der: support SetOf sorting on heapless no_std targets
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Rust's built-in `[T]::sort_by` requires both `alloc` and a panic handler, which is limiting for `no_std` targets. This commit switches to using a heapless-friendly insertion sort implementation which is also fallible and can bubble up errors from `DerOrd::der_cmp`. Insertion sort is small (<10loc) and should perform well when the input is mostly in order, which should hopefully often be the case for `SetOf` constructors. It also works completely in-place and is thus friendly to heapless targets. This PR replaces the previous `[T]::sort_by` approach used by `SetOfVec`, which included a workaround which handled `DerOrd::der_cmp` errors by returning `Ordering::Less` and doing a second pass over the results to check they were correctly ordered. This might provide better performance and be worth bringing back, but for now this PR chooses instead to consolidate on a single, well-tested implementation. Testing is performed using proptest, to check equivalence with Rust's built-in `[T]::sort_by`.
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Rust's built-in
[T]::sort_byrequires bothallocand a panic handler, which is limiting forno_stdtargets.This commit switches to using a heapless-friendly insertion sort implementation which is also fallible and can bubble up errors from
DerOrd::der_cmp.Insertion sort is small (<10loc) and should perform well when the input is mostly in order, which should hopefully often be the case for
SetOfconstructors. It also works completely in-place and is thus friendly to heapless targets.This PR replaces the previous
[T]::sort_byapproach used bySetOfVec, which included a workaround which handledDerOrd::der_cmperrors by returningOrdering::Lessand doing a second pass over the results to check they were correctly ordered. This might provide better performance and be worth bringing back, but for now this PR chooses instead to consolidate on a single, well-tested implementation.Testing is performed using proptest, to check equivalence with Rust's built-in
[T]::sort_by.