[ICE] With closure on mutable ref

[leonardo-m]

fn foo(x: &mut u32) {
    let bar = || { foo(x); };
    bar();
}
fn main() {}

Compiled with rustc bug2.rs gives:

error: internal compiler error: compiler\rustc_mir\src\borrow_check\diagnostics\mutability_errors.rs:525:22: upvar `x` borrowed, but not mutably

thread 'rustc' panicked at 'Box<Any>', compiler\rustc_errors\src\lib.rs:958:9
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
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::bug_fmt
   7: rustc_mir::borrow_check::diagnostics::mutability_errors::<impl rustc_mir::borrow_check::MirBorrowckCtxt>::show_mutating_upvar
   8: rustc_mir::borrow_check::diagnostics::mutability_errors::<impl rustc_mir::borrow_check::MirBorrowckCtxt>::report_mutability_error
   9: rustc_mir::borrow_check::MirBorrowckCtxt::access_place
  10: <rustc_mir::borrow_check::MirBorrowckCtxt as rustc_mir::dataflow::framework::visitor::ResultsVisitor>::visit_statement_before_primary_effect
  11: <rustc_mir::dataflow::framework::direction::Forward as rustc_mir::dataflow::framework::direction::Direction>::visit_results_in_block
  12: rustc_mir::dataflow::framework::visitor::visit_results
  13: rustc_mir::borrow_check::do_mir_borrowck
  14: rustc_infer::infer::InferCtxtBuilder::enter
  15: rustc_mir::borrow_check::mir_borrowck
  16: core::ops::function::FnOnce::call_once
  17: 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
  18: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  19: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}::{{closure}}
  20: rustc_query_system::query::plumbing::force_query_with_job
  21: rustc_query_system::query::plumbing::get_query_impl
  22: rustc_query_system::query::plumbing::ensure_query_impl
  23: rustc_session::utils::<impl rustc_session::session::Session>::time
  24: rustc_interface::passes::analysis
  25: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  26: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  27: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}::{{closure}}
  28: rustc_query_system::query::plumbing::force_query_with_job
  29: rustc_query_system::query::plumbing::get_query_impl
  30: rustc_interface::passes::QueryContext::enter
  31: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  32: rustc_span::with_source_map
  33: rustc_interface::interface::create_compiler_and_run
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose 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 1.51.0-nightly (d4e3570db 2021-02-01) running on x86_64-pc-windows-gnu

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

[matthiaskrgr]

This was caused by 74500b9
@rustbot prioritize

edit:
in beta, this gives the following error:

error[E0596]: cannot borrow `bar` as mutable, as it is not declared as mutable
 --> src/main.rs:3:5
  |
2 |     let bar = || { foo(x); };
  |         --- help: consider changing this to be mutable: `mut bar`
3 |     bar();
  |     ^^^ cannot borrow as mutable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.

[apiraino]

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

[camelid]

Looks likely that #81158 is the cause. cc @Aaron1011

[camelid]

Re-prioritizing as P-high and removing I-prioritize as discussed in the prioritization working group.

@rustbot label: +regression-from-stable-to-nightly +P-high -P-medium -I-prioritize