pola-rs/polars · error

must have an upper bound

Error message

must have an upper bound

What it means

extend_trusted_len_unchecked relies on TrustedLen's contract that size_hint().1 is Some; it unwraps the upper bound and panics when the iterator's size hint has no upper bound — the iterator passed does not actually implement TrustedLen.

Solutions

1. Only pass iterators that genuinely implement TrustedLen to extend_trusted_len_unchecked
2. Use the safe extend API when the iterator's length is not statically known
3. Fix custom iterator size_hint implementations to return an exact upper bound

When it happens

Trigger: Thrown at crates/polars-arrow/src/legacy/trusted_len/push_unchecked.rs:75 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of pola-rs/polars@fe841f959e (2026-09-18). Data as JSON: /api/errors/37d95344fe2d2fac. Report an issue: GitHub.

Appendix: source

Thrown at crates/polars-arrow/src/legacy/trusted_len/push_unchecked.rs:75

        iter: I,
    ) -> Result<Self, E>
    where
        Self: Sized;
}

impl<T> TrustedLenPush<T> for Vec<T> {
    #[inline(always)]
    unsafe fn push_unchecked(&mut self, value: T) {
        debug_assert!(self.capacity() > self.len());
        let end = self.as_mut_ptr().add(self.len());
        std::ptr::write(end, value);
        self.set_len(self.len() + 1);
    }

    #[inline]
    unsafe fn extend_trusted_len_unchecked<I: IntoIterator<Item = T>>(&mut self, iter: I) {
        let iter = iter.into_iter();
        let upper = iter.size_hint().1.expect("must have an upper bound");
        self.reserve(upper);

        let mut dst = self.as_mut_ptr().add(self.len());
        for value in iter {
            std::ptr::write(dst, value);
            dst = dst.add(1)
        }
        self.set_len(self.len() + upper)
    }

    unsafe fn try_extend_trusted_len_unchecked<E, I: IntoIterator<Item = Result<T, E>>>(
        &mut self,
        iter: I,
    ) -> Result<(), E> {
        let iter = iter.into_iter();
        let upper = iter.size_hint().1.expect("must have an upper bound");
        self.reserve(upper);

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