Pushable array datatype for solidity

pushable_array.sol

// SPDX-License-Identifier: UNLICENSE

pragma solidity >=0.8.0 <0.9.0;

// NOT TESTED - USE AT YOUR OWN RISK


// Supports 32 byte word types. Could be easily extended to multiword types by 
// passing in the size of the elements as well though
struct PushableArray {
    bytes self;
}

library PushArray {
    function newArray(uint8 len) internal pure returns (PushableArray memory s) {
        assembly {
            // grab free mem ptr
            s := mload(0x40)
            // update free mem ptr to 32*(len+2)+curr_free_memptr
            mstore(0x40, add(s, mul(add(0x02, len), 0x20)))
            mstore(add(0x20, s), len)
        }
    }

    function push(PushableArray memory self, uint256 elem) internal pure {
        assembly {
            if eq(mload(self), mload(add(0x20, self))) {
                revert (0,0)
            }
            mstore(
                add(
                    self, // self + (2+curr_len)*32
                    mul(
                        add(0x02, mload(self)), // 2 + curr len * 32
                        0x20
                    )
                ), elem
            )
            mstore(self, add(0x01, mload(self)))
        }
    }

    function push_unsafe(PushableArray memory self, uint256 elem) internal pure {
        assembly {
            mstore(add(self, mul(add(0x02, mload(self)), 0x20)), elem)
            mstore(self, add(0x01, mload(self)))
        }
    }

    function get(PushableArray memory self, uint8 i) internal pure returns (uint256 s) {
        assembly {
            s := mload(add(self, mul(0x20, add(0x02, i))))
        }
    } 
}

contract Push {
    using PushArray for PushableArray;

    function t2() public returns (uint256){
        PushableArray memory pa = PushArray.newArray(5);
        // unsafe pushes
        pa.push_unsafe(125);
        pa.push_unsafe(126);
        pa.push_unsafe(127);
        pa.push_unsafe(128);
        pa.push_unsafe(129);
        // one more push *would not* revert
        uint256 last = pa.get(4);
        assert(last == 129);
        return last;
    }

    function t() public returns (uint256){
        PushableArray memory pa = PushArray.newArray(5);
        // safe pushes
        pa.push(125);
        pa.push(126);
        pa.push(127);
        pa.push(128);
        pa.push(129);
        // one more push would revert
        uint256 last = pa.get(4);
        assert(last == 129);
        return last;
    }
}