ICE: Unexpected panic in libcollections/vec.rs

[Robbepop]

I was editing the following code when the bug appeared:

fn transform_bvult(&mut self, mut ult: Lt) -> Expr {
	self.transform_assign(&mut ult.left);
	self.transform_assign(&mut ult.right);

	// Symbolic contradiction: `(< x x)` to `false`
	if ult.left == ult.right {
		return Expr::boolconst(false)
	}

	match (ult.left, ult.right) {

		// - `(< x (-y))` to `(< y x)`
		(left, box Expr::Neg(right)) => {
			Expr::bvult(left.ty(), right.inner, left)
		}

		// - `(< (-x) y)` to `(< y x)`
		(box Expr::Neg(left), right) => {
			Expr::bvult(left.ty(), right, left.inner)

		}

		// - `(< (-x) (-y))` to `(< x y)`
		(box Expr::Neg(left), box Expr::Neg(right)) => {
			Expr::bvult(left.ty(), left.inner, right.inner)
		}

		// Constant evaluation
		// (Expr::BitVecConst(left), Expr::BitVecConst(right)) => {
			// TODO: `x < y where x < y and x,y consteval => true`
			// TODO: `x < y where not(x < y) and x,y consteval => false`
		// }

		// Do nothing since no simplification pattern matches
		_ => {
			ult.into_expr()
		}
	}
}

Apparently swapping

// - `(< (-x) (-y))` to `(< x y)`
(box Expr::Neg(left), box Expr::Neg(right)) => {
	Expr::bvult(left.ty(), left.inner, right.inner)
}

To the front of the match expression results in a sane compile error which is equally unfortunate to me since nothing moved in theory when the default case triggers.

error[E0382]: use of partially moved value: `ult`
   --> src/ast/simplifier.rs:413:5
    |
387 | 		match (ult.left, ult.right) {
    | 		       -------- value moved here
...
413 | 				ult.into_expr()
    | 		  ^^^ value used here after move
    |
    = note: move occurs because `ult.left` has type `std::boxed::Box<ast::variants::Expr>`, which does not implement the `Copy` trait

error: aborting due to previous error(s)

Output with RUST_BACKTRACE=1

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: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', /checkout/src/libcollections/vec.rs:1561
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:60
             at /checkout/src/libstd/panicking.rs:355
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:365
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:549
   5: std::panicking::begin_panic
             at /checkout/src/libstd/panicking.rs:511
   6: std::panicking::begin_panic_fmt
             at /checkout/src/libstd/panicking.rs:495
   7: rust_begin_unwind
             at /checkout/src/libstd/panicking.rs:471
   8: core::panicking::panic_fmt
             at /checkout/src/libcore/panicking.rs:69
   9: core::panicking::panic_bounds_check
             at /checkout/src/libcore/panicking.rs:56
  10: rustc_const_eval::_match::specialize
  11: rustc_const_eval::_match::is_useful_specialized
  12: rustc_const_eval::_match::is_useful::{{closure}}
  13: <core::option::Option<T>>::map
  14: rustc_const_eval::_match::is_useful
  15: rustc_const_eval::_match::is_useful_specialized
  16: rustc_const_eval::_match::is_useful::{{closure}}
  17: <core::option::Option<T>>::map
  18: rustc_const_eval::_match::is_useful
  19: rustc_const_eval::_match::is_useful_specialized
  20: rustc_const_eval::_match::is_useful::{{closure}}
  21: <core::option::Option<T>>::map
  22: rustc_const_eval::_match::is_useful
  23: <rustc_const_eval::check_match::MatchVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_expr
  24: <rustc_const_eval::check_match::MatchVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_expr
  25: <rustc_const_eval::check_match::OuterVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_fn
  26: rustc::hir::intravisit::walk_impl_item
  27: rustc_const_eval::check_match::check_crate
  28: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}
  29: rustc_driver::driver::phase_3_run_analysis_passes
  30: rustc_driver::driver::compile_input
  31: rustc_driver::run_compiler

Output with RUST_BACKTRACE=full

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: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'index out of bounds: the len is 0 but the index is 0', /checkout/src/libcollections/vec.rs:1561
stack backtrace:
   0:     0x7fdc4be1cee3 - std::sys::imp::backtrace::tracing::imp::unwind_backtrace::hf6993ec76a3b39d8
                               at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1:     0x7fdc4be18504 - std::sys_common::backtrace::_print::h3d630ba2218f7ec3
                               at /checkout/src/libstd/sys_common/backtrace.rs:71
   2:     0x7fdc4be2a82a - std::panicking::default_hook::{{closure}}::hd0df6de62c549847
                               at /checkout/src/libstd/sys_common/backtrace.rs:60
                               at /checkout/src/libstd/panicking.rs:355
   3:     0x7fdc4be2a27d - std::panicking::default_hook::h9bbcc81c50b40ea3
                               at /checkout/src/libstd/panicking.rs:365
   4:     0x7fdc4be2ac3b - std::panicking::rust_panic_with_hook::hcb9f41173fe183d3
                               at /checkout/src/libstd/panicking.rs:549
   5:     0x7fdc4be2aac4 - std::panicking::begin_panic::h6af6db04028173d3
                               at /checkout/src/libstd/panicking.rs:511
   6:     0x7fdc4be2aa49 - std::panicking::begin_panic_fmt::h2ea6b56a88fe4f9f
                               at /checkout/src/libstd/panicking.rs:495
   7:     0x7fdc4be2a9d7 - rust_begin_unwind
                               at /checkout/src/libstd/panicking.rs:471
   8:     0x7fdc4be675dd - core::panicking::panic_fmt::h883a028e9f4b4457
                               at /checkout/src/libcore/panicking.rs:69
   9:     0x7fdc4be67588 - core::panicking::panic_bounds_check::hc3c2775988732c0b
                               at /checkout/src/libcore/panicking.rs:56
  10:     0x7fdc4a2b34ad - rustc_const_eval::_match::specialize::hadc7768d150b8c58
  11:     0x7fdc4a2b0be3 - rustc_const_eval::_match::is_useful_specialized::hdb71f059cfe9dea5
  12:     0x7fdc4a2b03fe - rustc_const_eval::_match::is_useful::{{closure}}::ha30ce4b8498e1202
  13:     0x7fdc4a28e88f - <core::option::Option<T>>::map::ha82a0cc47260c3a2
  14:     0x7fdc4a2adb7a - rustc_const_eval::_match::is_useful::h9052c30b8ec0f26e
  15:     0x7fdc4a2b0c34 - rustc_const_eval::_match::is_useful_specialized::hdb71f059cfe9dea5
  16:     0x7fdc4a2b03fe - rustc_const_eval::_match::is_useful::{{closure}}::ha30ce4b8498e1202
  17:     0x7fdc4a28e88f - <core::option::Option<T>>::map::ha82a0cc47260c3a2
  18:     0x7fdc4a2adb7a - rustc_const_eval::_match::is_useful::h9052c30b8ec0f26e
  19:     0x7fdc4a2b0c34 - rustc_const_eval::_match::is_useful_specialized::hdb71f059cfe9dea5
  20:     0x7fdc4a2b03fe - rustc_const_eval::_match::is_useful::{{closure}}::ha30ce4b8498e1202
  21:     0x7fdc4a28e88f - <core::option::Option<T>>::map::ha82a0cc47260c3a2
  22:     0x7fdc4a2adb7a - rustc_const_eval::_match::is_useful::h9052c30b8ec0f26e
  23:     0x7fdc4a2b530d - <rustc_const_eval::check_match::MatchVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_expr::h6e23fe3a012213fd
  24:     0x7fdc4a2b496b - <rustc_const_eval::check_match::MatchVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_expr::h6e23fe3a012213fd
  25:     0x7fdc4a2b3ac6 - <rustc_const_eval::check_match::OuterVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_fn::h6c18c5eb9d1d2370
  26:     0x7fdc4a294ec7 - rustc::hir::intravisit::walk_impl_item::h8d26ab20edc8822c
  27:     0x7fdc4a2b41f7 - rustc_const_eval::check_match::check_crate::h53dfb4d18dc8c3c9
  28:     0x7fdc4c1b96f8 - rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}::h68eccaf0b2f82b34
  29:     0x7fdc4c1ab2ef - rustc_driver::driver::phase_3_run_analysis_passes::hbb243e50bc4b822e
  30:     0x7fdc4c18d610 - rustc_driver::driver::compile_input::h6125b69645e50cce
  31:     0x7fdc4c1d8466 - rustc_driver::run_compiler::hc6d7ae0444eeb671
  32:     0x7fdc4c0fb9ab - std::sys_common::backtrace::__rust_begin_short_backtrace::hf40d79e5a79af0b6
  33:     0x7fdc4be33e8a - __rust_maybe_catch_panic
                               at /checkout/src/libpanic_unwind/lib.rs:98
  34:     0x7fdc4c1209a0 - <F as alloc::boxed::FnBox<A>>::call_box::hd023aca2401e451f
  35:     0x7fdc4be29455 - std::sys::imp::thread::Thread::new::thread_start::h9e7dc9c59c04ba25
                               at /checkout/src/liballoc/boxed.rs:658
                               at /checkout/src/libstd/sys_common/thread.rs:21
                               at /checkout/src/libstd/sys/unix/thread.rs:84
  36:     0x7fdc47355296 - start_thread
  37:     0x7fdc4bae325e - __clone
  38:                0x0 - <unknown>

[tirr-c]

Minimal testcase that I found: https://play.rust-lang.org/?gist=e5f4ef65f86afc84b1eca43ac545c3ec&version=nightly&backtrace=1

---

Matching like this is fine:

match (a, b) {
    (box Test::Foo(x), box Test::Bar(y)) => println!("Foo {}, Bar {}", x, y),
    _ => println!("Some other things"),
}

This is also fine:

match (a, b) {
    (a, box Test::Bar(y)) => println!("Some other {:?}, Bar {}", a, y),
    (box a, b) => println!("Some unboxed {:?}, some boxed {:?}", a, b),
}