solidity/docs/ir/ir-breaking-changes.rst

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Solidity IR-based Codegen Changes
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This section highlights the main differences between the old and the IR-based codegen,
along with the reasoning behind the changes and how to update affected code.

Semantic Only Changes
=====================

This section lists the changes that are semantic-only, thus potentially
hiding new and different behavior in existing code.

 * When storage structs are deleted, every storage slot that contains a member of the struct is set to zero entirely. Formally, padding space was left untouched.
Consequently, if the padding space within a struct is used to store data (e.g. in the context of a contract upgrade), you have to be aware that ``delete`` will now also clear the added member (while it wouldn't have been cleared in the past).

::
    // SPDX-License-Identifier: GPL-3.0
    pragma solidity >0.7.0;

    contract C {
        struct S {
            uint64 y;
            uint64 z;
        }
        S s;
        function f() public {
            // ...
            delete s;
            // s occupies only first 16 bytes of the 32 bytes slot
            // delete will write zero to the full slot
        }
    }

We have the same behavior for implicit delete, for example when array of structs is shortened.