Investigate recent ICE on nightly about rlsl HOT 5 CLOSED

env RUSTFLAGS="-Z treat-err-as-bug" RUST_BACKTRACE=1 cargo build revealed that it is most likely the impl trait that causes this error. Sadly I use the impl trait feature pretty often.

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#0 [normalize_projection_ty] normalizing Canonical { variables: Slice([CanonicalVarInfo { kind: Region }, CanonicalVarInfo { kind: Region }, CanonicalVarInfo { kind: Region }]), value: ParamEnvAnd { param_env: ParamEnv { caller_bounds: Slice([]), reveal: UserFacing }, value: ProjectionTy { substs: Slice([std::iter::FlatMap<std::slice::Iter<'_, EntryPoint<'_, '_>>, impl std::iter::Iterator, fn(&'_ EntryPoint<'_, '_>) -> impl std::iter::Iterator {EntryPoint<'_, '_>::input_iter}>]), item_def_id: DefId(2/0:1904 ~ core[fc79]::iter[0]::iterator[0]::Iterator[0]::Item[0]) } } }
#1 [mir_const] processing <Entry<'tcx, Input<'tcx>>>::input
#2 [mir_validated] processing <Entry<'tcx, Input<'tcx>>>::input
#3 [borrowck] processing <Entry<'tcx, Input<'tcx>>>::input

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After a long debugging session, I could reduce the error to the following lines

    let set: HashSet<_> = entry_points
        .iter()
        .flat_map(EntryPoint::input_iter)
        .collect();

Where input_iter uses the impl trait

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Hm this seems to fix the error

    let set: HashSet<_> = entry_points
        .iter()
        .flat_map(|entry| EntryPoint::input_iter(entry))
        .collect();

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Fixed by eaf9edf

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