feat!: make options.limit a compile-time constant (#7027)

This will guarantee that we can do things such as `for i in
0..options.limit` - I changed some of the callsites to iterate over the
notes using it. This will reduce gate count for callers that set a limit
value.

docs/docs/migration_notes.md

@@ -8,6 +8,10 @@ Aztec is in full-speed development. Literally every version breaks compatibility
 
 ## TBD
 
+### [Aztec.nr] `options.limit` has to be constant
+
+The `limit` parameter in `NoteGetterOptions` and `NoteViewerOptions` is now required to be a compile-time constant. This allows performing loops over this value, which leads to reduced circuit gate counts when setting a `limit` value.
+
 ### [Aztec.nr] emit encrypted logs
 
 Emitting or broadcasting encrypted notes are no longer done as part of the note creation, but must explicitly be either emitted or discarded instead.

noir-projects/aztec-nr/aztec/src/note/note_getter.nr

@@ -155,9 +155,8 @@ fn constrain_get_notes_internal<Note, N, M, FILTER_ARGS>(
             num_notes += 1;
         };
     }
-    if options.limit != 0 {
-        assert(num_notes <= options.limit, "Got more notes than limit.");
-    }
+
+    assert(num_notes <= options.limit, "Got more notes than limit.");
 
     assert(num_notes != 0, "Cannot return zero notes");
 

noir-projects/aztec-nr/aztec/src/note/note_getter_options.nr

@@ -156,6 +156,10 @@ impl<Note, N, M, FILTER_ARGS> NoteGetterOptions<Note, N, M, FILTER_ARGS> {
     // This method lets you set a limit for the maximum number of notes to be retrieved in a single query result.
     pub fn set_limit(&mut self, limit: u32) -> Self {
         assert(limit <= MAX_NOTE_HASH_READ_REQUESTS_PER_CALL as u32);
+        // By requesting that the limit is a constant, we guarantee that it will be possible to loop over it, reducing
+        // gate counts when a limit has been set.
+        assert_constant(limit);
+
         self.limit = limit;
         *self
     }

noir-projects/aztec-nr/aztec/src/note/note_viewer_options.nr

@@ -54,6 +54,10 @@ impl<Note, N, M> NoteViewerOptions<Note, N, M> {
 
     pub fn set_limit(&mut self, limit: u32) -> Self {
         assert(limit <= MAX_NOTES_PER_PAGE as u32);
+        // By requesting that the limit is a constant, we guarantee that it will be possible to loop over it, reducing
+        // gate counts when a limit has been set. This isn't required in unconstrained code, but we still keep this
+        // requirement here for API consistency.
+        assert_constant(limit);
         self.limit = limit;
         *self
     }

noir-projects/aztec-nr/easy-private-state/src/easy_private_uint.nr

@@ -45,7 +45,7 @@ impl<Context> EasyPrivateUint<&mut PrivateContext> {
         // docs:end:get_notes
 
         let mut minuend: u64 = 0;
-        for i in 0..maybe_notes.len() {
+        for i in 0..options.limit {
             if maybe_notes[i].is_some() {
                 let note = maybe_notes[i].unwrap_unchecked();
 

noir-projects/aztec-nr/value-note/src/utils.nr

@@ -58,7 +58,7 @@ pub fn decrement_by_at_most(
     let opt_notes = balance.get_notes(options);
 
     let mut decremented = 0;
-    for i in 0..opt_notes.len() {
+    for i in 0..options.limit {
         if opt_notes[i].is_some() {
             let note = opt_notes[i].unwrap_unchecked();
 

noir-projects/noir-contracts/contracts/card_game_contract/src/cards.nr

@@ -125,7 +125,7 @@ impl Deck<&mut PrivateContext> {
         let options = NoteGetterOptions::with_filter(filter_cards, cards);
         let maybe_notes = self.set.get_notes(options);
         let mut found_cards = [Option::none(); N];
-        for i in 0..maybe_notes.len() {
+        for i in 0..options.limit {
             if maybe_notes[i].is_some() {
                 let card_note = CardNote::from_note(maybe_notes[i].unwrap_unchecked());
 

noir-projects/noir-contracts/contracts/token_blacklist_contract/src/types/balances_map.nr

@@ -88,7 +88,7 @@ impl<T> BalancesMap<T, &mut PrivateContext> {
         // docs:end:get_notes
 
         let mut minuend: U128 = U128::from_integer(0);
-        for i in 0..maybe_notes.len() {
+        for i in 0..options.limit {
             if maybe_notes[i].is_some() {
                 let note = maybe_notes[i].unwrap_unchecked();
 

noir-projects/noir-contracts/contracts/token_contract/src/types/balances_map.nr

@@ -91,7 +91,7 @@ impl<T> BalancesMap<T, &mut PrivateContext> {
         // docs:end:get_notes
 
         let mut minuend: U128 = U128::from_integer(0);
-        for i in 0..maybe_notes.len() {
+        for i in 0..options.limit {
             if maybe_notes[i].is_some() {
                 let note = maybe_notes[i].unwrap_unchecked();