[braedenfoster]

Fixes #12644.

List.keep_if, drop_if, keep_oks and keep_errs were a fold over List.concat(acc, [item]), so each kept item allocated a one-item list and the result grew by reallocating. This moves them onto the reuse pattern that List.map (#9610, #10594) and sort_impl use.

Changes

keep_if

• Unique list: list_map_prepare_reuse, then list_owned_unique. Each kept item is swapped down to the end of the kept prefix with list_swap_unsafe, then List.drop_last releases the dropped items. Order is kept, each dropped item is released once, capacity is unchanged, and an all-kept list comes back as it was. No allocation.
• Shared list: items are tested up to the first dropped one. If none is dropped, the input is returned as it is. Otherwise the kept items are copied with list_append_unsafe into one List.with_capacity(len) buffer, allocated when the first kept item needs a place, so keeping nothing allocates nothing. The predicate still runs once per item.

drop_if calls keep_if with the predicate negated.

keep_oks and keep_errs build their result in one buffer allocated on the first match. They do not work in place: when before and after share a layout it would be possible, but the moved-out tail slots would need a primitive that shortens a list without releasing them. That seems better as a follow-up.

The in-place path uses only list_swap_unsafe and drop_last. listSwap copies a shared list unless the call is marked in place, so a wrong uniqueness answer costs a copy rather than a wrong result (the #10368 failure mode).

The second commit (all-kept shortcut and lazy allocation on the shared path) can be dropped or split out if you prefer a smaller change.

Allocation calls

roc_alloc plus roc_realloc made by the call alone, 12-item list, test/alloc-count, same for --opt=dev and --opt=speed:

Call	Before	After
keep_if, all kept	16	0
keep_if, 6 of 12 kept	9	0
keep_if, none kept	0	0
drop_if, none dropped	16	0
keep_oks, all Ok	16	1

Tests

• src/eval/test/eval_tests.zig: order kept; all, none and some kept; empty and emptied lists; a unique seamless slice; a shared list used again afterwards; a predicate that captures the list being filtered; heap Str items; drop_if, keep_oks and keep_errs; allocation counts.
• src/eval/test/rc_conformance_tests.zig: keep_if, drop_if and keep_oks over unique and shared Str lists.
• test/alloc-count/app_list_keep_if.roc with dev and speed CLI entries.
• 8 snapshots changed only in Builtin node ids.

The full eval suite, rc_conformance and the Builtin doc tests pass locally.

[Anton-4]

Thanks @braedenfoster :)
Just a heads-up, I plan to do some minor cleanup on this tomorrow.

[braedenfoster]

Thanks @braedenfoster :)
Just a heads-up, I plan to do some minor cleanup on this tomorrow.

Awesome thanks 🙏

[Anton-4]

Thanks @braedenfoster! It's nice to have these optimized 🚀