ICE: polonius loan scopes differ from NLL borrow scopes

[matthiaskrgr]

auto-reduced (treereduce-rust):

#![feature(inherent_associated_types)]

type Function = for<'a> fn(&'a i32) -> S<'a>::P;

struct S<'a>(&'a ());

impl<'a> S {
    type P = &'a i32;
}

fn ret_ref_local<'e>() -> &'e i32 {
    let f: Function = |x| x;

    let local = 0;
    f(&local)
}

original:

#![feature(inherent_associated_types)]

type Function = for<'a> fn(&'a i32) -> S<'a>::P;

struct S<'a>(&'a ());

impl<'a> S<{ fn bar(&self) {} }> {
    type P = &'a i32;
}

fn ret_ref_local<'e>() -> &'e i32 {
    let f: Function = |x| x;

    let local = 0;
    f(&local)
}

fn main() {}

Version information

rustc 1.80.0-nightly (44701e070 2024-06-04)
binary: rustc
commit-hash: 44701e070c8453df10df1984944258018ac21610
commit-date: 2024-06-04
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.6

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc -Zpolonius=next

Program output

error[E0726]: implicit elided lifetime not allowed here
 --> /tmp/icemaker_global_tempdir.j09zTpiMeB10/rustc_testrunner_tmpdir_reporting.6BA5IqLroDn9/mvce.rs:7:10
  |
7 | impl<'a> S {
  |          ^ expected lifetime parameter
  |
help: indicate the anonymous lifetime
  |
7 | impl<'a> S<'_> {
  |           ++++

warning: the feature `inherent_associated_types` is incomplete and may not be safe to use and/or cause compiler crashes
 --> /tmp/icemaker_global_tempdir.j09zTpiMeB10/rustc_testrunner_tmpdir_reporting.6BA5IqLroDn9/mvce.rs:1:12
  |
1 | #![feature(inherent_associated_types)]
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #8995 <https://github.com/rust-lang/rust/issues/8995> for more information
  = note: `#[warn(incomplete_features)]` on by default

error[E0601]: `main` function not found in crate `mvce`
  --> /tmp/icemaker_global_tempdir.j09zTpiMeB10/rustc_testrunner_tmpdir_reporting.6BA5IqLroDn9/mvce.rs:16:2
   |
16 | }
   |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.j09zTpiMeB10/rustc_testrunner_tmpdir_reporting.6BA5IqLroDn9/mvce.rs`

error: lifetime may not live long enough
  --> /tmp/icemaker_global_tempdir.j09zTpiMeB10/rustc_testrunner_tmpdir_reporting.6BA5IqLroDn9/mvce.rs:12:27
   |
12 |     let f: Function = |x| x;
   |                        -- ^ returning this value requires that `'1` must outlive `'2`
   |                        ||
   |                        |return type of closure is &'2 i32
   |                        has type `&'1 i32`

thread 'rustc' panicked at compiler/rustc_borrowck/src/dataflow.rs:418:13:
assertion `left == right` failed: polonius loan scopes differ from NLL borrow scopes, for body /tmp/icemaker_global_tempdir.j09zTpiMeB10/rustc_testrunner_tmpdir_reporting.6BA5IqLroDn9/mvce.rs:11:1: 16:2 (#0)
  left: {bb0[16]: [bw0]}
 right: {bb2[0]: [bw0], bb1[0]: [bw0]}
stack backtrace:
   0:     0x78e818c02d45 - std::backtrace_rs::backtrace::libunwind::trace::he27724ab9ba077ae
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/../../backtrace/src/backtrace/libunwind.rs:116:5
   1:     0x78e818c02d45 - std::backtrace_rs::backtrace::trace_unsynchronized::h8e764d5b7d96c928
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x78e818c02d45 - std::sys_common::backtrace::_print_fmt::h3b015d1dd0132d35
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x78e818c02d45 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h5d1014338738062e
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x78e818c534eb - core::fmt::rt::Argument::fmt::h60a123cf0371ab3c
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/core/src/fmt/rt.rs:165:63
   5:     0x78e818c534eb - core::fmt::write::h83e73de2f751b956
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/core/src/fmt/mod.rs:1168:21
   6:     0x78e818bf7acf - std::io::Write::write_fmt::hc31dd1d0eb90cbf9
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/io/mod.rs:1835:15
   7:     0x78e818c02b1e - std::sys_common::backtrace::_print::hc36ef09893adcde7
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x78e818c02b1e - std::sys_common::backtrace::print::hff629cd88cbfd188
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x78e818c05559 - std::panicking::default_hook::{{closure}}::h9d71a1d8151e4423
  10:     0x78e818c052fa - std::panicking::default_hook::h8b6ebfe091a035c5
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/panicking.rs:298:9
  11:     0x78e81537c3ff - std[b127da73d53c36b6]::panicking::update_hook::<alloc[38d31d85db1a046c]::boxed::Box<rustc_driver_impl[45af4acf2cc78f06]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x78e818c05c8b - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::hae8bef6fc4aacccb
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/alloc/src/boxed.rs:2077:9
  13:     0x78e818c05c8b - std::panicking::rust_panic_with_hook::h1bc8a1911adf5d38
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/panicking.rs:799:13
  14:     0x78e818c05a04 - std::panicking::begin_panic_handler::{{closure}}::hd4b966e58e008e0c
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/panicking.rs:664:13
  15:     0x78e818c03209 - std::sys_common::backtrace::__rust_end_short_backtrace::hef31567bf2c1bb30
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/sys_common/backtrace.rs:171:18
  16:     0x78e818c05737 - rust_begin_unwind
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/panicking.rs:652:5
  17:     0x78e818c4fa83 - core::panicking::panic_fmt::h00800dd6c6e334a4
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/core/src/panicking.rs:72:14
  18:     0x78e818c5007f - core::panicking::assert_failed_inner::h377882419abf48f3
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/core/src/panicking.rs:403:23
  19:     0x78e8150b5347 - core[680d2829b3045f23]::panicking::assert_failed::<indexmap[18e14f81f0668936]::map::IndexMap<rustc_middle[349545b1af2ab62b]::mir::Location, alloc[38d31d85db1a046c]::vec::Vec<rustc_borrowck[46ee65c78ca61b43]::dataflow::BorrowIndex>, core[680d2829b3045f23]::hash::BuildHasherDefault<rustc_hash[338ae66e5de4b634]::FxHasher>>, indexmap[18e14f81f0668936]::map::IndexMap<rustc_middle[349545b1af2ab62b]::mir::Location, alloc[38d31d85db1a046c]::vec::Vec<rustc_borrowck[46ee65c78ca61b43]::dataflow::BorrowIndex>, core[680d2829b3045f23]::hash::BuildHasherDefault<rustc_hash[338ae66e5de4b634]::FxHasher>>>
  20:     0x78e817759932 - rustc_borrowck[46ee65c78ca61b43]::do_mir_borrowck
  21:     0x78e81773dbbe - rustc_query_impl[b18d7974ec639165]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b18d7974ec639165]::query_impl::mir_borrowck::dynamic_query::{closure#2}::{closure#0}, rustc_middle[349545b1af2ab62b]::query::erase::Erased<[u8; 8usize]>>
  22:     0x78e816b867b1 - rustc_query_system[6a27861ca80f3c7f]::query::plumbing::try_execute_query::<rustc_query_impl[b18d7974ec639165]::DynamicConfig<rustc_query_system[6a27861ca80f3c7f]::query::caches::VecCache<rustc_span[97aaaa995e2f92fe]::def_id::LocalDefId, rustc_middle[349545b1af2ab62b]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[b18d7974ec639165]::plumbing::QueryCtxt, false>
  23:     0x78e816b8620d - rustc_query_impl[b18d7974ec639165]::query_impl::mir_borrowck::get_query_non_incr::__rust_end_short_backtrace
  24:     0x78e816b64904 - rustc_interface[411eb75ca6615201]::passes::analysis
  25:     0x78e816b63a95 - rustc_query_impl[b18d7974ec639165]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[b18d7974ec639165]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[349545b1af2ab62b]::query::erase::Erased<[u8; 1usize]>>
  26:     0x78e8175da965 - rustc_query_system[6a27861ca80f3c7f]::query::plumbing::try_execute_query::<rustc_query_impl[b18d7974ec639165]::DynamicConfig<rustc_query_system[6a27861ca80f3c7f]::query::caches::SingleCache<rustc_middle[349545b1af2ab62b]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[b18d7974ec639165]::plumbing::QueryCtxt, false>
  27:     0x78e8175da6cf - rustc_query_impl[b18d7974ec639165]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  28:     0x78e8173db092 - rustc_interface[411eb75ca6615201]::interface::run_compiler::<core[680d2829b3045f23]::result::Result<(), rustc_span[97aaaa995e2f92fe]::ErrorGuaranteed>, rustc_driver_impl[45af4acf2cc78f06]::run_compiler::{closure#0}>::{closure#1}
  29:     0x78e8173c8589 - std[b127da73d53c36b6]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[411eb75ca6615201]::util::run_in_thread_with_globals<rustc_interface[411eb75ca6615201]::util::run_in_thread_pool_with_globals<rustc_interface[411eb75ca6615201]::interface::run_compiler<core[680d2829b3045f23]::result::Result<(), rustc_span[97aaaa995e2f92fe]::ErrorGuaranteed>, rustc_driver_impl[45af4acf2cc78f06]::run_compiler::{closure#0}>::{closure#1}, core[680d2829b3045f23]::result::Result<(), rustc_span[97aaaa995e2f92fe]::ErrorGuaranteed>>::{closure#0}, core[680d2829b3045f23]::result::Result<(), rustc_span[97aaaa995e2f92fe]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[680d2829b3045f23]::result::Result<(), rustc_span[97aaaa995e2f92fe]::ErrorGuaranteed>>
  30:     0x78e8173c8342 - <<std[b127da73d53c36b6]::thread::Builder>::spawn_unchecked_<rustc_interface[411eb75ca6615201]::util::run_in_thread_with_globals<rustc_interface[411eb75ca6615201]::util::run_in_thread_pool_with_globals<rustc_interface[411eb75ca6615201]::interface::run_compiler<core[680d2829b3045f23]::result::Result<(), rustc_span[97aaaa995e2f92fe]::ErrorGuaranteed>, rustc_driver_impl[45af4acf2cc78f06]::run_compiler::{closure#0}>::{closure#1}, core[680d2829b3045f23]::result::Result<(), rustc_span[97aaaa995e2f92fe]::ErrorGuaranteed>>::{closure#0}, core[680d2829b3045f23]::result::Result<(), rustc_span[97aaaa995e2f92fe]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[680d2829b3045f23]::result::Result<(), rustc_span[97aaaa995e2f92fe]::ErrorGuaranteed>>::{closure#2} as core[680d2829b3045f23]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  31:     0x78e818c0fc7b - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h4ef5945c2429999c
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/alloc/src/boxed.rs:2063:9
  32:     0x78e818c0fc7b - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h5dd441b71d9c7ff9
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/alloc/src/boxed.rs:2063:9
  33:     0x78e818c0fc7b - std::sys::pal::unix::thread::Thread::new::thread_start::hac40c46ba02b627e
                               at /rustc/44701e070c8453df10df1984944258018ac21610/library/std/src/sys/pal/unix/thread.rs:108:17
  34:     0x78e8120a6ded - <unknown>
  35:     0x78e81212a0dc - <unknown>
  36:                0x0 - <unknown>

error: 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: please make sure that you have updated to the latest nightly

note: rustc 1.80.0-nightly (44701e070 2024-06-04) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z polonius=next -Z dump-mir-dir=dir

query stack during panic:
#0 [mir_borrowck] borrow-checking `ret_ref_local`
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to 3 previous errors; 1 warning emitted

Some errors have detailed explanations: E0601, E0726.
For more information about an error, try `rustc --explain E0601`.

@rustbot label +F-inherent_associated_types

[matthiaskrgr]

Regression in nightly-2024-06-05
#125667 🤔 cc @oli-obk

[lqd]

Amanda (and I, when I have a bit more time) will also look into this soon-ish to try and see where NLLs differ from the location-insensitive pass in this new situation.

[amandasystems]

I can narrow that down to specifically:

outlives_constraints = Default::default();

Looking into a work-around!

[amandasystems]

The problem is that NLL has an optimisation for computing loans out of scope that shortens the scope to skip the two final reborrows in the generated MIR body because borrowed_place.ignore_borrow() returns true, but Polonius doesn't so the scope for NLL is slightly shorter while the Polonius kill happens at the start of the cleanup/return blocks.

[amandasystems]

This only happens when the "remove constraints" trick of @oli-obk's is engaged because liveness values are merged into SCCs during construction of RegionInferenceContext, which happens after the constraints have been removed. This means that region liveness in Polonius still has the old view of the constraint graph via BorrowSet while NLL considers the rewritten graph.

Ironically this means that NLL is now slightly context-sensitive in the sense that parts of the constraint graph disappears between MIR statements.

[amandasystems]

I don't think I can safely convert the work-around to Polonius since I'd have to either move the liveness logic to a later stage of computation (design change) or undo parts of the computation of the BorrowSet to exclude certain variables as holders of loans (scary, above my current power level).