bugc: string and bytes calldata parameters

[gnidan]

A function parameter may now be string calldata or bytes calldata, so a contract can hand a reference into its call data to a function without copying it:

define {
  function join(name: string calldata) {
    players[msg.sender].name = name;
  };
}
code {
  let offset = msg.data[4:36] as bytes32 as uint256;
  let n = msg.data[4 + offset:36 + offset] as bytes32 as uint256;
  let name: string calldata =
    msg.data[36 + offset:36 + offset + n] as string calldata;
  join(name);
}

#367 and #379 limited these types to a let or a cast (TYPE016) to keep their scope small; nothing in the calling convention needs it. A calldata reference is one word (offset in the high 128 bits, length in the low 128), so it passes as any word argument does. The call site now skips the copy to memory when the parameter is in calldata, and a parameter of the plain string or bytes type still gets a copy. The argument must itself be in calldata, with the same rules as a let of that type: a string in memory, or bytes calldata without a cast, is a type error for a string calldata parameter. A return type, struct field, storage variable or nested type of calldata is still TYPE016.

The parameter's IR type is the calldata reference, so its debug info is a calldata let's: the parameter's word, then a calldata region whose offset and length are computed from it.

Tests:

• Type checking accepts both parameter types, passing them on to another function, and rejects a calldata return type, a nested calldata parameter type, and a memory or mis-typed argument.
• At O0–O3, a string calldata and a bytes calldata parameter of 0, 31, 32, 33 and 65 bytes are stored and indexed, through a leaf function (inlined at O2 and O3) and through a function with two returns that passes its parameter on (a real call at every level). The IR has calldata parameter types and no extra copy.
• Soundness: the locals harness runs both shapes and, at every step that lists a parameter, reads its pointer against the trace. The pointer must decode to the string's bytes in calldata, and each parameter must be located at every level.

The BUG case study and the changelog are updated.

[github-actions]

PR Preview Action v1.8.1
Preview removed because the pull request was closed.
2026-10-15 00:40 UTC