Merge pull request #5459 from daira/fix-drop-warning

Avoid a warning by explicitly calling drop

Cargo.lock

@@ -815,6 +815,7 @@ dependencies = [
  "zcash_note_encryption",
  "zcash_primitives",
  "zcash_proofs",
+ "zeroize",
 ]
 
 [[package]]

Cargo.toml

@@ -49,6 +49,7 @@ zcash_note_encryption = "0.1"
 zcash_primitives = "0.5"
 zcash_proofs = "0.5"
 ed25519-zebra = "3"
+zeroize = "1.4.2"
 
 # Metrics
 hyper = { version = "=0.14.2", default-features = false, features = ["server", "tcp", "http1"] }

src/rust/src/zip339_ffi.rs

@@ -4,6 +4,7 @@ use std::{
     ffi::{CStr, CString},
     ptr, slice,
 };
+use zeroize::Zeroize;
 
 use zcash_primitives::zip339;
 
@@ -63,7 +64,9 @@ pub extern "C" fn zip339_free_phrase(phrase: *const c_char) {
     if !phrase.is_null() {
         unsafe {
             // It is correct to cast away const here; the memory is not actually immutable.
-            CString::from_raw(phrase as *mut c_char);
+            CString::from_raw(phrase as *mut c_char)
+                .into_bytes()
+                .zeroize();
         }
     }
 }