custom: Expose cpurand directly

This allows users to get CPU-based randomness directly, even on
platforms that have a normal getrandom implementation.

Signed-off-by: Joe Richey <[email protected]>

Author: josephlr

custom/cpurand/src/lib.rs

@@ -21,4 +21,13 @@ cfg_if! {
     }
 }
 
-register_custom_getrandom!(imp::getrandom_inner);
+/// Fill `dest` with random bytes via the CPU's randomness instructions.
+///
+/// This function returns an error on any failure, including partial reads. This
+/// can happen if your CPU does not support the required instructions, or if the
+/// CPU has encountered an internal failure.
+pub fn cpurand(dest: &mut [u8]) -> Result<(), Error> {
+    imp::getrandom_inner(dest)
+}
+
+register_custom_getrandom!(cpurand);