ICE 'allocation missing in dead_alloc_map' in rustc_mir::interpret when compiling Servo

[SimonSapin]

thread 'main' panicked at 'allocation missing in dead_alloc_map', libcore/option.rs:1008:5
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:480
   6: std::panicking::continue_panic_fmt
             at libstd/panicking.rs:390
   7: rust_begin_unwind
             at libstd/panicking.rs:325
   8: core::panicking::panic_fmt
             at libcore/panicking.rs:77
   9: core::option::expect_failed
             at libcore/option.rs:1008
  10: <rustc_mir::interpret::memory::Memory<'a, 'mir, 'tcx, M>>::check_align
  11: rustc_mir::interpret::validity::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::validate_primitive_type
  12: rustc_mir::interpret::validity::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::validate_operand
  13: rustc_mir::interpret::validity::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::validate_operand
  14: rustc_mir::interpret::validity::<impl rustc_mir::interpret::eval_context::EvalContext<'a, 'mir, 'tcx, M>>::validate_operand
  15: rustc_lint::builtin::check_const
  16: <rustc_lint::register_builtins::BuiltinCombinedLateLintPass as rustc::lint::LateLintPass<'a, 'tcx>>::check_item
  17: <rustc::lint::context::LateContext<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  18: <rustc::lint::context::LateContext<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_mod
  19: rustc::hir::intravisit::walk_item
  20: <rustc::lint::context::LateContext<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  21: <rustc::lint::context::LateContext<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_mod
  22: rustc::hir::intravisit::walk_item
  23: <rustc::lint::context::LateContext<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  24: <rustc::lint::context::LateContext<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_mod
  25: rustc::hir::intravisit::walk_item
  26: <rustc::lint::context::LateContext<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  27: <rustc::lint::context::LateContext<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_mod
  28: rustc::hir::intravisit::walk_item
  29: <rustc::lint::context::LateContext<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  30: <rustc::lint::context::LateContext<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_mod
  31: rustc::hir::intravisit::walk_crate
  32: rustc::lint::context::check_crate
  33: rustc::util::common::time
  34: rustc::ty::context::tls::enter_context
  35: <std::thread::local::LocalKey<T>>::with
  36: rustc::ty::context::TyCtxt::create_and_enter
  37: rustc_driver::driver::compile_input
  38: rustc_driver::run_compiler_with_pool
  39: rustc_driver::driver::spawn_thread_pool
  40: rustc_driver::run_compiler
  41: <scoped_tls::ScopedKey<T>>::set
  42: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  43: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  44: rustc_driver::run
  45: rustc_driver::main
  46: std::rt::lang_start::{{closure}}
  47: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  48: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  49: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  50: main
  51: __libc_start_main
  52: <unknown>
query stack during panic:
end of query stack

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 1.31.0-nightly (78ff609d7 2018-10-19) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C link-args=-fuse-ld=gold --crate-type lib

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

�[0m�[0m�[1m�[31merror:�[0m Could not compile `script`.

[SimonSapin]

Commit range: e7f5d48...78ff609. Merged PRs:

• Custom E0277 diagnostic for Path #54979 Custom E0277 diagnostic for Path
• Check the type of statics and constants for Sizedness #55004 Check the type of statics and constants for Sizedness
• Replace ParamEnv with a new type in chalk context. #55040 Replace ParamEnv with a new type in chalk context
• Rollup of 7 pull requests #55194, which is a rollup of:
  • Updated RELEASES.md for 1.30.0 #54300 (Updated RELEASES.md for 1.30.0)
  • [NLL] Propagate bounds from generators #55013 ([NLL] Propagate bounds from generators)
  • Fix ICE and report a human readable error #55071 (Fix ICE and report a human readable error)
  • Cleanup resolve #55144 (Cleanup resolve)
  • Don't warn about parentheses on match (return) #55166 (Don't warn about parentheses on match (return))
  • Add a copysign function to f32 and f64 #55169 (Add a copysign function to f32 and f64)
  • Stabilize slice::chunks_exact(), chunks_exact_mut(), rchunks(), rchunks_mut(), rchunks_exact(), rchunks_exact_mut() #55178 (Stabilize slice::chunks_exact(), chunks_exact_mut(), rchunks(), rchunks_mut(), rchunks_exact(), rchunks_exact_mut())
• support type annot in constants, casts #55152 support type annot in constants, casts

[oli-obk]

I honestly don't know what your code does, but from the error message and the backtrace I was able to create

#![feature(const_let)]

union Foo<'a> {
    y: &'a (),
    long_live_the_unit: &'static (),
}

const FOO: &() = {
    let y = ();
    unsafe { Foo { y: &y }.long_live_the_unit }
};

fn main() {
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2015&gist=b23ea8c8d50f4a31a52780e27ef55fa5

[oli-obk]

I have a repro now: https://github.com/rust-lang/rust/pull/55262/files#diff-c1369fee5bc12990c0cb1629b00a9893

[oli-obk]

cc @nikomatsakis @pnkfelix I think (haven't bisected yet) that #55152 is the likelyest candidate to have regressed the following (previously successfully compiling code example):

struct Slice(&'static [&'static [u8]]);
static MAP: Slice = Slice(&[
    b"CloseEvent" as &'static [u8],
]);

the following warning will keep the code compiling but cause dangling safe references in downstream code

warning[E0716]: temporary value dropped while borrowed
  --> $DIR/dangling-alloc-id-ice-2.rs:5:28
   |
LL |    static MAP: Slice = Slice(&[
   |   ___________________________-^
   |  |___________________________|
   | ||
LL | ||     b"CloseEvent" as &'static [u8],
LL | || ]);
   | || -- temporary value is freed at the end of this statement
   | ||_|
   | |__creates a temporary which is freed while still in use
   |    cast requires that borrow lasts for `'static`
   |
   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
           It represents potential unsoundness in your code.
           This warning will become a hard error in the future.

[pnkfelix]

"dangling safe references" ? Oh, you must mean "dangling &-references" ? Sorry, wasn't sure if the word "safe" there was meant to mean "this code is fine and should be accepted by the compiler.

[pnkfelix]

Do we not guarantee that an expression like [ b"CloseEvent" as &'static [u8], ] can be promoted to a static value?

[pnkfelix]

in particular, the code seems to work fine if I replace [ b"CloseEvent" as &'static [u8], ] with just [ b"CloseEvent", ]

[pnkfelix]

(We probably should fork off separate issues to track the distinct problems that @oli-obk has uncovered here...)

[nikomatsakis]

cc @eddyb -- this is related to the drop of temporaries in static

[nikomatsakis]

Er, never mind, my bad. The bug here is that promotion is not happening due to the cast, which is something that @oli-obk and I discussed on the PR -- it's a result presumably of needing to "loosen" the promotion rules to permit user type ascriptions.

[oli-obk]

In #55152 we were talking about const propagation, not const promotion ;)

I have no idea why your PR would break promotion in this case. I checked the const promotion code, and we're already treating type ascription as a nop.

[pnkfelix]

closing as duplicate of #55288, which is where the bulk of the remaining work is