Compiler assertion failed: tcx.migrate_borrowck() when failing to move #[thread_local] static vars.

[MaulingMonkey]

Hello! I have a compiler assertion on rustc 1.31.0-nightly (2bd5993 2018-10-02) when combining #[thread_local] and bad code that tries to move from the thread local global. This also occurred on an earlier nightly I tried upgrading from (months old? I didn't think to save that compiler version, sorry!)

#![feature(thread_local)]

pub struct CommandWriter {}

#[thread_local] static mut TL_COMMAND_BUFFER_WRITER : Option<Box<CommandWriter>> = Option::None;
static mut G_COMMAND_BUFFER_WRITER : Option<Box<CommandWriter>> = Option::None;

impl CommandWriter {
    pub fn with_this_thread<F: FnOnce(&CommandWriter)> (f: F) {
        unsafe {
            f(&TL_COMMAND_BUFFER_WRITER.unwrap()); // [1] Compiler panic: thread 'main' panicked at 'assertion failed: tcx.migrate_borrowck()', librustc_mir\transform\elaborate_drops.rs:52:17
            //f(&G_COMMAND_BUFFER_WRITER.unwrap()); // [2] Proper compiler error:  "cannot move out of static item"
            //f(&TL_COMMAND_BUFFER_WRITER.as_ref().unwrap()); // Builds fine
        }
    }
}

(Playground)

Compiler output with the code as posted and RUST_BACKTRACE=1:

warning: static item is never used: `G_COMMAND_BUFFER_WRITER`
 --> src\lib.rs:6:1
  |
6 | static mut G_COMMAND_BUFFER_WRITER : Option<Box<CommandWriter>> = Option::None;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

thread 'main' panicked at 'assertion failed: tcx.migrate_borrowck()', librustc_mir\transform\elaborate_drops.rs:52:17
stack backtrace:
   0: <std::sync::mpsc::RecvTimeoutError as core::fmt::Debug>::fmt
   1: <std::path::Iter<'a> as core::convert::AsRef<std::path::Path>>::as_ref
   2: std::panicking::take_hook
   3: std::panicking::take_hook
   4: <rustc::ty::sty::Binder<rustc::ty::ProjectionPredicate<'tcx>> as rustc::ty::ToPredicate<'tcx>>::to_predicate
   5: std::panicking::rust_panic_with_hook
   6: <rustc_mir::borrow_check::nll::constraints::graph::Reverse as rustc_mir::borrow_check::nll::constraints::graph::ConstraintGraphDirecton>::end_region
   7: <rustc_mir::transform::elaborate_drops::ElaborateDrops as rustc_mir::transform::MirPass>::run_pass
   8: <rustc_mir::transform::MirSource as core::fmt::Debug>::fmt
   9: rustc_mir::transform::optimized_mir
  10: <rustc::ty::query::job::QueryInfo<'tcx> as core::fmt::Debug>::fmt
  11: <rustc_target::spec::RelroLevel as rustc::session::config::dep_tracking::DepTrackingHash>::hash
  12: rustc::ty::context::tls::track_diagnostic
  13: rustc::dep_graph::graph::DepGraph::assert_ignored
  14: rustc::ty::context::tls::track_diagnostic
  15: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_print_query_stack
  16: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_print_query_stack
  17: rustc::ty::query::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::optimized_mir
  18: <rustc_metadata::schema::LazyState as core::fmt::Debug>::fmt
  19: <rustc_metadata::schema::LazyState as core::fmt::Debug>::fmt
  20: rustc_metadata::decoder::__ty_decoder_impl::<impl serialize::serialize::Decoder for rustc_metadata::decoder::DecodeContext<'a, 'tcx>>::read_u64
  21: rustc_metadata::index::<impl rustc_metadata::schema::LazySeq<rustc_metadata::index::Index>>::lookup
  22: rustc_metadata::dynamic_lib::dl::symbol
  23: <rustc_metadata::encoder::ImplVisitor<'a, 'tcx> as rustc::hir::itemlikevisit::ItemLikeVisitor<'v>>::visit_item
  24: rustc_metadata::cstore_impl::<impl rustc::middle::cstore::CrateStore for rustc_metadata::cstore::CStore>::encode_metadata
  25: rustc::ty::context::TyCtxt::encode_metadata
  26: rustc_codegen_llvm::base::codegen_instance
  27: rustc_codegen_llvm::type_::<impl rustc_codegen_llvm::llvm::ffi::::Type>::uint_from_ty
  28: <rustc_codegen_llvm::base::ValueIter<'ll> as core::iter::iterator::Iterator>::next
  29: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  30: rustc_driver::target_features::add_configuration
  31: rustc_driver::driver::phase_4_codegen
  32: rustc_driver::profile::trace::write_style
  33: <humantime::duration::Error as std::error::Error>::cause
  34: <rustc_driver::pretty::NoAnn<'hir> as rustc_driver::pretty::HirPrinterSupport<'hir>>::sess
  35: <rustc_driver::CompilationFailure as core::fmt::Debug>::fmt
  36: rustc_driver::driver::compile_input
  37: rustc_driver::run_compiler
  38: <env_logger::Logger as log::Log>::flush
  39: rustc_driver::run_compiler
  40: rustc_driver::target_features::add_configuration
  41: _rust_maybe_catch_panic
  42: rustc_driver::profile::dump
  43: rustc_driver::main
  44: <unknown>
  45: std::panicking::update_panic_count
  46: _rust_maybe_catch_panic
  47: std::rt::lang_start_internal
  48: <unknown>
  49: <unknown>
  50: BaseThreadInitThunk
  51: RtlUserThreadStart
query stack during panic:
#0 [optimized_mir] processing `CommandWriter::with_this_thread`
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 (2bd5993ca 2018-10-02) running on x86_64-pc-windows-msvc

note: compiler flags: -C debuginfo=2 -C incremental --crate-type lib

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

Reasonable/expected errors when the line marked [1] is commented out and line marked [2] is uncommented:

error[E0507]: cannot move out of static item
  --> src\lib.rs:12:16
   |
12 |             f(&G_COMMAND_BUFFER_WRITER.unwrap()); // [2] Proper compiler error:  "cannot move out of static item"
   |                ^^^^^^^^^^^^^^^^^^^^^^^ cannot move out of static item

error: aborting due to previous error

For more information about this error, try `rustc --explain E0507`.

Meta

rustc --version --verbose:
rustc 1.31.0-nightly (2bd5993 2018-10-02)
binary: rustc
commit-hash: 2bd5993
commit-date: 2018-10-02
host: x86_64-pc-windows-msvc
release: 1.31.0-nightly
LLVM version: 8.0

[pnkfelix]

assigning to self. will investigate.

[pnkfelix]

If you turn on the 2018 edition, the ICE goes away (because that makes the migrate_borrowck function start returning true:

https://play.rust-lang.org/?gist=c141f8d0adaa3c4e945bdbfa052e2fee&version=nightly&mode=debug&edition=2018

[pnkfelix]

((interestingly, our fallback to the AST-borrowck here signals no error on the code in question, so the migrate mode downgrades the error to a warning. will investigate...))

---

Update: in case people care, the error that is downgraded to a warning here is the following:

warning[E0507]: cannot move out of static item
  --> src/lib.rs:11:16
   |
11 |             f(&TL_COMMAND_BUFFER_WRITER.unwrap());
   |                ^^^^^^^^^^^^^^^^^^^^^^^^ cannot move out of static item
   |
   = 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]

Curiouser and curiouser: the failing assertion is preceded by an interesting comment:

rust/src/librustc_mir/transform/elaborate_drops.rs

Lines 42 to 53 in 5ea8eb5

	let move_data = match MoveData::gather_moves(mir, tcx) {
	Ok(move_data) => move_data,
	Err((move_data, _move_errors)) => {
	// The only way we should be allowing any move_errors
	// in here is if we are in the migration path for the
	// NLL-based MIR-borrowck.
	//
	// If we are in the migration path, we have already
	// reported these errors as warnings to the user. So
	// we will just ignore them here.
	assert!(tcx.migrate_borrowck());
	move_data

[pnkfelix]

discussed at NLL triage meeting. Not an RC2 blocker. Putting on the Release milestone.

[pnkfelix]

hey @estebank, why is this tagged as a stable-to-nightly regression? This can't be on stable, since its using a #![feature(..)], right?

[estebank]

@pnkfelix you're absolutely right. Missed that. Removing.

[pnkfelix]

Aha!

I was flummoxed by my failures to reproduce the problem locally...

turns out you have to pass --crate-type lib to reproduce. It won't happen if you e.g. add an fn main() { } and make a binary.

[pnkfelix]

Okay, so now I know how to reproduce the problem.

I guess the "right" thing would be to report the errors nicely from elaborate_drops when we are in borrowck=ast mode. I'm not sure whether they should be errors or warnings though.

[pnkfelix]

(Another option might be to plug in a fix to AST-borrowck...)

Update: this is what I went ahead with, see PR #55150

[pnkfelix]

This is actually a dupe of #47215.

We just hit it a bit differently here because instead of unwrapping the result, I started asserting that we must be in borrowck=migrate for this to occur. (Which I now know is not the case.)

[pnkfelix]

closing as duplicate of #47215. (And assigning the latter to myself.)