const new

build.rs

@@ -4,11 +4,17 @@ use std::env;
 
 fn main() {
     let ac = autocfg::new();
+
     if ac.probe_type("i128") {
         println!("cargo:rustc-cfg=has_i128");
     } else if env::var_os("CARGO_FEATURE_I128").is_some() {
         panic!("i128 support was not detected!");
     }
 
+    // autocfg doesn't have a direct way to probe for `const fn` yet.
+    if ac.probe_rustc_version(1, 31) {
+        autocfg::emit("has_const_fn");
+    }
+
     autocfg::rerun_path(file!());
 }

src/lib.rs

@@ -92,13 +92,23 @@ pub struct Complex<T> {
 pub type Complex32 = Complex<f32>;
 pub type Complex64 = Complex<f64>;
 
-impl<T: Clone + Num> Complex<T> {
+impl<T> Complex<T> {
+    #[cfg(has_const_fn)]
+    /// Create a new Complex
+    #[inline]
+    pub const fn new(re: T, im: T) -> Self {
+        Complex { re: re, im: im }
+    }
+
+    #[cfg(not(has_const_fn))]
     /// Create a new Complex
     #[inline]
     pub fn new(re: T, im: T) -> Self {
         Complex { re: re, im: im }
     }
+}
 
+impl<T: Clone + Num> Complex<T> {
     /// Returns imaginary unit
     #[inline]
     pub fn i() -> Self {