zeroize: Implement Zeroize for CString (RustCrypto#650)

This implements Zeroize for std::ffi::CString. As CString is not in
allocate but in std, a new optional std feature is added to depend
on the std library.

zeroize/Cargo.toml

@@ -24,6 +24,7 @@ default = ["alloc"]
 aarch64 = []
 alloc = []
 derive = ["zeroize_derive"]
+std = []
 
 [package.metadata.docs.rs]
 all-features = true

zeroize/src/lib.rs

@@ -233,6 +233,9 @@
 #[cfg_attr(test, macro_use)]
 extern crate alloc;
 
+#[cfg(feature = "std")]
+extern crate std;
+
 #[cfg(feature = "zeroize_derive")]
 #[cfg_attr(docsrs, doc(cfg(feature = "zeroize_derive")))]
 pub use zeroize_derive::{Zeroize, ZeroizeOnDrop};
@@ -253,6 +256,9 @@ use core::{ops, ptr, slice::IterMut, sync::atomic};
 #[cfg(feature = "alloc")]
 use alloc::{boxed::Box, string::String, vec::Vec};
 
+#[cfg(feature = "std")]
+use std::ffi::CString;
+
 /// Trait for securely erasing types from memory
 pub trait Zeroize {
     /// Zero out this object from memory using Rust intrinsics which ensure the
@@ -511,6 +517,15 @@ impl Zeroize for String {
     }
 }
 
+#[cfg(feature = "std")]
+#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
+impl Zeroize for CString {
+    fn zeroize(&mut self) {
+        let this = std::mem::take(self);
+        this.into_bytes().zeroize();
+    }
+}
+
 /// Fallible trait for representing cases where zeroization may or may not be
 /// possible.
 ///
@@ -700,6 +715,9 @@ mod tests {
     #[cfg(feature = "alloc")]
     use alloc::boxed::Box;
 
+    #[cfg(feature = "std")]
+    use std::ffi::CString;
+
     #[derive(Clone, Debug, PartialEq)]
     struct ZeroizedOnDrop(u64);
 
@@ -858,6 +876,14 @@ mod tests {
         assert!(as_vec.iter().all(|byte| *byte == 0));
     }
 
+    #[cfg(feature = "std")]
+    #[test]
+    fn zeroize_c_string() {
+        let mut cstring = CString::new("Hello, world!").expect("CString::new failed");
+        cstring.zeroize();
+        assert!(cstring.into_bytes().is_empty());
+    }
+
     #[cfg(feature = "alloc")]
     #[test]
     fn zeroize_box() {