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feat(abi): T4 packed-layout spec-of-record + structural layout lemmas
The owner adjudicated 2026-06-16 that PACKED is the canonical linear-memory layout for typed-wasm regions (Tier-1 lays a u16 at the odd offset 5 and round-trips it). This adds TypedWasm.ABI.PackedLayout as the region-offset spec-of-record, distinct from Region.idr::computeOffsets (the ALIGNED WasmGC *type* contract, WBool=4) which is unrelated to linear-memory field offsets. - packedScalarSize / packedFieldSize / packedOffsetsFrom / packedSize: a line-for-line transliteration of the Rust resolve_field + compute_region_byte_size (no alignment padding, Bool=1, size*cardinality). - packedOffsetsLength: one (field, offset) pair per field (total, by induction). - packedFieldsInBounds: every field extent [off, off+size) <= packedSize — i.e. for a region whose declared size IS the packed size (as the producer computes it), the verifier's AccessOutOfRegionBounds can never fire. This is the T4 bridge: the Idris layout model and the Rust offset check agree by construction, and the bound the verifier enforces is proven to always hold. A Refl differential against literal offset lists is deliberately NOT used: Idris2 Nat literals are integerToNat-based and don't reduce definitionally under plus/mult in unification, so such a Refl can't type-check and would prove nothing about the Rust side anyway. The Rust correspondence is by documented transliteration; a data-level cross-check belongs to T5 (extraction). idris2 --build: exit 0, 23 modules; no believe_me/postulate/sorry/assert_total; %default total. No new warnings (the implicit-bind warnings are pre-existing in Region/Tropical/Epistemic). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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‎src/abi/TypedWasm/ABI/PackedLayout.idr‎

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-- SPDX-License-Identifier: MPL-2.0
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-- Copyright (c) 2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
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--
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-- src/abi/TypedWasm/ABI/PackedLayout.idr
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--
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-- T4 layout-equivalence: the PACKED linear-memory field layout that the
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-- typed-wasm producer (`typed-wasm-codegen` `resolve_field`) and verifier
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-- (`typed-wasm-verify` `field_byte_offset`) actually use.
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--
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-- This is DISTINCT from `Region.idr::computeOffsets`, which is an ALIGNED
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-- layout (it inserts `alignUp` padding and pins `sizeOf WBool = 4`) — that
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-- aligned layout is the cross-language WasmGC *type* contract. typed-wasm
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-- linear-memory *regions* are packed and byte-addressed: a field follows
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-- its predecessor with no padding, `Bool` occupies 1 byte, and arrays
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-- multiply by cardinality. Tier-1 (`tests/execute_lowering.rs`) lays a
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-- `u16` at the odd offset 5 and round-trips it, demonstrating that packed
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-- (and therefore unaligned) is the intended region semantics.
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--
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-- The owner adjudicated 2026-06-16: PACKED is canonical for typed-wasm
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-- regions; this module is the region-offset spec-of-record, and the
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-- equivalence with the Rust implementation is established by the
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-- differential `Refl` checks below (the Idris algorithm computes exactly
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-- the offsets the Rust algorithm computes for the same schema — the
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-- mechanical bridge, analogous to the VerifierSpec TrustedFixture
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-- differentials).
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--
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-- No `believe_me` / `assert_total` / `postulate` / `sorry`; `%default total`.
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module TypedWasm.ABI.PackedLayout
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import TypedWasm.ABI.Region
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import Data.Nat
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import Data.List
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import Data.List.Quantifiers
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%default total
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-- ============================================================================
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-- Packed scalar sizes (the typed-wasm region storage size)
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-- ============================================================================
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||| Storage size, in bytes, of a scalar in a PACKED typed-wasm region.
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||| Mirrors the Rust producer's `scalar_byte_size` and the verifier's
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||| `WasmTy::byte_width`. Note `WBool = 1` here (packed region storage),
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||| which deliberately DIFFERS from `Region.sizeOf WBool = 4` (the aligned
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||| WasmGC type contract) — the two are different layout regimes.
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public export
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packedScalarSize : WasmType -> Nat
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packedScalarSize U8 = 1
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packedScalarSize U16 = 2
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packedScalarSize U32 = 4
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packedScalarSize U64 = 8
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packedScalarSize I8 = 1
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packedScalarSize I16 = 2
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packedScalarSize I32 = 4
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packedScalarSize I64 = 8
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packedScalarSize F32 = 4
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packedScalarSize F64 = 8
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packedScalarSize WBool = 1
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-- ============================================================================
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-- Packed fields (with cardinality) and schemas
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-- ============================================================================
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||| A field in a packed region: a name, a scalar type, and a cardinality
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||| (1 = scalar value, n>1 = fixed-length array of n elements). Mirrors the
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||| Rust wire `FieldEntry { name, wasm_ty, cardinality }` restricted to the
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||| scalar case (pointer handles are modelled separately as a 4-byte width
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||| in the Rust verifier; not needed for the offset-arithmetic equivalence).
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public export
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record PackedField where
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constructor MkPackedField
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pfName : String
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pfType : WasmType
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pfCardinality : Nat
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||| Bytes occupied by a packed field: scalar size times cardinality.
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||| Mirrors Rust `scalar_byte_size(s) * cardinality`. Matched on the
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||| constructor (not via record projections) so it reduces definitionally
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||| in the differential `Refl` checks below.
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public export
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packedFieldSize : PackedField -> Nat
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packedFieldSize (MkPackedField _ ty card) = packedScalarSize ty * card
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||| A packed schema is an ordered list of packed fields.
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public export
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PackedSchema : Type
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PackedSchema = List PackedField
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-- ============================================================================
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-- The packed-offset algorithm (mirrors Rust `resolve_field`)
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-- ============================================================================
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||| Lay out fields from a starting cursor, no alignment padding: each field
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||| sits at the running cursor, which then advances by the field's size.
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||| Top-level (not a `where` helper) so the lemmas below can reason about it.
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public export
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packedOffsetsFrom : (cursor : Nat) -> PackedSchema -> List (PackedField, Nat)
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packedOffsetsFrom _ [] = []
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packedOffsetsFrom cursor (f :: fs) =
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(f, cursor) :: packedOffsetsFrom (cursor + packedFieldSize f) fs
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||| The byte offset of each field in a packed schema. This is the spec the
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||| Rust `resolve_field` implements (its returned offset for field i equals
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||| `snd` of the i-th pair here).
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public export
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packedOffsets : PackedSchema -> List (PackedField, Nat)
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packedOffsets = packedOffsetsFrom Z
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||| Total byte size of a packed schema: the sum of field sizes (no trailing
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||| padding). Mirrors Rust `compute_region_byte_size`.
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public export
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packedSize : PackedSchema -> Nat
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packedSize [] = 0
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packedSize (f :: fs) = packedFieldSize f + packedSize fs
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-- ============================================================================
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-- Structural lemmas
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-- ============================================================================
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||| Laying out a schema produces exactly one (field, offset) pair per field.
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export
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packedOffsetsLength : (cursor : Nat) -> (s : PackedSchema)
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-> length (packedOffsetsFrom cursor s) = length s
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packedOffsetsLength cursor [] = Refl
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packedOffsetsLength cursor (f :: fs) =
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cong S (packedOffsetsLength (cursor + packedFieldSize f) fs)
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||| Every field's extent `[offset, offset + size)` stays within the region's
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||| total size: `offset + packedFieldSize f <= cursor + packedSize s` for
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||| each laid-out field. Consequence: for a packed-COMPUTED region the
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||| verifier's `AccessOutOfRegionBounds` can never fire — it only catches
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||| hand-built / malformed schemas whose declared size is too small.
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export
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packedFieldsInBounds : (cursor : Nat) -> (s : PackedSchema)
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-> All (\fo => LTE (snd fo + packedFieldSize (fst fo))
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(cursor + packedSize s))
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(packedOffsetsFrom cursor s)
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packedFieldsInBounds cursor [] = []
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packedFieldsInBounds cursor (f :: fs) =
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let headBound : LTE (cursor + packedFieldSize f)
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(cursor + (packedFieldSize f + packedSize fs))
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headBound = plusLteMonotoneLeft cursor (packedFieldSize f)
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(packedFieldSize f + packedSize fs)
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(lteAddRight (packedFieldSize f))
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tail : All (\fo => LTE (snd fo + packedFieldSize (fst fo))
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((cursor + packedFieldSize f) + packedSize fs))
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(packedOffsetsFrom (cursor + packedFieldSize f) fs)
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tail = packedFieldsInBounds (cursor + packedFieldSize f) fs
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tail' : All (\fo => LTE (snd fo + packedFieldSize (fst fo))
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(cursor + (packedFieldSize f + packedSize fs)))
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(packedOffsetsFrom (cursor + packedFieldSize f) fs)
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tail' = mapProperty
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(\pf => rewrite plusAssociative cursor (packedFieldSize f) (packedSize fs)
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in pf)
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tail
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in headBound :: tail'
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-- ============================================================================
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-- Rust correspondence (canonical reference schemas)
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-- ============================================================================
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-- This module's `packedOffsetsFrom` / `packedSize` are a line-for-line
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-- transliteration of the Rust `resolve_field` (codegen `parser.rs`) and
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-- `field_byte_offset` / `compute_region_byte_size`: same accumulator, same
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-- per-field size (`packedScalarSize` = `scalar_byte_size`, Bool = 1, no
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-- alignment padding), same `size * cardinality`. The two implementations
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-- therefore compute identical offsets by construction; the structural
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-- theorems above (`packedOffsetsLength`, `packedFieldsInBounds`) are the
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-- mechanized guarantees that hold for BOTH.
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--
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-- (A `Refl` differential against literal offset lists is not used: Idris2
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-- Nat literals are `integerToNat`-based and do not reduce definitionally
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-- under `plus`/`mult` in unification, so such a `Refl` cannot type-check —
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-- it would prove nothing about the Rust side regardless. The correspondence
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-- below is by transliteration; a future cross-check would extract the Rust
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-- offsets and compare as data, not as a type-level proof.)
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--
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-- Canonical reference schemas (offsets as the Rust producer computes them):
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--
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-- Tier-1 `Mix` (tests/execute_lowering.rs), packedSize = 28:
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-- head:i32@0 flag:u8@4 small:u16@5 sign:i8@7 big:i64@8
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-- fx:f32@16 fy:f64@20
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-- Array example, packedSize = 28:
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-- hp:i32@0 name:u8[24]@4
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||| The Tier-1 `Mix` region as a packed schema (reference data).
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public export
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mixSchema : PackedSchema
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mixSchema =
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[ MkPackedField "head" I32 1
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, MkPackedField "flag" U8 1
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, MkPackedField "small" U16 1
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, MkPackedField "sign" I8 1
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, MkPackedField "big" I64 1
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, MkPackedField "fx" F32 1
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, MkPackedField "fy" F64 1
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]
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||| An array-bearing schema (`name: u8[24]` after `hp: i32`) exercising the
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||| `size * cardinality` rule (reference data).
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public export
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arraySchema : PackedSchema
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arraySchema = [ MkPackedField "hp" I32 1, MkPackedField "name" U8 24 ]

‎src/abi/typed-wasm.ipkg‎

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@@ -24,6 +24,7 @@ modules = TypedWasm.ABI.Region
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, TypedWasm.ABI.Tropical
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, TypedWasm.ABI.Epistemic
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, TypedWasm.ABI.Layout
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, TypedWasm.ABI.PackedLayout
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, TypedWasm.ABI.Proofs
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, TypedWasm.ABI.VerifierSpec
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-- Cross-language layout contracts (aggregate library role, ADR-004).

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