Associated type bound has requirement overflow with impossible type

Unfortunately I do not have a minimal reproducer, but I do have a reproducer... on MacOS systems.

If you clone the Polars repository and check out 908c99a654fb9c46c63635c211962ac57ffc3719 on the main branch and try to run (the repository contains rust-toolchain.toml):

cargo check --all-features --bin dsl-schema

compilation fails with the following error:

error[E0275]: overflow evaluating the requirement `&itertools::groupbylazy::ChunkBy<_, _, _>: IntoIterator`
   --> crates/polars-ops/src/frame/join/hash_join/single_keys_dispatch.rs:331:49
    |
331 | ...rhs)) => group_join_inner(&lhs, &rhs, validate, nulls_equal),
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`polars_ops`)
    = note: required for `&Retained<ChunkBy<_, _, _>>` to implement `IntoIterator`
    = note: 125 redundant requirements hidden
    = note: required for `&Retained<Retained<Retained<...>>>` to implement `for<'a> IntoIterator`
note: required by a bound in `group_join_inner`
   --> crates/polars-ops/src/frame/join/hash_join/single_keys_dispatch.rs:470:27
    |
462 | fn group_join_inner<T>(
    |    ---------------- required by a bound in this function
...
470 |     for<'a> &'a T::Array: IntoIterator<Item = Option<&'a T::Physical<'a>>>,
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `group_join_inner`
    = note: the full name for the type has been written to '...'

where the full type is

&objc2::rc::retained::Retained<itertools::groupbylazy::ChunkBy<_, _, _>>
&'a objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<objc2::rc::retained::Retained<itertools::groupbylazy::ChunkBy<_, _, _>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

The thing is... there is no way that T::Array could ever be objc2::rc::retained::Retained. Polars only has an indirect dependency on objc2 through arboard for clipboard support, and T::Array here comes from T: PolarsDataType which is only implemented for a handful for Polars-specific types. They are in completely different parts of the codebase and do not interact. These are the relevant bounds:

    T: PolarsDataType,
    for<'a> &'a T::Array: IntoIterator<Item = Option<&'a T::Physical<'a>>>,

Meta

The rust-toolchain.toml of the reproducer specifies nightly-2026-01-09, but I've tested it with nightly-2026-01-15 as well, still reproducing.

Check out Polars commit 908c99a654fb9c46c63635c211962ac57ffc3719 and run cargo check --all-features --bin dsl-schema on macOS. Start with crates/polars-ops/src/frame/join/hash_join/single_keys_dispatch.rs at lines 331 and 462-470, focusing on the IntoIterator bound and reported objc2 type. Done means the reproducer no longer produces the requirement-overflow error without weakening the stated bounds.