ICE with nested slice pattern

[eefriedman]

Testcase:

#![feature(slice_patterns)]
fn main() { let x: &[i32] = &[]; let &[[_a, ref _b..]..] = x; }

Gives:

error: internal compiler error: ../src/librustc_mir/build/matches/mod.rs:155: match pairs [MatchPair { lvalue: (*var0), pattern: Pattern { ty: [i32], span: <anon>:2:39: 2:57, kind: Slice { prefix: [], slice: Some(Pattern { ty: [i32], span: <anon>:2:40: 2:54, kind: Slice { prefix: [Pattern { ty: i32, span: <anon>:2:41: 2:43, kind: Binding { mutability: Not, name: _a(61), mode: ByValue, var: 20, ty: i32, subpattern: None } }], slice: Some(Pattern { ty: [i32], span: <anon>:2:45: 2:51, kind: Binding { mutability: Not, name: _b(62), mode: ByRef(ReScope(CodeExtent(14/Remainder(BlockRemainder { block: 6, first_statement_index: 1 }))), Shared), var: 21, ty: &[i32], subpattern: None } }), suffix: [] } }), suffix: [] } }, slice_len_checked: false }] remaining after simplifying irrefutable pattern
 --> <anon>:2:39
  |
2 | fn main() { let x: &[i32] = &[]; let &[[_a, ref _b..]..] = x; }
  |                                       ^^^^^^^^^^^^^^^^^^

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 'Box<Any>', ../src/librustc_errors/lib.rs:580
note: Run with `RUST_BACKTRACE=1` for a backtrace.

CC @arielb1

[eefriedman]

Actually, taking another look, closing this in favor of the original bug; it looks like the original issue still exists. The ICE is just a symptom.