traits: Introduce std feature

This makes it possible to build `traits` without `std`. For this a new
trait `BasicFloat` was introduced, implementing some basic functionality
that works with `core`. Most notably this is lacking functions like
`cos`, `sin`, etc.

`Float` is not available without `std`.

Refs rust-num#216.

traits/Cargo.toml

@@ -10,3 +10,7 @@ name = "num-traits"
 version = "0.1.37"
 
 [dependencies]
+
+[features]
+default = ["std"]
+std = []

traits/src/bounds.rs

@@ -1,7 +1,7 @@
-use std::{usize, u8, u16, u32, u64};
-use std::{isize, i8, i16, i32, i64};
-use std::{f32, f64};
-use std::num::Wrapping;
+use core::{usize, u8, u16, u32, u64};
+use core::{isize, i8, i16, i32, i64};
+use core::{f32, f64};
+use core::num::Wrapping;
 
 /// Numbers which have upper and lower bounds
 pub trait Bounded {

traits/src/cast.rs

@@ -1,5 +1,5 @@
-use std::mem::size_of;
-use std::num::Wrapping;
+use core::mem::size_of;
+use core::num::Wrapping;
 
 use identities::Zero;
 use bounds::Bounded;
@@ -218,6 +218,26 @@ impl_to_primitive_uint!(u16);
 impl_to_primitive_uint!(u32);
 impl_to_primitive_uint!(u64);
 
+// Those have to be reimplemented, because `core::num::Float` is not stable.
+
+/// Returns `true` if the number is NaN.
+#[inline]
+fn is_nan(x: f64) -> bool {
+    x != x
+}
+
+/// Returns `true` if the number is infinite.
+#[inline]
+fn is_infinite(x: f64) -> bool {
+    x == ::core::f64::INFINITY || x == ::core::f64::NEG_INFINITY
+}
+
+/// Returns `true` if the number is neither infinite or NaN.
+#[inline]
+fn is_finite(x: f64) -> bool {
+    !(is_nan(x) || is_infinite(x))
+}
+
 macro_rules! impl_to_primitive_float_to_float {
     ($SrcT:ident, $DstT:ident, $slf:expr) => (
         if size_of::<$SrcT>() <= size_of::<$DstT>() {
@@ -226,8 +246,8 @@ macro_rules! impl_to_primitive_float_to_float {
             // Make sure the value is in range for the cast.
             // NaN and +-inf are cast as they are.
             let n = $slf as f64;
-            let max_value: $DstT = ::std::$DstT::MAX;
-            if !n.is_finite() || (-max_value as f64 <= n && n <= max_value as f64) {
+            let max_value: $DstT = ::core::$DstT::MAX;
+            if !is_finite(n) || (-max_value as f64 <= n && n <= max_value as f64) {
                 Some($slf as $DstT)
             } else {
                 None
@@ -454,8 +474,8 @@ impl<T: NumCast> NumCast for Wrapping<T> {
 
 #[test]
 fn to_primitive_float() {
-    use std::f32;
-    use std::f64;
+    use core::f32;
+    use core::f64;
 
     let f32_toolarge = 1e39f64;
     assert_eq!(f32_toolarge.to_f32(), None);