Improve error message when futures-test does not have std available

Author: Nemo157

futures-test/Cargo.toml

@@ -22,3 +22,7 @@ pin-utils = { version = "0.1.0-alpha.4", default-features = false }
 
 [dev-dependencies]
 futures-preview = { version = "0.3.0-alpha.2", path = "../futures", default-features = false, features = ["std"] }
+
+[features]
+default = ["std"]
+std = ["futures-core-preview/std", "futures-util-preview/std", "futures-executor-preview/std"]

futures-test/src/lib.rs

@@ -11,16 +11,24 @@
     html_root_url = "https://rust-lang-nursery.github.io/futures-doc/0.3.0-alpha.5/futures_test"
 )]
 
+#[cfg(not(feature = "std"))]
+compile_error!("`futures-test` must have the `std` feature activated, this is a default-active feature");
+
 #[doc(hidden)]
+#[cfg(feature = "std")]
 pub use std as std_reexport;
 
 #[doc(hidden)]
+#[cfg(feature = "std")]
 pub extern crate futures_core as futures_core_reexport;
 
 #[macro_use]
 #[doc(hidden)]
+#[cfg(feature = "std")]
 pub mod assert;
 
+#[cfg(feature = "std")]
 pub mod task;
 
+#[cfg(feature = "std")]
 pub mod future;