Add support for Custom RNGs

Author: josephlr

src/custom.rs

@@ -0,0 +1,31 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! An implementation which calls out to an externally defined function.
+use crate::Error;
+
+/// Reister a function to be invoked by getrandom on custom targets. This macro
+/// is only avalible on custom targets to prevent registering a function that
+/// will not be called.
+#[macro_export]
+macro_rules! register_custom_getrandom {
+    ($path:path) => {
+        #[no_mangle]
+        fn __getrandom_custom(dest: &mut [u8]) -> Result<(), Error> {
+            $path(dest)
+        }
+    };
+}
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    extern "Rust" {
+        #[allow(improper_ctypes)] // See rust-lang/rust#64593
+        fn __getrandom_custom(dest: &mut [u8]) -> Result<(), Error>;
+    }
+    unsafe { __getrandom_custom(dest) }
+}

src/lib.rs

@@ -230,6 +230,8 @@ cfg_if! {
                   target_env = "sgx",
               )))] {
         #[path = "rdrand.rs"] mod imp;
+    } else if #[cfg(feature = "custom")] {
+        #[path = "custom.rs"] mod imp;
     } else {
         compile_error!("\
             target is not supported, for more information see: \