Fix for newest breakage, closes #3

riscv-rt/Cargo.toml

@@ -11,3 +11,7 @@ license = "ISC"
 [dependencies]
 r0 = "^0.2.1"
 riscv = { path = "../riscv" }
+
+[build-dependencies]
+rustc_version = "0.2.1"
+chrono = "0.4.0"

riscv-rt/build.rs

@@ -1,10 +1,20 @@
+extern crate chrono;
+extern crate rustc_version;
+
 // NOTE: Adapted from cortex-m/build.rs
 use std::env;
 use std::fs::File;
 use std::io::Write;
 use std::path::PathBuf;
 
+use chrono::NaiveDate;
+
 fn main() {
+    let meta = rustc_version::version_meta().unwrap();
+	// newest nightlies don't need 'extern crate compiler_builtins'
+    if meta.commit_date.unwrap().parse::<NaiveDate>().unwrap() < NaiveDate::from_ymd(2018, 04, 07) {
+        println!("cargo:rustc-cfg=needs_cb")
+    }
     // Put the linker script somewhere the linker can find it
     let out = &PathBuf::from(env::var_os("OUT_DIR").unwrap());
     File::create(out.join("link.x"))

riscv-rt/src/lib.rs

@@ -160,7 +160,7 @@
 #![deny(missing_docs)]
 #![deny(warnings)]
 #![feature(asm)]
-#![feature(compiler_builtins_lib)]
+#![cfg_attr(needs_cb, feature(compiler_builtins_lib))]
 #![feature(const_fn)]
 #![feature(global_asm)]
 #![feature(lang_items)]
@@ -168,6 +168,7 @@
 #![feature(naked_functions)]
 #![feature(used)]
 
+#[cfg(needs_cb)]
 extern crate compiler_builtins;
 extern crate riscv;
 extern crate r0;