Use `zeroize_derive` feature instead of impl zeroize

Cargo.toml

@@ -46,7 +46,7 @@ rand_core = "0.4.0"
 unicode-normalization = { version = "=0.1.9", optional = true }
 rand = { version = "0.6.0", optional = true }
 serde = { version = "1.0", default-features = false, optional = true }
-zeroize = {version = "1.2", optional = true}
+zeroize = {version = "1.2", features = ["zeroize_derive"], optional = true}
 
 [dev-dependencies]
 rand = { version = "0.6.0", optional = false }

src/language/mod.rs

@@ -55,6 +55,12 @@ pub enum Language {
 	Spanish,
 }
 
+impl Default for Language {
+	fn default() -> Self {
+		Language::English
+	}
+}
+
 impl Language {
 	/// The list of supported languages.
 	/// Language support is managed by compile features.

src/lib.rs

@@ -157,6 +157,7 @@ impl error::Error for Error {}
 ///
 /// Supported number of words are 12, 18 and 24.
 #[derive(Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
+#[cfg_attr(feature = "zeroize", derive(Zeroize), zeroize(drop))]
 pub struct Mnemonic {
 	/// The language the mnemonic.
 	lang: Language,
@@ -166,11 +167,7 @@ pub struct Mnemonic {
 }
 
 #[cfg(feature = "zeroize")]
-impl Zeroize for Mnemonic {
-	fn zeroize(&mut self) {
-		self.words.zeroize()
-	}
-}
+impl zeroize::DefaultIsZeroes for Language {}
 
 serde_string_impl!(Mnemonic, "a BIP-39 Mnemonic Code");