Improve Platform and Feature Detection

- Add cfg aliases to simplify feature checks and make sure that
  platform specific features will not break when enabled on
  unsupported platforms.
- Disable X11 on MacOS. It appears it was meant to be supported, but
  never actually was. Disabling it so that MacOS builds can run with
  the X11 feature ( when desired for Linux ) without breaking.
- Make the platform::unix module Linux specific ( for now? ) as Android
  and MacOS have their own modules and all unix backends aren't
  supported for Android or MacOS yet.

surfman/Cargo.toml

@@ -14,6 +14,7 @@ build = "build.rs"
 
 [build-dependencies]
 gl_generator = "0.13"
+cfg_aliases = "0.1.0"
 
 [features]
 default = ["sm-winit"]
@@ -73,11 +74,7 @@ features = ["client", "dlopen", "egl"]
 [target.'cfg(all(unix, not(any(target_os = "macos", target_os = "android"))))'.dependencies.x11]
 version = "2.3.0"
 features = ["xlib"]
-
-[target.'cfg(any(not(unix), target_os = "macos", target_os = "android"))'.dependencies.x11]
 optional = true
-version = "2.3.0"
-features = ["xlib"]
 
 # Ensure that we have a static libEGL.lib present for linking with EGL bindings.
 [target.'cfg(target_os = "windows")'.dependencies.mozangle]

surfman/build.rs

@@ -3,11 +3,32 @@
 //! The `surfman` build script.
 
 use gl_generator::{Api, Fallbacks, Profile, Registry, StructGenerator};
+use cfg_aliases::cfg_aliases;
 use std::env;
 use std::fs::File;
 use std::path::PathBuf;
 
 fn main() {
+    // Setup aliases for #[cfg] checks
+    cfg_aliases! {
+        // Platforms
+        windows: { target_os = "windows" },
+        macos: { target_os = "macos" },
+        android: { target_os = "android" },
+        // TODO: is `target_os = "linux"` the same as the following check?
+        linux: { all(unix, not(any(macos, android))) },
+
+        // Features:
+        // Here we collect the features that are only valid on certain platforms and
+        // we add aliases that include checks for the correct platform.
+        angle: { all(windows, feature = "sm-angle") },
+        angle_builtin: { all(windows, feature = "sm-angle-builtin") },
+        angle_default: { all(windows, feature = "sm-angle-default") },
+        no_wgl: { all(windows, feature = "sm-no-wgl") },
+        wayland_default: { all(linux, feature = "sm-wayland-default") },
+        x11: { all(linux, feature = "sm-x11") },
+    }
+
     let target_os = env::var("CARGO_CFG_TARGET_OS").unwrap();
     let target_family = env::var("CARGO_CFG_TARGET_FAMILY").unwrap();
     let dest = PathBuf::from(&env::var("OUT_DIR").unwrap());

surfman/src/platform/generic/mod.rs

@@ -2,9 +2,7 @@
 //
 //! Backends that are not specific to any operating system.
 
-#[cfg(any(target_os = "android",
-          all(target_os = "windows", feature = "sm-angle"),
-          all(unix, not(target_os = "macos"))))]
+#[cfg(any(android, angle, linux))]
 pub(crate) mod egl;
 
 pub mod multi;

surfman/src/platform/mod.rs

@@ -4,26 +4,26 @@
 
 pub mod generic;
 
-#[cfg(target_os = "android")]
+#[cfg(android)]
 pub mod android;
-#[cfg(target_os = "android")]
+#[cfg(android)]
 pub use android as default;
 
-#[cfg(target_os = "macos")]
+#[cfg(macos)]
 pub mod macos;
-#[cfg(all(target_os = "macos", not(feature = "sm-x11")))]
+#[cfg(macos)]
 pub use macos::cgl as default;
-#[cfg(target_os = "macos")]
+#[cfg(macos)]
 pub use macos::system;
 
-#[cfg(unix)]
+#[cfg(linux)]
 pub mod unix;
-#[cfg(all(unix, not(target_os = "macos"), not(target_os = "android")))]
+#[cfg(linux)]
 pub use unix::default;
 
-#[cfg(target_os = "windows")]
+#[cfg(windows)]
 pub mod windows;
-#[cfg(all(target_os = "windows", feature = "sm-angle-default"))]
+#[cfg(angle_default)]
 pub use windows::angle as default;
-#[cfg(all(target_os = "windows", not(feature = "sm-angle-default")))]
+#[cfg(all(windows, not(angle_default)))]
 pub use windows::wgl as default;

surfman/src/platform/unix/mod.rs

@@ -2,12 +2,18 @@
 //
 //! Backends specific to Unix-like systems, particularly Linux.
 
-#[cfg(all(unix, not(any(target_os = "macos", target_os = "android"))))]
+// The default when x11 is enabled
+#[cfg(x11)]
 pub mod default;
-#[cfg(all(unix, not(any(target_os = "macos", target_os = "android"))))]
+
+// The default when x11 is not enabled
+#[cfg(not(x11))]
+pub use wayland as default;
+
+#[cfg(linux)]
 pub mod generic;
-#[cfg(all(unix, not(any(target_os = "macos", target_os = "android"))))]
+
+#[cfg(linux)]
 pub mod wayland;
-#[cfg(all(any(feature = "sm-x11",
-              all(unix, not(any(target_os = "macos", target_os = "android"))))))]
+#[cfg(x11)]
 pub mod x11;