Derive various num traits for newtypes

Cargo.toml

@@ -10,6 +10,7 @@ name = "num-derive"
 repository = "https://github.com/rust-num/num-derive"
 version = "0.2.2"
 readme = "README.md"
+build = "build.rs"
 
 [dependencies]
 num-traits = "0.2"

RELEASES.md

@@ -1,3 +1,10 @@
+# Unreleased
+
+- [Added newtype deriving][17] for `FromPrimitive`, `ToPrimitive`,
+  `NumOps<Self, Self>`, `NumCast`, `Zero`, `One`, `Num`, and `Float`.
+
+[17]: https://github.com/rust-num/num-derive/pull/17
+
 # Release 0.2.2 (2018-05-22)
 
 - [Updated dependencies][14].

build.rs

@@ -0,0 +1,35 @@
+use std::env;
+use std::io::Write;
+use std::process::{Command, Stdio};
+
+fn main() {
+    if probe("fn main() { 0i128; }") {
+        println!("cargo:rustc-cfg=has_i128");
+    } else if env::var_os("CARGO_FEATURE_I128").is_some() {
+        panic!("i128 support was not detected!");
+    }
+}
+
+/// Test if a code snippet can be compiled
+fn probe(code: &str) -> bool {
+    let rustc = env::var_os("RUSTC").unwrap_or_else(|| "rustc".into());
+    let out_dir = env::var_os("OUT_DIR").expect("environment variable OUT_DIR");
+
+    let mut child = Command::new(rustc)
+        .arg("--out-dir")
+        .arg(out_dir)
+        .arg("--emit=obj")
+        .arg("-")
+        .stdin(Stdio::piped())
+        .spawn()
+        .expect("rustc probe");
+
+    child
+        .stdin
+        .as_mut()
+        .expect("rustc stdin")
+        .write_all(code.as_bytes())
+        .expect("write rustc stdin");
+
+    child.wait().expect("rustc probe").success()
+}