Add mem tracking primitives

src/codec.rs

@@ -85,6 +85,11 @@ pub trait Input {
 	/// This is called when decoding reference-based type is finished.
 	fn ascend_ref(&mut self) {}
 
+	/// Hook that is called before allocating memory on the heap.
+	fn on_before_alloc_mem(&mut self, _size: usize) -> Result<(), Error> {
+		Ok(())
+	}
+
 	/// !INTERNAL USE ONLY!
 	///
 	/// Decodes a `bytes::Bytes`.

src/lib.rs

@@ -57,6 +57,7 @@ mod joiner;
 mod keyedvec;
 #[cfg(feature = "max-encoded-len")]
 mod max_encoded_len;
+mod mem_tracking;
 
 #[cfg(feature = "std")]
 pub use self::codec::IoReader;

src/mem_tracking.rs

@@ -0,0 +1,20 @@
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use crate::Decode;
+
+/// Marker trait used for identifying types that call the mem tracking hooks exposed by `Input`
+/// while decoding.
+pub trait DecodeWithMemTracking: Decode {}