[PATCH v3] patch-ids: document intentional const-casting in patch_id_neq()

[Tian Yuchen <cat@malon.dev>]

The hashmap API requires the comparison function to take const pointers.
However, patch_id_neq() uses lazy evaluation to compute patch IDs on
demand. As established in b3dfeebb (rebase: avoid computing unnecessary
patch IDs, 2016-07-29), this avoids unnecessary work since not all
objects in the hashmap will eventually be compared.

Remove the ten-year-old "NEEDSWORK" comment and formally document
this intentional design trade-off.

Signed-off-by: Tian Yuchen <cat@malon.dev>
---
 patch-ids.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/patch-ids.c b/patch-ids.c
index a5683b462c..1fbc88cbec 100644
--- a/patch-ids.c
+++ b/patch-ids.c
@@ -41,7 +41,14 @@ static int patch_id_neq(const void *cmpfn_data,
 			const struct hashmap_entry *entry_or_key,
 			const void *keydata UNUSED)
 {
-	/* NEEDSWORK: const correctness? */
+	/*
+	 * We drop the 'const' modifier here intentionally.
+	 *
+	 * Even though eptr and entry_or_key are const, we want to
+	 * lazily compute their .patch_id members; see b3dfeebb (rebase:
+	 * avoid computing unnecessary patch IDs, 2016-07-29). So we cast
+	 * the constness away with container_of().
+	 */
 	struct diff_options *opt = (void *)cmpfn_data;
 	struct patch_id *a, *b;
 
-- 
2.43.0