Use cfg instead of feature

quinn-udp/Cargo.toml

@@ -18,8 +18,6 @@ default = ["tracing", "log"]
 # Configure `tracing` to log events via `log` if no `tracing` subscriber exists.
 log = ["tracing/log"]
 direct-log = ["dep:log"]
-# Use private Apple APIs to send multiple packets in a single syscall.
-fast-apple-datapath = []
 
 [dependencies]
 libc = "0.2.158"

quinn-udp/build.rs

@@ -1,6 +1,11 @@
 use cfg_aliases::cfg_aliases;
 
 fn main() {
+    if std::env::var("CARGO_CFG_QUINN_UDP_APPLE_FAST_DATAPATH").is_ok() {
+        // Use private Apple APIs to send multiple packets in a single syscall.
+        println!("cargo:rustc-cfg=quinn_udp_apple_fast_datapath");
+    };
+
     // Setup cfg aliases
     cfg_aliases! {
         // Platforms
@@ -20,7 +25,7 @@ fn main() {
             )
         },
         // Convenience aliases
-        apple_fast: { all(apple, feature = "fast-apple-datapath") },
-        apple_slow: { all(apple, not(feature = "fast-apple-datapath")) },
+        apple_fast: { all(apple, quinn_udp_apple_fast_datapath) },
+        apple_slow: { all(apple, not(quinn_udp_apple_fast_datapath)) },
     }
 }