ICE: cannot convert `'{erased}` to a region vid

[matthiaskrgr]

auto-reduced (treereduce-rust):

#![feature(adt_const_params, generic_const_exprs)]

const fn concat_strs() -> &'static str {
    const fn concat_arr<const M: usize, const N: usize>(a: [u8; M], b: [u8; N]) -> [u8; M + N] {}

    impl<const A: &'static str, const B: &'static str> Inner<A, B>
    where
        [(); A.len()]:,
        [(); B.len()]:,
        [(); A.len() + B.len()]:,
    {
        const ABSTR: &'static str = unsafe {
            std::str::from_utf8_unchecked(&concat_arr(
                A.as_ptr().cast().read(),
                B.as_ptr().cast().read(),
            ))
        };
    }
}

original:

#![feature(adt_const_params, generic_const_exprs)]

const fn concat_strs<const A: &'static str>() -> &'static str {
    const fn concat_arr<const M: usize, const N: usize>(a: [u8; M], b: [u8; N]) -> [u8; M + N] {}

    impl<const A: &'static str, const B: &'static str> Inner<A, B>
    where
        [(); A.len()]:,
        [(); B.len()]:,
        [(); A.len() + B.len()]:,
    {
        const ABSTR: &'static str = unsafe {
            std::str::from_utf8_unchecked(&concat_arr(
                A.as_ptr().cast().read(),
                B.as_ptr().cast().read(),
            ))
        };
    }
}

Version information

rustc 1.80.0-nightly (1ba35e9bb 2024-05-25)
binary: rustc
commit-hash: 1ba35e9bb44d416fc2ebf897855454258b650b01
commit-date: 2024-05-25
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.6

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zincremental-verify-ich=yes -Cincremental=<dir> -Cdebuginfo=2

Program output

error[E0412]: cannot find type `Inner` in this scope
 --> /tmp/icemaker_global_tempdir.VrW7PdSYIB8H/rustc_testrunner_tmpdir_reporting.79VNYkEwgPVK/mvce.rs:6:56
  |
6 |     impl<const A: &'static str, const B: &'static str> Inner<A, B>
  |                                                        ^^^^^ not found in this scope

warning: the feature `adt_const_params` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.VrW7PdSYIB8H/rustc_testrunner_tmpdir_reporting.79VNYkEwgPVK/mvce.rs:1:12
  |
1 | #![feature(adt_const_params, generic_const_exprs)]
  |            ^^^^^^^^^^^^^^^^
  |
  = note: see issue #95174 <https://github.com/rust-lang/rust/issues/95174> for more information
  = note: `#[warn(incomplete_features)]` on by default

warning: the feature `generic_const_exprs` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.VrW7PdSYIB8H/rustc_testrunner_tmpdir_reporting.79VNYkEwgPVK/mvce.rs:1:30
  |
1 | #![feature(adt_const_params, generic_const_exprs)]
  |                              ^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #76560 <https://github.com/rust-lang/rust/issues/76560> for more information

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.VrW7PdSYIB8H/rustc_testrunner_tmpdir_reporting.79VNYkEwgPVK/mvce.rs:19:2
   |
19 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.VrW7PdSYIB8H/rustc_testrunner_tmpdir_reporting.79VNYkEwgPVK/mvce.rs`

error[E0308]: mismatched types
 --> /tmp/icemaker_global_tempdir.VrW7PdSYIB8H/rustc_testrunner_tmpdir_reporting.79VNYkEwgPVK/mvce.rs:3:27
  |
3 | const fn concat_strs() -> &'static str {
  |          -----------      ^^^^^^^^^^^^ expected `&str`, found `()`
  |          |
  |          implicitly returns `()` as its body has no tail or `return` expression

error[E0308]: mismatched types
 --> /tmp/icemaker_global_tempdir.VrW7PdSYIB8H/rustc_testrunner_tmpdir_reporting.79VNYkEwgPVK/mvce.rs:4:84
  |
4 |     const fn concat_arr<const M: usize, const N: usize>(a: [u8; M], b: [u8; N]) -> [u8; M + N] {}
  |              ----------                                                            ^^^^^^^^^^^ expected `[u8; M + N]`, found `()`
  |              |
  |              implicitly returns `()` as its body has no tail or `return` expression
  |
note: consider returning one of these bindings
 --> /tmp/icemaker_global_tempdir.VrW7PdSYIB8H/rustc_testrunner_tmpdir_reporting.79VNYkEwgPVK/mvce.rs:4:57
  |
4 |     const fn concat_arr<const M: usize, const N: usize>(a: [u8; M], b: [u8; N]) -> [u8; M + N] {}
  |                                                         ^           ^

warning: type annotations needed
  --> /tmp/icemaker_global_tempdir.VrW7PdSYIB8H/rustc_testrunner_tmpdir_reporting.79VNYkEwgPVK/mvce.rs:14:35
   |
14 |                 A.as_ptr().cast().read(),
   |                                   ^^^^
   |
   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
   = note: for more information, see issue #46906 <https://github.com/rust-lang/rust/issues/46906>
   = note: `#[warn(tyvar_behind_raw_pointer)]` on by default

warning: type annotations needed
  --> /tmp/icemaker_global_tempdir.VrW7PdSYIB8H/rustc_testrunner_tmpdir_reporting.79VNYkEwgPVK/mvce.rs:15:35
   |
15 |                 B.as_ptr().cast().read(),
   |                                   ^^^^
   |
   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
   = note: for more information, see issue #46906 <https://github.com/rust-lang/rust/issues/46906>

error: internal compiler error: compiler/rustc_borrowck/src/universal_regions.rs:880:36: cannot convert `'{erased}` to a region vid

thread 'rustc' panicked at compiler/rustc_borrowck/src/universal_regions.rs:880:36:
Box<dyn Any>
stack backtrace:
   0:     0x786c185c8075 - std::backtrace_rs::backtrace::libunwind::trace::h90fcfe28ddb0241a
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/std/src/../../backtrace/src/backtrace/libunwind.rs:105:5
   1:     0x786c185c8075 - std::backtrace_rs::backtrace::trace_unsynchronized::h295e5065b6a00265
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x786c185c8075 - std::sys_common::backtrace::_print_fmt::hcc6f2b94476e542d
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x786c185c8075 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hed2c5e0a0ce6836c
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x786c1861718b - core::fmt::rt::Argument::fmt::h1df61bd36deed924
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/core/src/fmt/rt.rs:165:63
   5:     0x786c1861718b - core::fmt::write::h846d656f7222a8c0
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/core/src/fmt/mod.rs:1172:21
   6:     0x786c185bcdcf - std::io::Write::write_fmt::h67d939b7212f6891
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/std/src/io/mod.rs:1835:15
   7:     0x786c185c7e4e - std::sys_common::backtrace::_print::hcb12f24eca79ee8b
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x786c185c7e4e - std::sys_common::backtrace::print::h7a02369029f2cb19
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x786c185ca879 - std::panicking::default_hook::{{closure}}::he817c31d2eff8a0b
  10:     0x786c185ca61a - std::panicking::default_hook::h9dce7ab19378e0f9
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/std/src/panicking.rs:298:9
  11:     0x786c150f0160 - std[6f3c54f961bcd653]::panicking::update_hook::<alloc[6e197ba0e7511b9e]::boxed::Box<rustc_driver_impl[297cb1e57032d44d]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x786c185cafab - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h178099f3431bb0b9
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/alloc/src/boxed.rs:2077:9
  13:     0x786c185cafab - std::panicking::rust_panic_with_hook::h114664f4a33b113f
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/std/src/panicking.rs:799:13
  14:     0x786c151200b4 - std[6f3c54f961bcd653]::panicking::begin_panic::<rustc_errors[9eb651f4c119cf9e]::ExplicitBug>::{closure#0}
  15:     0x786c1511ce66 - std[6f3c54f961bcd653]::sys_common::backtrace::__rust_end_short_backtrace::<std[6f3c54f961bcd653]::panicking::begin_panic<rustc_errors[9eb651f4c119cf9e]::ExplicitBug>::{closure#0}, !>
  16:     0x786c1511cb46 - std[6f3c54f961bcd653]::panicking::begin_panic::<rustc_errors[9eb651f4c119cf9e]::ExplicitBug>
  17:     0x786c15129651 - <rustc_errors[9eb651f4c119cf9e]::diagnostic::BugAbort as rustc_errors[9eb651f4c119cf9e]::diagnostic::EmissionGuarantee>::emit_producing_guarantee
  18:     0x786c155f81cc - rustc_middle[1e04d1e697039279]::util::bug::opt_span_bug_fmt::<rustc_span[4b65ca1f0ca9c763]::span_encoding::Span>::{closure#0}
  19:     0x786c155dc59a - rustc_middle[1e04d1e697039279]::ty::context::tls::with_opt::<rustc_middle[1e04d1e697039279]::util::bug::opt_span_bug_fmt<rustc_span[4b65ca1f0ca9c763]::span_encoding::Span>::{closure#0}, !>::{closure#0}
  20:     0x786c155dc41b - rustc_middle[1e04d1e697039279]::ty::context::tls::with_context_opt::<rustc_middle[1e04d1e697039279]::ty::context::tls::with_opt<rustc_middle[1e04d1e697039279]::util::bug::opt_span_bug_fmt<rustc_span[4b65ca1f0ca9c763]::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
  21:     0x786c1397b620 - rustc_middle[1e04d1e697039279]::util::bug::bug_fmt
  22:     0x786c167357fb - <rustc_borrowck[edc41e3fea2de417]::type_check::TypeChecker>::fully_perform_op::<(), rustc_middle[1e04d1e697039279]::ty::ParamEnvAnd<rustc_middle[1e04d1e697039279]::traits::query::type_op::ProvePredicate>>
  23:     0x786c1672dd06 - <rustc_borrowck[edc41e3fea2de417]::type_check::TypeVerifier as rustc_middle[1e04d1e697039279]::mir::visit::Visitor>::visit_constant
  24:     0x786c140202cb - <rustc_borrowck[edc41e3fea2de417]::type_check::TypeVerifier as rustc_middle[1e04d1e697039279]::mir::visit::Visitor>::visit_body
  25:     0x786c16719c71 - <rustc_borrowck[edc41e3fea2de417]::type_check::TypeVerifier as rustc_middle[1e04d1e697039279]::mir::visit::Visitor>::visit_operand
  26:     0x786c1401fb9d - <rustc_borrowck[edc41e3fea2de417]::type_check::TypeVerifier as rustc_middle[1e04d1e697039279]::mir::visit::Visitor>::visit_body
  27:     0x786c13ff462b - rustc_borrowck[edc41e3fea2de417]::type_check::type_check
  28:     0x786c13927fd7 - rustc_borrowck[edc41e3fea2de417]::nll::compute_regions
  29:     0x786c172cba6b - rustc_borrowck[edc41e3fea2de417]::do_mir_borrowck
  30:     0x786c172be03e - rustc_query_impl[2b30849480afbfc2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[2b30849480afbfc2]::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1e04d1e697039279]::query::erase::Erased<[u8; 8usize]>>
  31:     0x786c1676dc84 - rustc_query_system[f641f50659fb4646]::query::plumbing::try_execute_query::<rustc_query_impl[2b30849480afbfc2]::DynamicConfig<rustc_query_system[f641f50659fb4646]::query::caches::VecCache<rustc_span[4b65ca1f0ca9c763]::def_id::LocalDefId, rustc_middle[1e04d1e697039279]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[2b30849480afbfc2]::plumbing::QueryCtxt, true>
  32:     0x786c1688a074 - rustc_query_impl[2b30849480afbfc2]::query_impl::mir_borrowck::get_query_incr::__rust_end_short_backtrace
  33:     0x786c16749204 - rustc_interface[2ec5f1d4cd1ff974]::passes::analysis
  34:     0x786c16748395 - rustc_query_impl[2b30849480afbfc2]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[2b30849480afbfc2]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[1e04d1e697039279]::query::erase::Erased<[u8; 1usize]>>
  35:     0x786c171c38ce - rustc_query_system[f641f50659fb4646]::query::plumbing::try_execute_query::<rustc_query_impl[2b30849480afbfc2]::DynamicConfig<rustc_query_system[f641f50659fb4646]::query::caches::SingleCache<rustc_middle[1e04d1e697039279]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[2b30849480afbfc2]::plumbing::QueryCtxt, true>
  36:     0x786c171c34fa - rustc_query_impl[2b30849480afbfc2]::query_impl::analysis::get_query_incr::__rust_end_short_backtrace
  37:     0x786c16fb5bce - rustc_interface[2ec5f1d4cd1ff974]::interface::run_compiler::<core[5392e317ecfab49a]::result::Result<(), rustc_span[4b65ca1f0ca9c763]::ErrorGuaranteed>, rustc_driver_impl[297cb1e57032d44d]::run_compiler::{closure#0}>::{closure#1}
  38:     0x786c16fa2549 - std[6f3c54f961bcd653]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[2ec5f1d4cd1ff974]::util::run_in_thread_with_globals<rustc_interface[2ec5f1d4cd1ff974]::util::run_in_thread_pool_with_globals<rustc_interface[2ec5f1d4cd1ff974]::interface::run_compiler<core[5392e317ecfab49a]::result::Result<(), rustc_span[4b65ca1f0ca9c763]::ErrorGuaranteed>, rustc_driver_impl[297cb1e57032d44d]::run_compiler::{closure#0}>::{closure#1}, core[5392e317ecfab49a]::result::Result<(), rustc_span[4b65ca1f0ca9c763]::ErrorGuaranteed>>::{closure#0}, core[5392e317ecfab49a]::result::Result<(), rustc_span[4b65ca1f0ca9c763]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[5392e317ecfab49a]::result::Result<(), rustc_span[4b65ca1f0ca9c763]::ErrorGuaranteed>>
  39:     0x786c16fa2308 - <<std[6f3c54f961bcd653]::thread::Builder>::spawn_unchecked_<rustc_interface[2ec5f1d4cd1ff974]::util::run_in_thread_with_globals<rustc_interface[2ec5f1d4cd1ff974]::util::run_in_thread_pool_with_globals<rustc_interface[2ec5f1d4cd1ff974]::interface::run_compiler<core[5392e317ecfab49a]::result::Result<(), rustc_span[4b65ca1f0ca9c763]::ErrorGuaranteed>, rustc_driver_impl[297cb1e57032d44d]::run_compiler::{closure#0}>::{closure#1}, core[5392e317ecfab49a]::result::Result<(), rustc_span[4b65ca1f0ca9c763]::ErrorGuaranteed>>::{closure#0}, core[5392e317ecfab49a]::result::Result<(), rustc_span[4b65ca1f0ca9c763]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[5392e317ecfab49a]::result::Result<(), rustc_span[4b65ca1f0ca9c763]::ErrorGuaranteed>>::{closure#2} as core[5392e317ecfab49a]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  40:     0x786c185d4d8b - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hfe2f8026e195e1aa
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/alloc/src/boxed.rs:2063:9
  41:     0x786c185d4d8b - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h3bdc22bc8cf7a921
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/alloc/src/boxed.rs:2063:9
  42:     0x786c185d4d8b - std::sys::pal::unix::thread::Thread::new::thread_start::h9cbf23e48f3c3fdb
                               at /rustc/1ba35e9bb44d416fc2ebf897855454258b650b01/library/std/src/sys/pal/unix/thread.rs:108:17
  43:     0x786c11eaa1cf - <unknown>
  44:     0x786c11f2b6ec - <unknown>
  45:                0x0 - <unknown>

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: rustc 1.80.0-nightly (1ba35e9bb 2024-05-25) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z incremental-verify-ich=yes -C incremental=[REDACTED] -C debuginfo=2

query stack during panic:
#0 [mir_borrowck] borrow-checking `concat_strs::<impl at /tmp/icemaker_global_tempdir.VrW7PdSYIB8H/rustc_testrunner_tmpdir_reporting.79VNYkEwgPVK/mvce.rs:6:5: 10:34>::ABSTR`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 5 previous errors; 4 warnings emitted

Some errors have detailed explanations: E0308, E0412, E0601.
For more information about an error, try `rustc --explain E0308`.

@rustbot label +F-adt_const_params +F-generic_const_exprs

[matthiaskrgr]

#![allow(incomplete_features)]
#![feature(adt_const_params, generic_const_exprs)]
#![allow(dead_code)]

const fn catone<const M: usize>(_a: &[u8; M]) -> [u8; M + 1]
where
    [(); M + 1]:,
{
    unimplemented!()
}

struct Catter<const A: &'static [u8]>;
impl<const A: &'static [u8]> Catter<A>
where
    [(); A.len() + 1]:,
{
    const ZEROS: &'static [u8; A.len()] = &[0_u8; A.len()];
    const R: &'static [u8] = &catone(Self::ZEROS);
}

fn main() {}

[matthiaskrgr]

#125451

[Jaic1]

Is this ICE still reproducible now? I cannot reproduce it on my local machine.

Version info:

rustc 1.83.0-nightly (363ae4188 2024-09-24)
binary: rustc
commit-hash: 363ae4188316b8b22cf6c1890bc73d84d05f70a4
commit-date: 2024-09-24
host: x86_64-unknown-linux-gnu
release: 1.83.0-nightly
LLVM version: 19.1.0

Test case:

#![allow(incomplete_features)]
#![feature(unsized_const_params, generic_const_exprs)]

const fn concat_strs() -> &'static str {
    const fn concat_arr<const M: usize, const N: usize>(a: [u8; M], b: [u8; N]) -> [u8; M + N] {}

    impl<const A: &'static str, const B: &'static str> Inner<A, B>
    where
        [(); A.len()]:,
        [(); B.len()]:,
        [(); A.len() + B.len()]:,
    {
        const ABSTR: &'static str = unsafe {
            std::str::from_utf8_unchecked(&concat_arr(
                A.as_ptr().cast().read(),
                B.as_ptr().cast().read(),
            ))
        };
    }
}

fn main() {}

Command:

rustc +nightly ice-125564.rs -Zincremental-verify-ich=yes -Cincremental=dir -Cdebuginfo=2

Program Output

error[E0412]: cannot find type `Inner` in this scope
 --> ice-125564.rs:7:56
  |
7 |     impl<const A: &'static str, const B: &'static str> Inner<A, B>
  |                                                        ^^^^^ not found in this scope

error[E0308]: mismatched types
 --> ice-125564.rs:4:27
  |
4 | const fn concat_strs() -> &'static str {
  |          -----------      ^^^^^^^^^^^^ expected `&str`, found `()`
  |          |
  |          implicitly returns `()` as its body has no tail or `return` expression

error[E0308]: mismatched types
 --> ice-125564.rs:5:84
  |
5 |     const fn concat_arr<const M: usize, const N: usize>(a: [u8; M], b: [u8; N]) -> [u8; M + N] {}
  |              ----------                                                            ^^^^^^^^^^^ expected `[u8; M + N]`, found `()`
  |              |
  |              implicitly returns `()` as its body has no tail or `return` expression
  |
note: consider returning one of these bindings
 --> ice-125564.rs:5:57
  |
5 |     const fn concat_arr<const M: usize, const N: usize>(a: [u8; M], b: [u8; N]) -> [u8; M + N] {}
  |                                                         ^           ^

warning: type annotations needed
  --> ice-125564.rs:15:35
   |
15 |                 A.as_ptr().cast().read(),
   |                                   ^^^^
   |
   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
   = note: for more information, see issue #46906 <https://github.com/rust-lang/rust/issues/46906>
   = note: `#[warn(tyvar_behind_raw_pointer)]` on by default

warning: type annotations needed
  --> ice-125564.rs:16:35
   |
16 |                 B.as_ptr().cast().read(),
   |                                   ^^^^
   |
   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
   = note: for more information, see issue #46906 <https://github.com/rust-lang/rust/issues/46906>

error: aborting due to 3 previous errors; 2 warnings emitted

Some errors have detailed explanations: E0308, E0412.
For more information about an error, try `rustc --explain E0308`.

[jakehemmerle]

Running @Jaic1 's code above, I also got the same exact output.

rustc 1.83.0-nightly (06bb8364a 2024-10-01)
binary: rustc
commit-hash: 06bb8364aaffefb0ce67e5f5445e66ec99c1f66e
commit-date: 2024-10-01
host: aarch64-apple-darwin
release: 1.83.0-nightly
LLVM version: 19.1.0

Details

error[E0412]: cannot find type `Inner` in this scope
 --> test.rs:7:56
  |
7 |     impl<const A: &'static str, const B: &'static str> Inner<A, B>
  |                                                        ^^^^^ not found in this scope

error[E0308]: mismatched types
 --> test.rs:4:27
  |
4 | const fn concat_strs() -> &'static str {
  |          -----------      ^^^^^^^^^^^^ expected `&str`, found `()`
  |          |
  |          implicitly returns `()` as its body has no tail or `return` expression

error[E0308]: mismatched types
 --> test.rs:5:84
  |
5 |     const fn concat_arr<const M: usize, const N: usize>(a: [u8; M], b: [u8; N]) -> [u8; M + N] {}
  |              ----------                                                            ^^^^^^^^^^^ expected `[u8; M + N]`, found `()`
  |              |
  |              implicitly returns `()` as its body has no tail or `return` expression
  |
note: consider returning one of these bindings
 --> test.rs:5:57
  |
5 |     const fn concat_arr<const M: usize, const N: usize>(a: [u8; M], b: [u8; N]) -> [u8; M + N] {}
  |                                                         ^           ^

warning: type annotations needed
  --> test.rs:15:35
   |
15 |                 A.as_ptr().cast().read(),
   |                                   ^^^^
   |
   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
   = note: for more information, see issue #46906 <https://github.com/rust-lang/rust/issues/46906>
   = note: `#[warn(tyvar_behind_raw_pointer)]` on by default

warning: type annotations needed
  --> test.rs:16:35
   |
16 |                 B.as_ptr().cast().read(),
   |                                   ^^^^
   |
   = warning: this is accepted in the current edition (Rust 2015) but is a hard error in Rust 2018!
   = note: for more information, see issue #46906 <https://github.com/rust-lang/rust/issues/46906>

error: aborting due to 3 previous errors; 2 warnings emitted

Some errors have detailed explanations: E0308, E0412.
For more information about an error, try `rustc --explain E0308`.