ICE: Lifetime issue not detected for GAT

[battesonb]

I encountered this while trying to understand the relatively new GAT feature in Rust.

Code

trait LendingIterator {
    type Item<'a>
    where
        Self: 'a;

    fn next(&mut self) -> Option<Self::Item<'_>>;

    fn for_each(mut self, mut f: Box<dyn FnMut(Self::Item<'_>) + 'static>)
    where
        Self: Sized,
    {
        while let Some(next) = self.next() {
            f(next);
        }
    }
}

struct Query<'q> {
    inner: &'q [u32],
    index: usize,
}

impl<'q> Query<'q> {
    pub fn new(inner: &'q Vec<u32>) -> Self {
        Self { index: 0, inner }
    }
}

impl<'q> LendingIterator for Query<'q> {
    type Item<'a> = &'a u32 where Self: 'a;

    fn next(&mut self) -> Option<Self::Item<'_>> {
        if let Some(value) = self.inner.get(self.index) {
            self.index += 1;
            return Some(value);
        }

        None
    }
}

fn main() {
    let mut data = vec![1, 2, 3];

    LendingIterator::for_each(
        Query::new(&data),
        Box::new(|val| {
            eprintln!("{}", val);
        }),
    );

    // Picks up the 'static issue, doesn't compile/crash.
    // Query::new(&data).for_each(Box::new(|val| {
    //     eprintln!("{}", val);
    // }));
}

Meta

rustc --version --verbose:

rustc 1.73.0 (cc66ad468 2023-10-03)
binary: rustc
commit-hash: cc66ad468955717ab92600c770da8c1601a4ff33
commit-date: 2023-10-03
host: aarch64-apple-darwin
release: 1.73.0
LLVM version: 17.0.2

rustc +nightly --version --verbose:

rustc 1.75.0-nightly (9d83ac217 2023-10-31)
binary: rustc
commit-hash: 9d83ac217957eece2189eccf4a7232caec7232ee
commit-date: 2023-10-31
host: aarch64-apple-darwin
release: 1.75.0-nightly
LLVM version: 17.0.3

Error output

error: internal compiler error: compiler/rustc_borrowck/src/universal_regions.rs:880:36: cannot convert `RePlaceholder(!1_BoundRegion { var: 0, kind: BrAnon(None) })` to a region vid

Backtrace

thread 'rustc' panicked at /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/compiler/rustc_errors/src/lib.rs:1635:9:
Box<dyn Any>
stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: <rustc_errors::HandlerInner>::bug::<alloc::string::String>
   2: <rustc_errors::Handler>::bug::<alloc::string::String>
   3: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
   4: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
   5: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
   6: rustc_middle::util::bug::bug_fmt
   7: <rustc_borrowck::universal_regions::UniversalRegionIndices>::to_region_vid
   8: <rustc_borrowck::type_check::free_region_relations::UniversalRegionRelationsBuilder>::add_implied_bounds
   9: rustc_borrowck::type_check::free_region_relations::create
  10: rustc_borrowck::type_check::type_check
  11: rustc_borrowck::nll::compute_regions
  12: rustc_borrowck::do_mir_borrowck
  13: rustc_borrowck::mir_borrowck
      [... omitted 2 frames ...]
  14: <rustc_borrowck::type_check::TypeChecker>::prove_closure_bounds
  15: <rustc_borrowck::type_check::TypeChecker>::check_rvalue
  16: <rustc_borrowck::type_check::TypeChecker>::typeck_mir
  17: rustc_borrowck::type_check::type_check
  18: rustc_borrowck::nll::compute_regions
  19: rustc_borrowck::do_mir_borrowck
  20: rustc_borrowck::mir_borrowck
      [... omitted 2 frames ...]
  21: <core::panic::unwind_safe::AssertUnwindSafe<rustc_data_structures::sync::par_for_each_in<&[rustc_span::def_id::LocalDefId], <rustc_middle::hir::map::Map>::par_body_owners<rustc_interface::passes::analysis::{closure#1}::{closure#0}>::{closure#0}>::{closure#0}::{closure#0}> as core::ops::function::FnOnce<()>>::call_once
  22: rustc_data_structures::sync::par_for_each_in::<&[rustc_span::def_id::LocalDefId], <rustc_middle::hir::map::Map>::par_body_owners<rustc_interface::passes::analysis::{closure#1}::{closure#0}>::{closure#0}>
  23: <rustc_session::session::Session>::time::<(), rustc_interface::passes::analysis::{closure#1}>
  24: rustc_interface::passes::analysis
      [... omitted 2 frames ...]
  25: <rustc_middle::ty::context::GlobalCtxt>::enter::<rustc_driver_impl::run_compiler::{closure#1}::{closure#2}::{closure#6}, core::result::Result<(), rustc_span::ErrorGuaranteed>>
  26: <rustc_interface::interface::Compiler>::enter::<rustc_driver_impl::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.73.0 (cc66ad468 2023-10-03) running on aarch64-apple-darwin

note: compiler flags: --crate-type bin -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked -C incremental=[REDACTED]

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

query stack during panic:
#0 [mir_borrowck] borrow-checking `main::{closure#0}`
#1 [mir_borrowck] borrow-checking `main`
#2 [analysis] running analysis passes on this crate
end of query stack
error: could not compile `rust-playground` (bin "rust-playground")

[lqd]

Reduced

pub trait LendingIterator {
    type Item<'a>
    where
        Self: 'a;
    fn for_each(self, _: Box<dyn FnMut(Self::Item<'_>)>)
    where
        Self: Sized,
    {
    }
}
pub struct Query<'q>(&'q ());

impl<'q> LendingIterator for Query<'q> {
    type Item<'a> = &'a () where Self: 'a;
}

fn main() {
    let q = Query(&());
    LendingIterator::for_each(q, Box::new(|_| {}));
}

Started ICEing in nightly-2022-08-10, and the <rustc_borrowck::type_check::free_region_relations::UniversalRegionRelationsBuilder>::add_implied_bounds in the backtrace makes #99217 look pretty likely.

[lqd]

This may be more t-types than t-compiler per se.

[matthiaskrgr]

Might be related to #112250 ?

[lqd]

Yeah, implied bounds + placeholders so it seems to be around the same area indeed, and could likely be fixed by #112422 cc @aliemjay

[aliemjay]

fixed by #112422.