3:        0x1117d0fd7 - panicking::default_hook::had684d056a84b05cPgz
   4:        0x111798336 - sys_common::unwind::begin_unwind_inner::hfef527c9f2434064HGt
   5:        0x111799f4e - sys_common::unwind::begin_unwind_fmt::h02d40e945b8e457cNFt
   6:        0x1117c5807 - rust_begin_unwind
   7:        0x11180a6d0 - panicking::panic_fmt::h54d7b44acd72bb46zML
   8:        0x11180a8e0 - panicking::panic_bounds_check::hb99e0b711696a86cFLL
   9:        0x10e1227c0 - middle::infer::InferCtxt<'a, 'tcx>::shallow_resolve::h43c92678dc539950AtD
  10:        0x10e1fbc7b - middle::traits::fulfill::process_predicate::hc70860709ec86ab05US
  11:        0x10e1faa83 - obligation_forest::ObligationForest<O, T>::process_obligations::h18416070063450621727
  12:        0x10e1f619c - middle::traits::fulfill::FulfillmentContext<'tcx>::select_where_possible::h817133e3c0e6314baNS
  13:        0x10dc9b354 - check::FnCtxt<'a, 'tcx>::select_obligations_where_possible::h7ea8359c24588fcdiMq
  14:        0x10dcaeccc - check::FnCtxt<'a, 'tcx>::resolve_type_vars_if_possible::hfde6a734ee0b83c8GSp
  15:        0x10dcc570a - check::demand::coerce::h59fab50398c45a417Cf
  16:        0x10dcc30e6 - check::check_argument_types::h7425e24ea32ed15f6ar
  17:        0x10dd3d999 - check::callee::confirm_builtin_call::h47e6b2f48a31900cfkm
  18:        0x10dcdc120 - check::callee::check_call::h0c0b2518a65d933bkbm
  19:        0x10dcc69ca - check::check_expr_with_expectation_and_lvalue_pref::he02550a8ac3fc525bJr
  20:        0x10dcf6b52 - check::check_stmt::hfd7db92334e1e068IYs
  21:        0x10dca193a - check::check_block_with_expected::h29cb83403d5adabbv2s
  22:        0x10dc98348 - check::check_fn::hf8f01d7e0d5b7dcayWo
  23:        0x10dc95a2f - check::check_bare_fn::hf7c744d6fc7c32be1Lo
  24:        0x10dc908d9 - check::check_item_body::h2d3043343fe0fdd1Dbp
  25:        0x10dc8878b - check::check_item_bodies::h457431c5d3d2ced0OJo
  26:        0x10dc80005 - check_crate::h3e3d779ef330fadcCFC
  27:        0x10d323db1 - driver::phase_3_run_analysis_passes::_$u7b$$u7b$closure$u7d$$u7d$::closure.30095
  28:        0x10d3223e9 - middle::ty::context::TyCtxt<'tcx>::create_and_enter::h11812805895441873020
  29:        0x10d31edad - driver::phase_3_run_analysis_passes::h5261001332275006052
  30:        0x10d2f1df7 - driver::compile_input::he5def18759d9424cPca
  31:        0x10d2df6f6 - run_compiler::h61ff55f691b932efVMc
  32:        0x10d2dcca2 - sys_common::unwind::try::try_fn::h2984156204492186173
  33:        0x1117c579b - __rust_try
  34:        0x1117c5723 - sys_common::unwind::inner_try::hbef67b929e699248JDt
  35:        0x10d2dd539 - boxed::F.FnBox<A>::call_box::h8793623295134616532
  36:        0x1117d02c8 - sys::thread::Thread::new::thread_start::h68477ad6a3ba8036fvy
  37:     0x7fff89eb999c - _pthread_body
  38:     0x7fff89eb9919 - _pthread_start

[strega-nil]

I've done some testing: the important parts are: the passed type must have at least one type parameter, it must implement Deref, and the compiler must be unable to infer at least one type parameter. As another condition, the function must take a reference, and you must pass a reference, and these references must be compatible, i.e., you can coerce between the passed reference and the received reference. I believe those are the only conditions.

fn main() {
    // may have any number of type parameters greater than zero
    struct WithParam<T>(std::marker::PhantomData<T>); 
    // Target can be anything
    impl<T> std::ops::Deref for WithParam<T> {
        type Target = ();
        fn deref(&self) -> &() {
            panic!()
        }
    }
    // can be literally anything
    struct OtherType;
    fn doop(_: &OtherType) { }
    // must not be able to infer the inner type
    doop(&WithParam(std::marker::PhantomData))
    // the references must be able to coerce,
    // so either &mut -> &, & -> &, or &mut -> &mut
    // passing an &WithParam to a function expecting an &mut OtherType
    // will not create ICE conditions
}

https://play.rust-lang.org/?gist=1ff04f679e568535f7c9&version=stable

[mboecker]

On nightly, this is fixed.
https://is.gd/VKvrDI

[arielb1]

duplicate of that issue.

[calidion]

@mboecker

Your example is broken now!

[mboecker]

no, its supposed to throw an error, it just wasnt supposed to cause an internal compiler error.

note: expected type `&[u8; 32]`
found type `&std::vec::Vec<_>`

this is due to the function having the explicit length 32 in its signature. since the vector is dynamically sized, it cannot be coerced into a fixed size array. the function should take &[u32] as an argument, a slice (of, at compile-time, unknown length). I hope this answers your question, otherwise have a look at the book regarding slices.

[calidion]

how to easily convert an Vec<u8> to a fixed [u8;32]?

[mboecker]

you could try this solution by implementing your own function to convert it or use the recently stabilized TryInto trait like suggested in this answer.

Although I love answering your questions, please dont use the bug tracker for them but rather one of the other chat platforms like discord or IRC.