Add util/mem to zero out memory on drop. (#8356)

* Add `util/mem` to zero out memory on drop.

* Remove nonsense.

* Remove `Into` impls for `Memzero`.

* Update ethereum-types and remove H256Mut.

util/mem/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "mem"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+
+[dependencies]

util/mem/src/lib.rs

@@ -0,0 +1,57 @@
+// Copyright 2018 Parity Technologies (UK) Ltd.
+// This file is part of Parity.
+
+// Parity is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity.  If not, see <http://www.gnu.org/licenses/>.
+
+use std::ops::{Deref, DerefMut};
+use std::ptr;
+
+/// Wrapper to zero out memory when dropped.
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Memzero<T: AsMut<[u8]>> {
+	mem: T,
+}
+
+impl<T: AsMut<[u8]>> From<T> for Memzero<T> {
+	fn from(mem: T) -> Memzero<T> {
+		Memzero { mem }
+	}
+}
+
+impl<T: AsMut<[u8]>> Drop for Memzero<T> {
+	fn drop(&mut self) {
+		let n = self.mem.as_mut().len();
+		let p = self.mem.as_mut().as_mut_ptr();
+		for i in 0..n {
+			unsafe {
+				ptr::write_volatile(p.offset(i as isize), 0)
+			}
+		}
+	}
+}
+
+impl<T: AsMut<[u8]>> Deref for Memzero<T> {
+	type Target = T;
+
+	fn deref(&self) -> &Self::Target {
+		&self.mem
+	}
+}
+
+impl<T: AsMut<[u8]>> DerefMut for Memzero<T> {
+	fn deref_mut(&mut self) -> &mut Self::Target {
+		&mut self.mem
+	}
+}
+