Reject word sizes struct cannot express

[zardus]

THIS MESSAGE WAS GENERATED BY AN AUTOMATED PROCESS

Arch.struct_fmt() returned "Z" for a three-byte word, so every 24-bit p-code architecture — the PIC, HCS-12 and extended AVR languages among them — handed its callers a format string that raises "bad char in struct format" as soon as anything uses it.

struct has integer format characters for one, two, four and eight bytes and no way to name a three-byte integer, so the size really is invalid and this function should say so rather than invent a character for it.

The regression checks that ArchPcode("HCS-12:BE:24:default").struct_fmt() raises, and that every size struct can express still round-trips through calcsize.

angr/cle#721 is the other half of this. Validation: #364 (comment).

[zardus]

THIS MESSAGE WAS GENERATED BY AN AUTOMATED PROCESS

Validation record for head 6faa1ee404c78c908317aac60868f35955b2ea02 against baseline da171ca0dd8eec1e16dd9c4f04576b2c958e2aea.

• Reproducer: python -c "import archinfo, struct; print(archinfo.ArchPcode('HCS-12:BE:24:default').struct_fmt())" — prints >Z on baseline, and struct.calcsize('>Z') then raises bad char in struct format; raises ValueError on head
• Regression: python -m pytest tests/test_pcode.py::TestArchPcode::test_struct_fmt_24bit tests/test_struct_fmt.py — both fail on baseline (struct_fmt() returns a format character instead of raising), pass on head
• Full suite: python -m pytest tests/ — 20 passed
• Lint/type: pylint and pyright scored per changed file against the merge base, the way the hosted jobs score them — archinfo/arch.py 9.70 -> 9.70, tests/test_pcode.py 10.00 -> 10.00, tests/test_struct_fmt.py new file 10.00; pyright badness 0.2227 -> 0.2222, 0.0 -> 0.0, 0.0 -> 0.0
• Pre-commit: pre-commit run --all-files — every hook passes, no rewrites
• Workspace gate: archinfo and cle suites, hooks and workspace checks under Python 3.12 — exit 0 twice on this exact state

Scope: 14 of the 183 p-code languages pypcode 4.0.1.dev0 exposes report a three-byte word — HCS-12, HCS12, HCS-12X, PIC-18, PIC-24E/F/H, dsPIC30F, dsPIC33C/E/F and the three extended AVR languages. All returned Z on baseline, regardless of input. It surfaced in a corpus sweep running CFGFast, out of Clemory.unpack_word (angr/cle#721).

Caveats: test_struct_fmt_24bit lives in TestArchPcode, which skips without pypcode — an optional extra here, so the Windows and macOS uv run pytest jobs skip it. tests/test_struct_fmt.py covers the same contract on ArchAMD64 and runs everywhere.

[angr-bot]

Corpus decompilation diffs can be found at angr/dec-snapshots@master...angr/archinfo_364