Add build constants to reduce cfg expansion

build.rs

@@ -1,3 +1,5 @@
+use std::env;
+
 fn main() {
     let cfg = match autocfg::AutoCfg::new() {
         Ok(cfg) => cfg,
@@ -22,4 +24,81 @@ fn main() {
     if !cfg.probe_rustc_version(1, 53) {
         autocfg::emit("polling_no_unsupported_error_kind");
     }
+
+    // Probe for the target_os.
+    let target_os = TargetOs::new();
+
+    // kqueue is supported for most BSD-derives OSes.
+    let bsdlike = &[
+        "macos",
+        "ios",
+        "tvos",
+        "watchos",
+        "freebsd",
+        "netbsd",
+        "dragonfly",
+        "openbsd"
+    ];
+
+    // We fall back to poll() on these platforms.
+    let poll_fallback = &[
+        "vxworks",
+        "fuchsia",
+        "horizon"
+    ];
+
+    // If we're forced to use the polling fallback, use that and return.
+    if has_cfg("polling_test_poll_backend") {
+        return;
+    }
+
+    // epoll is supported on Linux and Android
+    if target_os.is("linux") || target_os.is("android") {
+        autocfg::emit("polling_epoll");
+    }
+
+    // Event ports are supported on illumos and Solaris
+    else if target_os.is("illumos") || target_os.is("solaris") {
+        autocfg::emit("polling_event_port");
+    } 
+
+    // kqueue is supported on most BSD-derives OSes.
+    else if bsdlike.iter().any(|os| target_os.is(os)) {
+        autocfg::emit("polling_kqueue");
+    }
+
+    // If we're not on a supported platform, fall back to poll().
+    else if poll_fallback.iter().any(|os| target_os.is(os)) || has_cfg("unix") {
+        autocfg::emit("polling_poll");
+    }
+
+    // Otherwise, check for windows.
+    else if target_os.is("windows") {
+        autocfg::emit("polling_iocp");
+    }
+}
+
+/// Tell whether the `cfg` directive is set.
+fn has_cfg(name: &str) -> bool {
+    let env_check = format!("CARGO_FEATURE_{}", cfg_name(name));
+    env::var_os(env_check).is_some()
+}
+
+struct TargetOs(String);
+
+impl TargetOs {
+    fn new() -> Self {
+        let target_os = env::var_os("CARGO_CFG_TARGET_OS")
+            .map(|s| s.to_string_lossy().into_owned())
+            .unwrap_or_else(|| "".to_string());
+        Self(target_os)
+    }
+
+    fn is(&self, os: &str) -> bool {
+        self.0.split(',').any(|s| s == os)
+    }
+}
+
+fn cfg_name(name: &str) -> String {
+    name.to_uppercase().replace('-', "_").replace('"', "")
 }

src/lib.rs

@@ -83,36 +83,19 @@ cfg_if! {
     if #[cfg(polling_test_poll_backend)] {
         mod poll;
         use poll as sys;
-    } else if #[cfg(any(target_os = "linux", target_os = "android"))] {
+    } else if #[cfg(polling_epoll)] {
         mod epoll;
         use epoll as sys;
-    } else if #[cfg(any(
-        target_os = "illumos",
-        target_os = "solaris",
-    ))] {
+    } else if #[cfg(polling_event_port)] {
         mod port;
         use port as sys;
-    } else if #[cfg(any(
-        target_os = "macos",
-        target_os = "ios",
-        target_os = "tvos",
-        target_os = "watchos",
-        target_os = "freebsd",
-        target_os = "netbsd",
-        target_os = "openbsd",
-        target_os = "dragonfly",
-    ))] {
+    } else if #[cfg(polling_kqueue)] {
         mod kqueue;
         use kqueue as sys;
-    } else if #[cfg(any(
-        target_os = "vxworks",
-        target_os = "fuchsia",
-        target_os = "horizon",
-        unix,
-    ))] {
+    } else if #[cfg(polling_poll)] {
         mod poll;
         use poll as sys;
-    } else if #[cfg(target_os = "windows")] {
+    } else if #[cfg(polling_iocp)] {
         mod wepoll;
         use wepoll as sys;
     } else {
@@ -542,18 +525,9 @@ impl Poller {
 
 #[cfg(all(
     any(
-        target_os = "linux",
-        target_os = "android",
-        target_os = "illumos",
-        target_os = "solaris",
-        target_os = "macos",
-        target_os = "ios",
-        target_os = "tvos",
-        target_os = "watchos",
-        target_os = "freebsd",
-        target_os = "netbsd",
-        target_os = "openbsd",
-        target_os = "dragonfly",
+        polling_epoll,
+        polling_kqueue,
+        polling_event_port
     ),
     not(polling_test_poll_backend),
 ))]
@@ -595,7 +569,7 @@ mod raw_fd_impl {
     }
 }
 
-#[cfg(windows)]
+#[cfg(polling_iocp)]
 #[cfg_attr(docsrs, doc(cfg(windows)))]
 mod raw_handle_impl {
     use crate::Poller;