Select future-core featue according to feature

Cargo.toml

@@ -38,16 +38,17 @@ docs = ["attributes", "unstable", "default"]
 unstable = ["std", "broadcaster", "futures-timer"]
 attributes = ["async-attributes"]
 std = [
+  "alloc",
   "crossbeam-utils",
+  "futures-core/std",
   "futures-io",
   "memchr",
   "once_cell",
   "pin-utils",
   "slab",
-  "alloc",
 ]
 alloc = [
-  "futures-core",
+  "futures-core/alloc",
   "pin-project-lite",
 ]
 
@@ -58,7 +59,7 @@ broadcaster = { version = "1.0.0", optional = true }
 crossbeam-channel = { version = "0.4.0", optional = true }
 crossbeam-deque = { version = "0.7.2", optional = true }
 crossbeam-utils = { version = "0.7.0", optional = true }
-futures-core = { version = "0.3.1", optional = true }
+futures-core = { version = "0.3.1", optional = true, default-features = false }
 futures-io = { version = "0.3.1", optional = true }
 futures-timer = { version = "2.0.2", optional = true }
 kv-log-macro = { version = "1.0.4", optional = true }