ICE when coercing `NonNull<[T; 0]>` to `NonNull<[T]>` in const fn

[rodrimati1992]

Code

use core::ptr::NonNull;

pub const fn dangling_slice<T>() -> NonNull<[T]> {
    NonNull::<[T; 0]>::dangling()
}

Meta

rustc --version --verbose:

1.46.0-beta.2

(2020-07-23 6f959902b3103c49ca98)

Error output

   Compiling playground v0.0.1 (/playground)
thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/librustc_mir/transform/qualify_min_const_fn.rs:196:58
note: run with `RUST_BACKTRACE=1` environment variable to display a 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 1.46.0-beta.2 (6f959902b 2020-07-23) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type lib

note: some of the compiler flags provided by cargo are hidden

error: could not compile `playground`.

To learn more, run the command again with --verbose.

Backtrace

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src/librustc_mir/transform/qualify_min_const_fn.rs:196:58
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   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:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:530
  12: rust_begin_unwind
             at src/libstd/panicking.rs:437
  13: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  14: core::panicking::panic
             at src/libcore/panicking.rs:50
  15: rustc_mir::transform::qualify_min_const_fn::is_min_const_fn
  16: rustc_mir::transform::check_consts::validation::Validator::check_body
  17: rustc_mir::transform::mir_const_qualif
  18: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_const_qualif>::compute
  19: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  20: rustc_data_structures::stack::ensure_sufficient_stack
  21: rustc_query_system::query::plumbing::get_query_impl
  22: rustc_mir::transform::mir_validated
  23: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_validated>::compute
  24: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  25: rustc_data_structures::stack::ensure_sufficient_stack
  26: rustc_query_system::query::plumbing::get_query_impl
  27: rustc_mir::borrow_check::mir_borrowck
  28: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_borrowck>::compute
  29: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  30: rustc_data_structures::stack::ensure_sufficient_stack
  31: rustc_query_system::query::plumbing::get_query_impl
  32: rustc_query_system::query::plumbing::ensure_query_impl
  33: rustc_session::utils::<impl rustc_session::session::Session>::time
  34: rustc_interface::passes::analysis
  35: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  36: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  37: rustc_query_system::query::plumbing::get_query_impl
  38: rustc_middle::ty::context::tls::enter_global
  39: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  40: rustc_span::with_source_map
  41: rustc_interface::interface::create_compiler_and_run
  42: scoped_tls::ScopedKey<T>::set
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 1.46.0-beta.2 (6f959902b 2020-07-23) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -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 [mir_const_qualif] const checking `dangling`
#1 [mir_validated] processing `dangling`
#2 [mir_borrowck] borrow-checking `dangling`
#3 [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.

[rodrimati1992]

@jonas-schievink
Is it a regression from stable to beta if it doesn't compile on stable?

I added this to the original comment:

On rust stable 1.45.1, it causes this compilation error instead:

error[E0723]: unsizing casts are not allowed in const fn
 --> src/lib.rs:4:5
  |
4 |     NonNull::<[T; 0]>::dangling()
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information

[jonas-schievink]

Yes, the error message regressed. I don't know if the code is supposed to compile on beta or if it should still fail with the same error, but an ICE is a regression either way.

[JohnTitor]

I think the failure is introduced in #73862 and this unwrap causes it:

rust/src/librustc_mir/transform/qualify_min_const_fn.rs

Line 196 in 829d69b

let pointee_ty = cast_ty.builtin_deref(true).unwrap().ty;

So, my thought is we shouldn't unwrap here and return Err instead if it's None. @oli-obk is this correct?

[oli-obk]

Yes, return an error mentioning that unsizing is only allowed for references right now.