refactor: use cfg-aliases for feature flags

Introduces aliases for each feature flag as well as for the main
combinations of http version and client/server roles. This makes it
significantly easier to understand which feature flags are required on
any given line as the aliases require less parsing of nested `cfg`
expressions.

Author: joshka

Cargo.toml

@@ -240,3 +240,6 @@ required-features = ["full"]
 name = "server"
 path = "tests/server.rs"
 required-features = ["full"]
+
+[build-dependencies]
+cfg_aliases = "0.2.1"

build.rs

@@ -0,0 +1,25 @@
+use cfg_aliases::cfg_aliases;
+
+fn main() {
+    cfg_aliases! {
+        http1: { feature = "http1" },
+        http2: { feature = "http2" },
+
+        client : { feature = "client" },
+        server : { feature = "server" },
+
+        ffi: { feature = "ffi" },
+        full: { feature = "full" },
+        nightly: { feature = "nightly" },
+        runtime: { feature = "runtime" },
+        tracing: { feature = "tracing" },
+
+        http_client: { all(any(http1, http2), client) },
+        http1_client: { all(http1, client) },
+        http2_client: { all(http2, client) },
+
+        http_server: { all(any(http1, http2), server) },
+        http1_server: { all(http1, server) },
+        http2_server: { all(http2, server) },
+    }
+}

examples/hello-http2.rs

@@ -3,7 +3,7 @@
 
 use http_body_util::Full;
 use hyper::body::Bytes;
-#[cfg(feature = "server")]
+#[cfg(server)]
 use hyper::server::conn::http2;
 use hyper::service::service_fn;
 use hyper::{Request, Response};
@@ -19,7 +19,7 @@ use support::TokioIo;
 
 // An async function that consumes a request, does nothing with it and returns a
 // response.
-#[cfg(feature = "server")]
+#[cfg(server)]
 async fn hello(_: Request<hyper::body::Incoming>) -> Result<Response<Full<Bytes>>, Infallible> {
     Ok(Response::new(Full::new(Bytes::from("Hello, World!"))))
 }
@@ -42,7 +42,7 @@ where
     }
 }
 
-#[cfg(feature = "server")]
+#[cfg(server)]
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
     pretty_env_logger::init();
@@ -83,7 +83,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
     }
 }
 
-#[cfg(not(feature = "server"))]
+#[cfg(not(server))]
 fn main() {
     panic!("This example requires the 'server' feature to be enabled");
 }