Panic in nightly rustc when compiling code that tries to borrow *v as mutable behind & reference

[KerfuffleV2]

Code

use std::collections::HashMap;
use async_std::{fs, io, prelude::*};

type Handle = io::BufWriter<fs::File>;
struct Thing(HashMap<String, Handle>);

impl Thing {
    pub async fn die_horribly(&mut self) {
        for v in self.0.values() {
            v.flush().await;
        }
    }
}

fn main() {
    println!("Hello, world!");
}

Dependencies from Cargo.toml

[dependencies]
async-std = "1.9.0"

Meta

rustc --version --verbose:

rustc 1.52.0-nightly (e9920ef77 2021-02-11)
binary: rustc
commit-hash: e9920ef7749d11fc71cc32ca4ba055bcfeaab945
commit-date: 2021-02-11
host: x86_64-unknown-linux-gnu
release: 1.52.0-nightly
LLVM version: 11.0.1

The internal compiler error does not occur in stable (rustc 1.50.0 (cb75ad5db 2021-02-10)):

% cargo build
   Compiling rusterr v0.1.0 (/home/user/devel/rusterr)
error[E0596]: cannot borrow `*v` as mutable, as it is behind a `&` reference
  --> src/main.rs:10:13
   |
9  |         for v in self.0.values() {
   |                  --------------- this iterator yields `&` references
10 |             v.flush().await;
   |             ^ `v` is a `&` reference, so the data it refers to cannot be borrowed as mutable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.
error: could not compile `rusterr`

The fix for compilation is to use values_mut() instead of values() and this also prevents the panic in the nightly version of rustc.

Error output

% cargo build
   Compiling rusterr v0.1.0 (/home/user/devel/rusterr)
error: internal compiler error: compiler/rustc_middle/src/hir/map/mod.rs:306:18: impossible case reached

thread 'rustc' panicked at 'Box<Any>', /rustc/e9920ef7749d11fc71cc32ca4ba055bcfeaab945/library/std/src/panic.rs:59:5
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 1.52.0-nightly (e9920ef77 2021-02-11) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
#0 [mir_borrowck] borrow-checking `Thing::die_horribly::{closure#0}`
#1 [mir_borrowck] borrow-checking `Thing::die_horribly`
end of query stack
error: aborting due to previous error

error: could not compile `rusterr`

Backtrace

% RUST_BACKTRACE=1 cargo build
   Compiling rusterr v0.1.0 (/home/user/devel/rusterr)
error: internal compiler error: compiler/rustc_middle/src/hir/map/mod.rs:306:18: impossible case reached

thread 'rustc' panicked at 'Box<Any>', /rustc/e9920ef7749d11fc71cc32ca4ba055bcfeaab945/library/std/src/panic.rs:59:5
stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::bug
   3: rustc_errors::Handler::bug
   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_middle::hir::map::Map::item
   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: 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
  17: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  18: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  19: rustc_data_structures::stack::ensure_sufficient_stack
  20: rustc_query_system::query::plumbing::force_query_with_job
  21: rustc_query_system::query::plumbing::get_query_impl
  22: rustc_mir::borrow_check::type_check::TypeChecker::check_rvalue
  23: rustc_mir::borrow_check::type_check::TypeChecker::typeck_mir
  24: rustc_mir::borrow_check::type_check::type_check
  25: rustc_mir::borrow_check::nll::compute_regions
  26: rustc_mir::borrow_check::do_mir_borrowck
  27: rustc_infer::infer::InferCtxtBuilder::enter
  28: rustc_mir::borrow_check::mir_borrowck
  29: 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
  30: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  31: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  32: rustc_data_structures::stack::ensure_sufficient_stack
  33: rustc_query_system::query::plumbing::force_query_with_job
  34: rustc_query_system::query::plumbing::get_query_impl
  35: rustc_typeck::collect::type_of::type_of
  36: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::type_of>::compute
  37: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  38: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  39: rustc_data_structures::stack::ensure_sufficient_stack
  40: rustc_query_system::query::plumbing::force_query_with_job
  41: rustc_query_system::query::plumbing::get_query_impl
  42: rustc_typeck::check::check::check_item_type
  43: rustc_middle::hir::map::Map::visit_item_likes_in_module
  44: rustc_typeck::check::check::check_mod_item_types
  45: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::check_mod_item_types>::compute
  46: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  47: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  48: rustc_data_structures::stack::ensure_sufficient_stack
  49: rustc_query_system::query::plumbing::force_query_with_job
  50: rustc_query_system::query::plumbing::get_query_impl
  51: rustc_query_system::query::plumbing::ensure_query_impl
  52: rustc_session::utils::<impl rustc_session::session::Session>::time
  53: rustc_typeck::check_crate
  54: rustc_interface::passes::analysis
  55: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  56: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  57: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  58: rustc_data_structures::stack::ensure_sufficient_stack
  59: rustc_query_system::query::plumbing::force_query_with_job
  60: rustc_query_system::query::plumbing::get_query_impl
  61: rustc_interface::passes::QueryContext::enter
  62: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  63: rustc_span::with_source_map
  64: rustc_interface::interface::create_compiler_and_run
  65: scoped_tls::ScopedKey<T>::set
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 1.52.0-nightly (e9920ef77 2021-02-11) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
#0 [mir_borrowck] borrow-checking `Thing::die_horribly::{closure#0}`
#1 [mir_borrowck] borrow-checking `Thing::die_horribly`
#2 [type_of] computing type of `Thing::die_horribly::{opaque#0}`
#3 [check_mod_item_types] checking item types in top-level module
#4 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

error: could not compile `rusterr`

[jyn514]

It would be nice to reproduce this without the async-std dependency.

[jyn514]

Slightly smaller:

use async_std::{fs, io, prelude::*};

type Handle = io::BufWriter<fs::File>;

async fn die_horribly(thing: &[Handle]) {
    for v in thing {
        v.flush().await;
    }
}

fn main() {}

[jyn514]

I bisected far enough to find it regressed between the 9th and 10th, then ran out of disk space 🤦

installing nightly-2021-02-10
testing...
RESULT: nightly-2021-02-10, ===> Yes

installing nightly-2021-02-08
installing nightly-2021-02-09
searched toolchains nightly-2020-10-01 through nightly-2021-02-10
installing nightly-2021-02-10
uninstalling nightly-2021-02-10
error: The regression was not found. Expanding the bounds may help.
fetching https://static.rust-lang.org/dist/2021-02-09/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2021-02-09: 40 B / 40 B [=============================================================================================================] 100.00 % 319.37 KB/s converted 2021-02-09 to 0fc6756b42e0556cc2e18079f5fc6b4d58f4e81a
fetching https://static.rust-lang.org/dist/2021-02-10/channel-rust-nightly-git-commit-hash.txt
nightly manifest 2021-02-10: 40 B / 40 B [=============================================================================================================] 100.00 % 313.29 KB/s converted 2021-02-10 to 097bc6a84f2280a889b9ab4b544f27851a978927
looking for regression commit between 2021-02-09 and 2021-02-10
cloning rust repository
ERROR: failed to make directory './rust.git': No space left on device; class=Os (2)

[Stupremee]

pub struct Handle {}
impl Handle {
    async fn flush(&mut self) {}
}

async fn die_horribly(thing: &[Handle]) {
    for v in thing {
        v.flush().await;
    }
}

Smaller example without async_std

[KerfuffleV2]

It would be nice to reproduce this without the async-std dependency.

Here you go.

The async part doesn't seem important and the issue still occurs with the std versions of File and BufWriter and async-std removed from dependencies:

use std::{fs, io::*};
use std::collections::HashMap;

type Handle = BufWriter<fs::File>;
struct Thing(HashMap<String, Handle>);

impl Thing {
    pub fn die_horribly(&mut self) {
        for v in self.0.values() {
            v.flush();
        }
    }
}

fn main() {
}

It could probably be reduced in size further but I'm not much of a Rust expert. I suspect anything which triggers that particular compiler error would do the trick.

[jyn514]

searched nightlies: from nightly-2021-02-10 to nightly-2021-02-11
regressed nightly: nightly-2021-02-11
searched commits: from 097bc6a to 07194ff
regressed commit: f7534b5

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --preserve --regress ice --start 2021-02-10 --end 2021-02-11