List.keep_if_try allocates once per kept item #12680

List.keep_if_try and List.keep_if_try! (src/build/roc/Builtin.roc) still build their result with List.concat($out, [item]):

keep_if_try = |list, predicate| {
	var $out = []
	for item in list {
		if predicate(item)? {
			$out = List.concat($out, [item])
		}
	}
	Ok($out)
}

Each kept item allocates a one-item list, and $out grows by reallocating. This is the same problem #12644 reported for List.keep_if, drop_if, keep_oks and keep_errs, which #12645 fixes.

Possible fix, following #12645:

• Unique list: compact in place with list_swap_unsafe and finish with List.drop_last, as keep_if does. Stopping early on Err must still release the whole list correctly.
• Shared list: copy the kept items into one List.with_capacity(len) buffer with list_append_unsafe, and return the input unchanged when every item is kept.

At minimum, replacing the concat with one List.with_capacity(list.len()) buffer and list_append_unsafe (as map_try does) would cut this to one allocation.

Issue authored by Claude Opus 5.5.