Move the unsize feature behind a nightly Cargo feature

This should fully stabilize the `wasm-bindgen` crate, preparing us for stable
Rust!

.travis.yml

@@ -56,6 +56,7 @@ matrix:
         - cargo test
         # Run the main body of the test suite
         - cargo test --target wasm32-unknown-unknown
+        - cargo test --target wasm32-unknown-unknown --features nightly
         # Rerun the test suite but disable `--debug` in generated JS
         - WASM_BINDGEN_NO_DEBUG=1 cargo test --target wasm32-unknown-unknown
         # Make sure our serde tests work

Cargo.toml

@@ -21,6 +21,7 @@ default = ["spans", "std"]
 spans = ["wasm-bindgen-macro/spans"]
 std = []
 serde-serialize = ["serde", "serde_json", "std"]
+nightly = []
 
 # This is only for debugging wasm-bindgen! No stability guarantees, so enable
 # this at your own peril!

examples/closures/Cargo.toml

@@ -7,5 +7,5 @@ authors = ["Alex Crichton <alex@alexcrichton.com>"]
 crate-type = ["cdylib"]
 
 [dependencies]
-wasm-bindgen = { path = "../.." }
+wasm-bindgen = { path = "../..", features = ['nightly'] }
 js-sys = { path = "../../crates/js-sys" }

src/closure.rs

@@ -7,6 +7,7 @@
 #![allow(const_err)] // FIXME(rust-lang/rust#52603)
 
 use std::cell::UnsafeCell;
+#[cfg(feature = "nightly")]
 use std::marker::Unsize;
 use std::mem::{self, ManuallyDrop};
 use std::prelude::v1::*;
@@ -90,6 +91,7 @@ impl<T> Closure<T>
     ///
     /// This is unfortunately pretty restrictive for now but hopefully some of
     /// these restrictions can be lifted in the future!
+    #[cfg(feature = "nightly")]
     pub fn new<F>(t: F) -> Closure<T>
         where F: Unsize<T> + 'static
     {

src/lib.rs

@@ -5,9 +5,9 @@
 //! this crate and this crate also provides JS bindings through the `JsValue`
 //! interface.
 
-#![feature(unsize)]
 #![no_std]
 #![doc(html_root_url = "https://docs.rs/wasm-bindgen/0.2")]
+#![cfg_attr(feature = "nightly", feature(unsize))]
 
 #[cfg(feature = "serde-serialize")]
 extern crate serde;

tests/wasm/closures.rs

@@ -1,3 +1,5 @@
+#![cfg(feature = "nightly")]
+
 use wasm_bindgen_test::*;
 use wasm_bindgen::prelude::*;
 use std::cell::Cell;