ICE: compiler/rustc_middle/src/mir/mod.rs:2306:49: could not find allocation for alloc1

[chengniansun]

Code

const FOO: *const u32 = {
    let x;
    &x
};
fn main() {
    let FOO = FOO;
}

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (4f7612ac1 2020-10-31)
binary: rustc
commit-hash: 4f7612ac1499258025077f1fd05d2f429f9accfb
commit-date: 2020-10-31
host: x86_64-unknown-linux-gnu
release: 1.49.0-nightly

Error output

error[E0381]: borrow of possibly-uninitialized variable: `x`
 --> reduced_mutant.rs:3:5
  |
3 |     &x
  |     ^^ use of possibly-uninitialized `x`

error: encountered dangling pointer in final constant
 --> reduced_mutant.rs:1:1
  |
1 | / const FOO: *const u32 = {
2 | |     let x;
3 | |     &x
4 | | };
  | |__^

error[E0005]: refutable pattern in local binding: `_` not covered
 --> reduced_mutant.rs:6:9
  |
1 | / const FOO: *const u32 = {
2 | |     let x;
3 | |     &x
4 | | };
  | |__- constant defined here
5 |   fn main() {
6 |       let FOO = FOO;
  |           ^^^
  |           |
  |           interpreted as a constant pattern, not a new variable
  |           help: introduce a variable instead: `foo_var`
  |
  = note: the matched value is of type `*const u32`

error: internal compiler error: compiler/rustc_middle/src/mir/mod.rs:2306:49: could not find allocation for alloc1

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:942:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.49.0-nightly (4f7612ac1 2020-10-31) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [mir_borrowck] borrow-checking `main`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 4 previous errors

Some errors have detailed explanations: E0005, E0381.
For more information about an error, try `rustc --explain E0005`.

Backtrace

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:942:9
stack backtrace:
   0: std::panicking::begin_panic
   1: rustc_errors::HandlerInner::bug
   2: rustc_errors::Handler::bug
   3: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
   4: rustc_middle::ty::context::tls::with_opt::{{closure}}
   5: rustc_middle::ty::context::tls::with_opt
   6: rustc_middle::util::bug::opt_span_bug_fmt
   7: rustc_middle::util::bug::bug_fmt
   8: rustc_middle::mir::Constant::check_static_ptr
   9: <rustc_mir::borrow_check::type_check::TypeVerifier as rustc_middle::mir::visit::Visitor>::visit_constant
  10: rustc_middle::mir::visit::Visitor::super_rvalue
  11: <rustc_mir::borrow_check::type_check::TypeVerifier as rustc_middle::mir::visit::Visitor>::visit_body
  12: rustc_mir::borrow_check::type_check::type_check
  13: rustc_mir::borrow_check::nll::compute_regions
  14: rustc_mir::borrow_check::do_mir_borrowck
  15: rustc_infer::infer::InferCtxtBuilder::enter
  16: rustc_mir::borrow_check::mir_borrowck
  17: core::ops::function::FnOnce::call_once
  18: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_borrowck>::compute
  19: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  20: rustc_data_structures::stack::ensure_sufficient_stack
  21: rustc_query_system::query::plumbing::get_query_impl
  22: rustc_query_system::query::plumbing::ensure_query_impl
  23: rustc_interface::passes::analysis
  24: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  25: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  26: rustc_data_structures::stack::ensure_sufficient_stack
  27: rustc_query_system::query::plumbing::get_query_impl
  28: rustc_interface::passes::QueryContext::enter
  29: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  30: rustc_span::with_source_map
  31: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

NOTE: The bug is found by our work-in-progress compiler testing tool Kira, and the test program is reduced/minimized by Perses

[JohnTitor]

This is ICE since 1.45.0, I think #71508 is the cause. This @RalfJung's comment seems related: #71508 (comment)

[RalfJung]

The intention was that "error: encountered dangling pointer in final constant" would stop compilation... but as usual when using a constant as a pattern like this code does, compilation doesn't actually stop, so bogus things end up hitting other parts of the compiler. :/ FWIW, this regression can only possible affect code that should not compile (but it should error cleanly, not ICE).

Cc @oli-obk

[oli-obk]

This specific case could become delay_span_bug by invoking get_global_alloc instead of global_alloc. But that's just fixing symptoms as noted by @RalfJung

I think the correct fix would be to make sure that intern_const_alloc_recursive bubbles up an ErrorReported to make sure we don't get that constant but either just get an error or potentially a ConstKind::Error

[RalfJung]

Found the issue for the underlying problem (or at least, something closely related): #76064

[JohnTitor]

Assigning P-medium as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.