M2compiler-V3/docs/summary_step3.md

V3 step 3 — composites with length-prefixed layout (planned)

Status: not started.

Scope (locked memory rule)

Every ARRAY = inline LONGCARD element count + packed elements; all arrays incl. fixed; each multi-dim level has its own header; records include headers in field offsets; SYSTEM.TSIZE covers the header; open formals = single descriptor pointer.

• ARRAY [lo..hi], multi-dim, RECORD, SET, POINTER, NIL, whole-array := (equal counts else Trap.RangeFault), HIGH/LEN (static for fixed, header read for open), index lowering base+8+(i-lo)*elemSize + bounds check.
• SymTab: ElemSize replaces slot counting; TypeSlots retired.
• Trap module introduced (range/NIL faults).

Acceptance (planned)

Layout tests (§5 of plan.md: fixed header bytes, nested headers, record offsets, open-formal HIGH, mismatched-assign trap, SYSTEM.BYTE I/O round-trip); suite green.

Files (planned)

• compiler/src/M2.atg § types growth; QbeGen aggregate emission (data $n = { l count, ... }); runtime/syslib/Trap.
• This file filled in on landing.