[PATCH v2] 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.

Pre-calculating all patch IDs to achieve true const correctness would
introduce an unacceptable performance penalty.

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

Signed-off-by: Tian Yuchen <cat@malon.dev>
---
 patch-ids.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/patch-ids.c b/patch-ids.c
index a5683b462c..35e6a974f1 100644
--- a/patch-ids.c
+++ b/patch-ids.c
@@ -41,7 +41,15 @@ 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.
+	 *
+	 * The hashmap API requires us to treat the entries as const.
+	 * However, to avoid performance regression, we lazily compute
+	 * the patch IDs inside this comparison function. This fundamentally
+	 * requires us to mutate the 'struct patch_id'. Therefore, we use
+	 * container_of() to cast away the constness from the hashmap_entry.
+	 */
 	struct diff_options *opt = (void *)cmpfn_data;
 	struct patch_id *a, *b;
 
-- 
2.43.0