diff --git a/lib/std/heap/ArenaAllocator.zig b/lib/std/heap/ArenaAllocator.zig
index 5901e6474fd88750a9592ab35934053b81b3fa4b..a9fea912841df99be85feae311cde1340b9d3088 100644
--- a/lib/std/heap/ArenaAllocator.zig
+++ b/lib/std/heap/ArenaAllocator.zig
@@ -78,7 +78,7 @@ fn countListCapacity(first_node: ?*Node) usize {
     while (it) |node| : (it = node.next) {
         // Compute the actually allocated size excluding the
         // linked list node.
-        capacity += node.size - @sizeOf(Node);
+        capacity += node.size.toInt() - @sizeOf(Node);
     }
     return capacity;
 }
@@ -164,7 +164,10 @@ pub fn reset(arena: *ArenaAllocator, mode: ResetMode) bool {
         };
         const allocated_slice = node.allocatedSliceUnsafe();
 
-        if (new_capacity == 0) {
+        // Align backwards to always stay below limit.
+        const new_size = mem.alignBackward(usize, @sizeOf(Node) + new_capacity, 2);
+
+        if (new_size == @sizeOf(Node)) {
             arena.child_allocator.rawFree(allocated_slice, .of(Node), @returnAddress());
             first_node_ptr.* = null;
             continue;
@@ -173,19 +176,17 @@ pub fn reset(arena: *ArenaAllocator, mode: ResetMode) bool {
         node.end_index = 0;
         first_node_ptr.* = node;
 
-        const adjusted_capacity: usize = mem.alignForward(usize, new_capacity, 2);
-
-        if (allocated_slice.len - @sizeOf(Node) == adjusted_capacity) {
+        if (allocated_slice.len == new_size) {
             // perfect, no need to invoke the child_allocator
             continue;
         }
 
-        if (arena.child_allocator.rawResize(allocated_slice, .of(Node), adjusted_capacity, @returnAddress())) {
+        if (arena.child_allocator.rawResize(allocated_slice, .of(Node), new_size, @returnAddress())) {
             // successful resize
-            node.size = adjusted_capacity;
+            node.size = .fromInt(new_size);
         } else {
             // manual realloc
-            const new_ptr = arena.child_allocator.rawAlloc(adjusted_capacity, .of(Node), @returnAddress()) orelse {
+            const new_ptr = arena.child_allocator.rawAlloc(new_size, .of(Node), @returnAddress()) orelse {
                 // we failed to preheat the arena properly, signal this to the user.
                 ok = false;
                 continue;
@@ -193,7 +194,7 @@ pub fn reset(arena: *ArenaAllocator, mode: ResetMode) bool {
             arena.child_allocator.rawFree(allocated_slice, .of(Node), @returnAddress());
             const new_first_node: *Node = @ptrCast(@alignCast(new_ptr));
             new_first_node.* = .{
-                .size = adjusted_capacity,
+                .size = .fromInt(new_size),
                 .end_index = 0,
                 .next = null,
             };
@@ -224,30 +225,49 @@ const Node = struct {
     /// accessing it.
     next: ?*Node,
 
-    const resize_bit: usize = 1;
+    const Size = packed struct(usize) {
+        resizing: bool,
+        _: @Int(.unsigned, @bitSizeOf(usize) - 1) = 0,
+
+        fn fromInt(int: usize) Size {
+            assert(int >= @sizeOf(Node));
+            const size: Size = @bitCast(int);
+            assert(!size.resizing);
+            return size;
+        }
+
+        fn toInt(size: Size) usize {
+            var int = size;
+            int.resizing = false;
+            return @bitCast(int);
+        }
+
+        comptime {
+            assert(Size{ .resizing = true } == @as(Size, @bitCast(@as(usize, 1))));
+        }
+    };
 
     fn loadBuf(node: *Node) []u8 {
         // monotonic is fine since `size` can only ever grow, so the buffer returned
         // by this function is always valid memory.
-        const size = @atomicLoad(usize, &node.size, .monotonic);
-        return @as([*]u8, @ptrCast(node))[0 .. size & ~resize_bit][@sizeOf(Node)..];
+        const size = @atomicLoad(Size, &node.size, .monotonic);
+        return @as([*]u8, @ptrCast(node))[0..size.toInt()][@sizeOf(Node)..];
     }
 
     /// Returns allocated slice or `null` if node is already (being) resized.
     fn beginResize(node: *Node) ?[]u8 {
-        const size = @atomicRmw(usize, &node.size, .Or, resize_bit, .acquire); // syncs with release in `endResize`
-        if (size & resize_bit != 0) return null;
-        return @as([*]u8, @ptrCast(node))[0..size];
+        const size = @atomicRmw(Size, &node.size, .Or, .{ .resizing = true }, .acquire); // syncs with release in `endResize`
+        if (size.resizing) return null;
+        return @as([*]u8, @ptrCast(node))[0..size.toInt()];
     }
 
     fn endResize(node: *Node, size: usize) void {
-        assert(size & resize_bit == 0);
-        return @atomicStore(usize, &node.size, size, .release); // syncs with acquire in `beginResize`
+        return @atomicStore(Size, &node.size, .fromInt(size), .release); // syncs with acquire in `beginResize`
     }
 
     /// Not threadsafe.
     fn allocatedSliceUnsafe(node: *Node) []u8 {
-        return @as([*]u8, @ptrCast(node))[0 .. node.size & ~resize_bit];
+        return @as([*]u8, @ptrCast(node))[0..node.size.toInt()];
     }
 };
 
@@ -408,7 +428,7 @@ fn alloc(ctx: *anyopaque, n: usize, alignment: Alignment, ret_addr: usize) ?[*]u
                     it = node.next;
                 }) {
                     last_free = node;
-                    assert(node.size & Node.resize_bit == 0);
+                    assert(!node.size.resizing);
                     const buf = node.allocatedSliceUnsafe()[@sizeOf(Node)..];
                     const aligned_index = alignedIndex(buf.ptr, 0, alignment);
 
@@ -433,7 +453,7 @@ fn alloc(ctx: *anyopaque, n: usize, alignment: Alignment, ret_addr: usize) ?[*]u
                         const new_size = mem.alignForward(usize, @sizeOf(Node) + aligned_index + n, 2);
 
                         if (arena.child_allocator.rawResize(allocated_slice, .of(Node), new_size, @returnAddress())) {
-                            node.size = new_size;
+                            node.size = .fromInt(new_size);
                             break :candidate .{ node, best_fit_prev };
                         }
                     }
@@ -486,12 +506,11 @@ fn alloc(ctx: *anyopaque, n: usize, alignment: Alignment, ret_addr: usize) ?[*]u
                 const big_enough_size = prev_size + min_size + 16;
                 break :size mem.alignForward(usize, big_enough_size + big_enough_size / 2, 2);
             };
-            assert(size & Node.resize_bit == 0);
             const ptr = arena.child_allocator.rawAlloc(size, .of(Node), @returnAddress()) orelse
                 return null;
             const new_node: *Node = @ptrCast(@alignCast(ptr));
             new_node.* = .{
-                .size = size,
+                .size = .fromInt(size),
                 .end_index = undefined, // set below
                 .next = undefined, // set below
             };
@@ -501,7 +520,7 @@ fn alloc(ctx: *anyopaque, n: usize, alignment: Alignment, ret_addr: usize) ?[*]u
 
         const buf = new_node.allocatedSliceUnsafe()[@sizeOf(Node)..];
         const aligned_index = alignedIndex(buf.ptr, 0, alignment);
-        assert(new_node.size >= @sizeOf(Node) + aligned_index + n);
+        assert(new_node.size.toInt() >= @sizeOf(Node) + aligned_index + n);
 
         new_node.end_index = aligned_index + n;
         new_node.next = first_node;
@@ -649,6 +668,7 @@ test "reset while retaining a buffer" {
     try std.testing.expect(arena_allocator.state.used_list.?.next != null);
 
     // This retains the first allocated buffer
-    try std.testing.expect(arena_allocator.reset(.{ .retain_with_limit = 1 }));
+    try std.testing.expect(arena_allocator.reset(.{ .retain_with_limit = 2 }));
     try std.testing.expect(arena_allocator.state.used_list.?.next == null);
+    try std.testing.expectEqual(2, arena_allocator.queryCapacity());
 }