ICE: &[str] in function signature

[coventry]

In lib.rs, I have:

fn testfn(s: &[str]) -> u32 {
    return 1;
}

coventry@uppekha:/tmp/test$ RUST_BACKTRACE=1 cargo test --verbose
   Compiling test v0.1.0 (file:///tmp/test)
     Running `rustc src/lib.rs --crate-name test --crate-type lib -g --test -C metadata=3649b7b644aa8a1f -C extra-filename=-3649b7b644aa8a1f --out-dir /tmp/test/target/debug --emit=dep-info,link -L dependency=/tmp/test/target/debug -L dependency=/tmp/test/target/debug/deps`
     Running `rustc src/lib.rs --crate-name test --crate-type lib -g --out-dir /tmp/test/target/debug --emit=dep-info,link -L dependency=/tmp/test/target/debug -L dependency=/tmp/test/target/debug/deps`
src/lib.rs:1:1: 3:2 warning: function is never used: `testfn`, #[warn(dead_code)] on by default
src/lib.rs:1 fn testfn(s: &[str]) -> u32 {
src/lib.rs:2     return 1;
src/lib.rs:3 }
src/lib.rs:1:11: 1:12 warning: unused variable: `s`, #[warn(unused_variables)] on by default
src/lib.rs:1 fn testfn(s: &[str]) -> u32 {
                       ^
src/lib.rs:1:1: 3:2 warning: function is never used: `testfn`, #[warn(dead_code)] on by default
src/lib.rs:1 fn testfn(s: &[str]) -> u32 {
src/lib.rs:2     return 1;
src/lib.rs:3 }
src/lib.rs:1:11: 1:12 warning: unused variable: `s`, #[warn(unused_variables)] on by default
src/lib.rs:1 fn testfn(s: &[str]) -> u32 {
                       ^
error: internal compiler error: error: internal compiler error: unexpected panicunexpected 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: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'assertion failed: slice_layout_is_correct(cx, &member_llvm_types[..], element_type)', /home/rustbuild/src/rust-buildbot/slave/stable-dist-rustc-linux/build/src/librustc_trans/trans/debuginfo.rs:2810

stack backtrace:
   1:     0x7f865b62f449 - sys::backtrace::write::hbc46dc0cfb3b9537d4r
   2:     0x7f865b637156 - panicking::on_panic::h74d3c14d86c58ac8jrw
   3:     0x7f865b5fa462 - rt::unwind::begin_unwind_inner::h382cea404b11eb00t6v
   4:     0x7f865acf137e - rt::unwind::begin_unwind::h11527883117899559464
   5:     0x7f865adf2f33 - trans::debuginfo::vec_slice_metadata::hf94e0b5f0c2f2a50cgy
   6:     0x7f865adde7fc - trans::debuginfo::type_metadata::h61fa8cb587a59ac0Ioy
   7:     0x7f865adf34c0 - trans::debuginfo::subroutine_type_metadata::h0a8adba26e117853iky
   8:     0x7f865adde1d4 - trans::debuginfo::type_metadata::h61fa8cb587a59ac0Ioy
   9:     0x7f865ad4a8b6 - trans::debuginfo::create_function_debug_context::hce33e3e89c0b549eG0w
  10:     0x7f865ad48d16 - trans::base::new_fn_ctxt::hc47263521fb5ebc7Djh
  11:     0x7f865ad51f10 - trans::base::trans_closure::h8e5687f4ccb96c5eLCh
  12:     0x7f865ad560ea - trans::base::trans_fn::h0bc8fb6dbe17d6adtNh
  13:     0x7f865ad59177 - trans::base::trans_item::h4f9c99b1e4474396Fbi
  14:     0x7f865ad6756d - trans::base::trans_crate::haa02506df24d5efcF0i
  15:     0x7f865bb7d0fa - driver::phase_4_translate_to_llvm::h86d6fb84c5c936d5hOa
  16:     0x7f865bb54faa - driver::compile_input::hb78754f2f33c01efQba
  17:     0x7f865bc164d1 - run_compiler::h258d36d5501c1cdfz4b
  18:     0x7f865bc14122 - boxed::F.FnBox<A>::call_box::h7239693171334256553
  19:     0x7f865bc13659 - rt::unwind::try::try_fn::h14329119008520845439
  20:     0x7f865b6a9ac8 - rust_try_inner
  21:     0x7f865b6a9ab5 - rust_try
  22:     0x7f865bc13908 - boxed::F.FnBox<A>::call_box::h17332056298259451807
  23:     0x7f865b636041 - sys::thread::create::thread_start::h490278b5c3c0b49faqv
  24:     0x7f8655eb8181 - start_thread
  25:     0x7f865b28047c - __clone
  26:                0x0 - <unknown>


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: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'assertion failed: slice_layout_is_correct(cx, &member_llvm_types[..], element_type)', /home/rustbuild/src/rust-buildbot/slave/stable-dist-rustc-linux/build/src/librustc_trans/trans/debuginfo.rs:2810

stack backtrace:
   1:     0x7fc298dfd449 - sys::backtrace::write::hbc46dc0cfb3b9537d4r
   2:     0x7fc298e05156 - panicking::on_panic::h74d3c14d86c58ac8jrw
   3:     0x7fc298dc8462 - rt::unwind::begin_unwind_inner::h382cea404b11eb00t6v
   4:     0x7fc2984bf37e - rt::unwind::begin_unwind::h11527883117899559464
   5:     0x7fc2985c0f33 - trans::debuginfo::vec_slice_metadata::hf94e0b5f0c2f2a50cgy
   6:     0x7fc2985ac7fc - trans::debuginfo::type_metadata::h61fa8cb587a59ac0Ioy
   7:     0x7fc2985c14c0 - trans::debuginfo::subroutine_type_metadata::h0a8adba26e117853iky
   8:     0x7fc2985ac1d4 - trans::debuginfo::type_metadata::h61fa8cb587a59ac0Ioy
   9:     0x7fc2985188b6 - trans::debuginfo::create_function_debug_context::hce33e3e89c0b549eG0w
  10:     0x7fc298516d16 - trans::base::new_fn_ctxt::hc47263521fb5ebc7Djh
  11:     0x7fc29851ff10 - trans::base::trans_closure::h8e5687f4ccb96c5eLCh
  12:     0x7fc2985240ea - trans::base::trans_fn::h0bc8fb6dbe17d6adtNh
  13:     0x7fc298527177 - trans::base::trans_item::h4f9c99b1e4474396Fbi
  14:     0x7fc29853556d - trans::base::trans_crate::haa02506df24d5efcF0i
  15:     0x7fc29934b0fa - driver::phase_4_translate_to_llvm::h86d6fb84c5c936d5hOa
  16:     0x7fc299322faa - driver::compile_input::hb78754f2f33c01efQba
  17:     0x7fc2993e44d1 - run_compiler::h258d36d5501c1cdfz4b
  18:     0x7fc2993e2122 - boxed::F.FnBox<A>::call_box::h7239693171334256553
  19:     0x7fc2993e1659 - rt::unwind::try::try_fn::h14329119008520845439
  20:     0x7fc298e77ac8 - rust_try_inner
  21:     0x7fc298e77ab5 - rust_try
  22:     0x7fc2993e1908 - boxed::F.FnBox<A>::call_box::h17332056298259451807
  23:     0x7fc298e04041 - sys::thread::create::thread_start::h490278b5c3c0b49faqv
  24:     0x7fc293686181 - start_thread
  25:     0x7fc298a4e47c - __clone
  26:                0x0 - <unknown>

Build failed, waiting for other jobs to finish...
Could not compile `test`.

Caused by:
  Process didn't exit successfully: `rustc src/lib.rs --crate-name test --crate-type lib -g --out-dir /tmp/test/target/debug --emit=dep-info,link -L dependency=/tmp/test/target/debug -L dependency=/tmp/test/target/debug/deps` (exit code: 101)
coventry@uppekha:/tmp/test$ 

Changing the signature of s to &[&str] results in compilation without error.

[Stebalien]

dup of #24707

[arielb1]

cc #25470

[coventry]

Closing as a dupe, then.