foo.diff

diff --git a/src/rust/tor_allocate/build.rs b/src/rust/tor_allocate/build.rs
new file mode 100644
index 000000000..c97fd55ce
--- /dev/null
+++ b/src/rust/tor_allocate/build.rs
@@ -0,0 +1,34 @@
+use std::env;
+use std::process::Command;
+use std::str;
+
+fn main() {
+    if let Some(version) = rustc_minor_version() {
+        if version >= 28 {
+            println!("cargo:rustc-cfg=global_allocator");
+        }
+    }
+}
+
+fn rustc_minor_version() -> Option<u32> {
+    macro_rules! otry {
+        ($e:expr) => {
+            match $e {
+                Some(e) => e,
+                None => return None,
+            }
+        };
+    }
+
+    let rustc = otry!(env::var_os("RUSTC"));
+    let output = otry!(Command::new(rustc).arg("--version").output().ok());
+    let version = otry!(str::from_utf8(&output.stdout).ok());
+    let mut pieces = version.split('.');
+
+    if pieces.next() != Some("rustc 1") {
+        return None;
+    }
+
+    otry!(pieces.next()).parse().ok()
+}
+
diff --git a/src/rust/tor_allocate/lib.rs b/src/rust/tor_allocate/lib.rs
index 1cfa0b517..0f3f00dd3 100644
--- a/src/rust/tor_allocate/lib.rs
+++ b/src/rust/tor_allocate/lib.rs
@@ -11,10 +11,12 @@
 
 extern crate libc;
 
+#[cfg(global_allocator)]
 use std::alloc::System;
 
 mod tor_allocate;
 pub use tor_allocate::*;
 
 #[global_allocator]
+#[cfg(global_allocator)]
 static A: System = System;