Add tensorflow_unstable feature gate

Author: adamcrume

Cargo.toml

@@ -11,3 +11,7 @@ repository = "https://github.com/google/tensorflow-rust"
 [dependencies]
 libc = "^0.2"
 libtensorflow-sys = { path = "libtensorflow-sys" }
+
+[features]
+default = []
+tensorflow_unstable = []

README.md

@@ -26,6 +26,24 @@ and $TENSORFLOW_SRC/bazel-bin/tensorflow to LD_LIBRARY_PATH.
 You may need to run `ldconfig` to reset `ld`'s cache after copying libtensorflow.so.
 
 Now run `cargo build` as usual.
+To include the unstable API (which is currently the entire API), use `--features tensorflow_unstable`.
+
+## FAQs
+
+#### Why are the docs empty?
+Run `cargo doc --features tensorflow_unstable`.
+See below.
+
+#### Why are no tests running?
+Run `cargo test --features tensorflow_unstable`.
+See below.
+
+#### Why does the compiler say that none of the API exists?
+The unstable parts of the API (which is currently the entire API) are
+feature-gated behind the feature `tensorflow_unstable` to prevent accidental
+use. See http://doc.crates.io/manifest.html#the-features-section.
+(We would prefer using an `#[unstable]` attribute, but that
+[doesn't exist](https://github.com/rust-lang/rfcs/issues/1491) yet.)
 
 ## Other
 

src/lib.rs

@@ -1,3 +1,5 @@
+#![cfg(feature = "tensorflow_unstable")]
+
 extern crate libc;
 extern crate libtensorflow_sys;