std.Io.Select: remove "outstanding" field

it is not fundamentally part of this abstraction

lib/std/Io.zig

@@ -1178,7 +1178,6 @@ pub fn Select(comptime U: type) type {
         io: Io,
         group: Group,
         queue: Queue(U),
-        outstanding: usize,
 
         const S = @This();
 
@@ -1191,7 +1190,6 @@ pub fn Select(comptime U: type) type {
                 .io = io,
                 .queue = .init(buffer),
                 .group = .init,
-                .outstanding = 0,
             };
         }
 
@@ -1235,7 +1233,6 @@ pub fn Select(comptime U: type) type {
                 }
             };
             const context: Context = .{ .select = s, .args = args };
-            _ = @atomicRmw(usize, &s.outstanding, .Add, 1, .monotonic);
             s.io.vtable.groupAsync(s.io.userdata, &s.group, @ptrCast(&context), .of(Context), Context.start);
         }
 
@@ -1276,16 +1273,12 @@ pub fn Select(comptime U: type) type {
             };
             const context: Context = .{ .select = s, .args = args };
             try s.io.vtable.groupConcurrent(s.io.userdata, &s.group, @ptrCast(&context), .of(Context), Context.start);
-            _ = @atomicRmw(usize, &s.outstanding, .Add, 1, .monotonic);
         }
 
         /// Blocks until another task of the select finishes.
         ///
-        /// Asserts there is at least one more `outstanding` task.
-        ///
         /// Not threadsafe.
         pub fn await(s: *S) Cancelable!U {
-            s.outstanding -= 1;
             return s.queue.getOne(s.io) catch |err| switch (err) {
                 error.Canceled => |e| return e,
                 error.Closed => unreachable,
@@ -1301,7 +1294,6 @@ pub fn Select(comptime U: type) type {
         ///
         /// Idempotent. Not threadsafe.
         pub fn cancel(s: *S) void {
-            s.outstanding = 0;
             s.group.cancel(s.io);
         }
     };

lib/std/crypto/kangarootwelve.zig

@@ -883,6 +883,7 @@ fn ktMultiThreaded(
         defer allocator.free(pending_cv_buf);
         var pending_cv_lens: [256]usize = .{0} ** 256;
 
+        var select_outstanding: usize = 0;
         var select: Select = .init(io, select_buf);
         defer select.cancel();
         var batches_spawned: usize = 0;
@@ -894,6 +895,7 @@ fn ktMultiThreaded(
                 const batch_leaves = @min(leaves_per_batch, full_leaves - batch_start_leaf);
                 const start_offset = chunk_size + batch_start_leaf * chunk_size;
 
+                select_outstanding += 1;
                 select.async(.batch, SelectLeafContext(Variant).process, .{SelectLeafContext(Variant){
                     .view = view,
                     .batch_idx = batches_spawned,
@@ -903,6 +905,7 @@ fn ktMultiThreaded(
                 batches_spawned += 1;
             }
 
+            select_outstanding -= 1;
             const result = try select.await();
             const batch = result.batch;
             const slot = batch.batch_idx % max_concurrent;
@@ -927,7 +930,7 @@ fn ktMultiThreaded(
             }
         }
 
-        assert(select.outstanding == 0);
+        assert(select_outstanding == 0);
     }
 
     if (has_partial_leaf) {