Declaring struct at null memory location

[zemse]

Abstract

Currently, memory is allocated always when someone is declaring a struct. Following is an example when someone might not want this behaviour.

struct MyStruct {
  uint256 a;
}

function run() {
  // fmp: 0x80
  bytes memory data;
  // fmp: 0x80, data: 0x60
  data = abi.encode(1); // allocates 2 words on memory i.e. len and actual data word
  // fmp: 0xC0, data: 0x80
  MyStruct memory v;
  // fmp: 0xE0, data: 0x80, v: 0xC0
  assembly { v := add(data, 0x20) }
  // fmp: 0xE0, data: 0x80, v: 0xA0
}

Motivation

In some cases, the dev knows that a certain memory slice will be always immutable (probably because the original variable will be dropped). However, the struct declaration advances free memory pointer on declaration. If there would be a way to prevent that, it would be great.

Specification

MyStruct memory v = null;
assembly { v := add(data, 0x20) }

A keyword null can be introduced in Solidity, which would make the struct variable point to 0x00 memory location.

Backwards Compatibility

None to my knowledge.