chore: use functional slice building (#8097)

Noticed some redundant procedural code.

tokio/src/runtime/scheduler/multi_thread/queue.rs

@@ -90,17 +90,13 @@ fn make_fixed_size<T>(buffer: Box<[T]>) -> Box<[T; LOCAL_QUEUE_CAPACITY]> {
 
 /// Create a new local run-queue
 pub(crate) fn local<T: 'static>() -> (Steal<T>, Local<T>) {
-    let mut buffer = Vec::with_capacity(LOCAL_QUEUE_CAPACITY);
-
-    for _ in 0..LOCAL_QUEUE_CAPACITY {
-        buffer.push(UnsafeCell::new(MaybeUninit::uninit()));
-    }
+    let buffer = std::iter::repeat_with(|| UnsafeCell::new(MaybeUninit::uninit()));
 
     let inner = Arc::new(Inner {
         head: AtomicUnsignedLong::new(0),
         tail: AtomicUnsignedShort::new(0),
         lifo: task::AtomicNotified::empty(),
-        buffer: make_fixed_size(buffer.into_boxed_slice()),
+        buffer: make_fixed_size(buffer.take(LOCAL_QUEUE_CAPACITY).collect()),
     });
 
     let local = Local {

tokio/src/runtime/time/wheel/mod.rs

@@ -50,13 +50,10 @@ pub(super) const MAX_DURATION: u64 = (1 << (6 * NUM_LEVELS)) - 1;
 impl Wheel {
     /// Creates a new timing wheel.
     pub(crate) fn new() -> Wheel {
-        let mut levels = Vec::with_capacity(NUM_LEVELS);
-        for i in 0..NUM_LEVELS {
-            levels.push(Level::new(i));
-        }
+        let levels = (0..NUM_LEVELS).map(Level::new).collect::<Box<_>>();
         Wheel {
             elapsed: 0,
-            levels: levels.into_boxed_slice().try_into().unwrap(),
+            levels: levels.try_into().unwrap(),
             pending: EntryList::new(),
         }
     }

tokio/src/runtime/time_alt/wheel/mod.rs

@@ -39,13 +39,10 @@ pub(super) const MAX_DURATION: u64 = (1 << (6 * NUM_LEVELS)) - 1;
 impl Wheel {
     /// Creates a new timing wheel.
     pub(crate) fn new() -> Wheel {
-        let mut levels = Vec::with_capacity(NUM_LEVELS);
-        for i in 0..NUM_LEVELS {
-            levels.push(Level::new(i));
-        }
+        let levels = (0..NUM_LEVELS).map(Level::new).collect::<Box<_>>();
         Wheel {
             elapsed: 0,
-            levels: levels.into_boxed_slice().try_into().unwrap(),
+            levels: levels.try_into().unwrap(),
         }
     }
 

tokio/src/sync/broadcast.rs

@@ -550,18 +550,16 @@ impl<T> Sender<T> {
         // Round to a power of two
         capacity = capacity.next_power_of_two();
 
-        let mut buffer = Vec::with_capacity(capacity);
-
-        for i in 0..capacity {
-            buffer.push(Mutex::new(Slot {
+        let buffer = (0..capacity).map(|i| {
+            Mutex::new(Slot {
                 rem: AtomicUsize::new(0),
                 pos: (i as u64).wrapping_sub(capacity as u64),
                 val: None,
-            }));
-        }
+            })
+        });
 
         let shared = Arc::new(Shared {
-            buffer: buffer.into_boxed_slice(),
+            buffer: buffer.collect(),
             mask: capacity - 1,
             tail: Mutex::new(Tail {
                 pos: 0,

tokio/src/util/sharded_list.rs

@@ -38,12 +38,9 @@ impl<L, T> ShardedList<L, T> {
         assert!(sharded_size.is_power_of_two());
 
         let shard_mask = sharded_size - 1;
-        let mut lists = Vec::with_capacity(sharded_size);
-        for _ in 0..sharded_size {
-            lists.push(Mutex::new(LinkedList::<L, T>::new()))
-        }
+        let lists = std::iter::repeat_with(|| Mutex::new(LinkedList::new()));
         Self {
-            lists: lists.into_boxed_slice(),
+            lists: lists.take(sharded_size).collect(),
             added: MetricAtomicU64::new(0),
             count: MetricAtomicUsize::new(0),
             shard_mask,