bugc: copy structs and strings between storage and memory

[gnidan]

This lets a BUG program copy a struct from storage, change it in memory, and write it back, as Arcade does with let player: Player = players[msg.sender]; ... players[msg.sender] = player;, and gives a struct local a pointer to its fields.

It fixes two silent miscompiles:

• let m: string = motd; loaded the slot's word and used it as a memory address. A string or bytes read from storage as a whole is now copied to memory: the length, then the data, from either the short (in-slot) or long (from keccak256(slot)) encoding.
• players[msg.sender] = player; wrote the struct's memory address into one slot. A struct in memory assigned to storage is now written field by field, each packed into its bytes in its slot, and a string or bytes field is copied as Solidity encodes it, with the old value's unused data words cleared (as in bugc: clear unused data words when a storage string shrinks #370).

Also:

• A struct copied from storage to memory may now have string and bytes fields (each field's word holds the address of its own copy). Nested structs copy recursively in both directions.
• A struct field of a struct in storage now takes all of its struct's slots. Before, it took one, so a multi-slot nested struct overlapped the fields after it.
• Copying an array between storage and memory is now a compile error. Before, it compiled to a copy of the slot's word.
• A struct local's pointer is a group: the local's word, then each field's region in its word in memory (player-score, a scalar right-aligned), and for a string field its word plus nested player-name-length and player-name-data regions, the length read from memory. The locals soundness test resolves it with @ethdebug/pointers at every step of a real trace, at every optimization level.

[github-actions]

PR Preview Action v1.8.1
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2026-10-09 05:57 UTC