std: ArenaAllocator tries to resize before allocating

Closes ziglang#5116

lib/std/heap/arena_allocator.zig

@@ -75,13 +75,29 @@ pub const ArenaAllocator = struct {
             const adjusted_addr = mem.alignForward(addr, ptr_align);
             const adjusted_index = self.state.end_index + (adjusted_addr - addr);
             const new_end_index = adjusted_index + n;
-            if (new_end_index > cur_buf.len) {
-                cur_node = try self.createNode(cur_buf.len, n + ptr_align);
-                continue;
+
+            if (new_end_index <= cur_buf.len) {
+                const result = cur_buf[adjusted_index..new_end_index];
+                self.state.end_index = new_end_index;
+                return result;
+            }
+
+            // Try to grow the buffer in-place
+            if (self.child_allocator.resizeFn(
+                self.child_allocator,
+                cur_node.data,
+                @alignOf(BufNode),
+                new_end_index,
+                1,
+                @returnAddress(),
+            )) |new_size| {
+                assert(new_size >= new_end_index);
+            } else |err| switch (err) {
+                error.OutOfMemory => {
+                    // Allocate a new node if that's not possible
+                    cur_node = try self.createNode(cur_buf.len, n + ptr_align);
+                },
             }
-            const result = cur_buf[adjusted_index..new_end_index];
-            self.state.end_index = new_end_index;
-            return result;
         }
     }