Merge #17

17: Derive various num traits for newtypes r=cuviper a=asayers

This PR adds deriving macros for the following traits:

* FromPrimitive
* ToPrimitive
* NumOps<Self, Self>
* NumCast
* Zero
* One
* Num
* Float

These macros only work when applied to a newtype where the inner type already
implements the trait in question.  They produce the obvious
"unwrap-call-rewrap" implementation.

The `FromPrimative` and `ToPrimative` macros now handle both newtypes and the
enums that they used to handle.

The odd one out is `NumOps`, for two reasons.  First, `NumOps` is just a trait
alias for `Add + Sub + Mul + Div + Rem`, so those are the traits which the
macro generates impls for.  I suppose it's not a great user experience to say
`#[derive(Foo)]` and end up with `impl Bar`, but I think in this case it's just
about OK.  After all, the user needs only to look at the definition of `NumOps`
to know that this is only possible behaviour - there's no other way to get an
impl for `NumOps`.

The other reason this one is strange is that when the user says
`#[derive(NumOps)]`, the macro decides that what they want is an impl for
`NumOps<Rhs=Self, Output=Self>`.  I think this makes sense though; these are
the default type parameters, so when you say `NumOps` unqualified this is what
it means.

As you can probably tell, my objective here was to get to `Float`.  That's why
this PR only provides macros for a subset of the traits in `num_traits`.  Our
codebase has a bunch of newtyped `f64`s, and I don't want people unwrapping
them all the time to work with them.

### To-do

- [x] docs
- [x] tests
- [x] RELEASES entry

Co-authored-by: Alex Sayers <alex.sayers@gmail.com>
Co-authored-by: Josh Stone <cuviper@gmail.com>

Author: 3 people

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()
+}