slice: make Partition clip its output

The result will still alias the input, but this ensures elements of the input
after the selected slice will not be modified by subsequent appends to the
result. This permits the caller to use the tail of the input slice more safely.

slice/slice.go

@@ -28,6 +28,9 @@ import (
 // returned slice is:
 //
 //	[6, 2, 8, 4]
+//
+// The capacity of the slice returned is clipped to its length, so that
+// appending to it will not modify the elements of vs after those kept.
 func Partition[T any](vs []T, keep func(T) bool) []T {
 	if len(vs) == 0 {
 		return vs
@@ -51,7 +54,7 @@ func Partition[T any](vs []T, keep func(T) bool) []T {
 		// If the right cursor reached the end, we're done: Everything left of i
 		// is kept, everything ≥ i is unkept.
 		if j == len(vs) {
-			return vs[:i]
+			return vs[:i:i]
 		}
 
 		// Reaching here, the elements under both cursors are out of
@@ -69,7 +72,7 @@ func Partition[T any](vs []T, keep func(T) bool) []T {
 		i++
 		j++
 	}
-	return vs[:i]
+	return vs[:i:i]
 }
 
 // Dedup rearranges the elements of vs in-place to deduplicate consecutive runs

slice/slice_test.go

@@ -489,6 +489,14 @@ func (tc *testCase[T]) partition(t *testing.T) {
 	if diff != "" {
 		t.Errorf("Partition result (-want, +got)\n%s", diff)
 	}
+
+	// Verify that the output is clipped to its length.
+	cp2 := copyOf(cp)
+	var zero T
+	_ = append(got, zero)
+	if diff := cmp.Diff(cp, cp2); diff != "" {
+		t.Errorf("After append to result (-got, +want):\n%s", diff)
+	}
 }
 
 func copyOf[T any](vs []T) []T {