# PBS Lowering IRBackend Specification

Status: Draft v1 (Frontend Scope)  
Applies to: first lowering boundary from bound PBS frontend model into `IRBackend`/`IRBackendFile`

## 1. Purpose

This document defines the normative frontend lowering contract from PBS source semantics to `IRBackend`.

Its purpose is to keep the first lowering deterministic and shared across implementations while PBS and backend design evolve.

## 2. Scope

This document defines:

- required preconditions before lowering starts,
- AST invariants that lowering may assume as hard contract,
- semantic obligations preserved in `IRBackend`,
- deterministic rejection behavior for unsupported frontend-lowering forms,
- and diagnostics attribution obligations for lowering failures in frontend scope.

This document does not define:

- VM lowering (`IRVM`),
- bytecode/PBX mapping,
- runtime execution behavior,
- verifier/loader internals.

Those concerns belong to shared acceptance specs under `docs/general/specs`.

## 3. Authority and Precedence

Normative precedence:

1. `3. Core Syntax Specification.md`
2. `4. Static Semantics Specification.md`
3. `11. AST Specification.md`
4. `12. Diagnostics Specification.md`
5. This document

If a lowering rule here conflicts with higher-precedence syntax/semantics rules, the higher-precedence rule wins.

## 4. Normative Inputs

This document depends on:

- `3. Core Syntax Specification.md`
- `4. Static Semantics Specification.md`
- `11. AST Specification.md`
- `12. Diagnostics Specification.md`

## 5. Lowering Preconditions

Frontend lowering into `IRBackend` may start only when:

1. source has been tokenized and parsed,
2. required syntax diagnostics have been emitted,
3. AST output satisfies `11` required invariants for this source unit, including:
   - one root per file,
   - deterministic child ordering,
   - mandatory node attribution (`file/start/end`) on nodes consumed by lowering,
   - mandatory declaration metadata on declaration nodes required by the active source slice,
   - and structural coherence after any parser recovery.
4. linking/name-resolution outcomes required for the current lowering slice are available,
5. and no unresolved semantic ambiguity is deferred to lowering.

Lowering must not invent unresolved semantic answers that belong to syntax/static/linking phases.

## 6. IRBackend Preserved Obligations

For each admitted source unit and callable in the current lowering slice, `IRBackend` must preserve at minimum:

1. callable identity (name/category as applicable),
2. callable arity,
3. declared return surface information,
4. source attribution anchor (`file + span`) for diagnostics and traceability,
5. source-observable parse intent for statement/expression structure (including precedence/associativity outcome already fixed by AST shape).

Lowering must not collapse source categories in a way that erases required declaration/callable identity needed by downstream diagnostics or conformance assertions.

The normative contract is obligation-based, not tied to one mandatory in-memory class graph.

## 7. Deterministic Rejection Policy

If a source form is outside current frontend-lowering support:

1. rejection must be deterministic,
2. diagnostics must expose stable `code`, `severity`, `phase`, `message template id`, and primary attribution,
3. and unsupported forms must not silently degrade into different valid lowered behavior.

Lowering must not convert a required syntax/static rejection into accepted lowered behavior.

## 8. Conformance Boundary

`IRBackend` is the first lowering boundary (frontend responsibility).

Conformance-valid claims at this boundary require Gate U evidence from `docs/general/specs/13. Conformance Test Specification.md`.

For this frontier, Gate U evidence is expected to cover at minimum:

- lexer behavior for covered inputs,
- parser behavior and AST shape invariants required by `11`,
- lowering outcomes at `IRBackend` boundary,
- and deterministic diagnostics identity/attribution for rejection cases.

Conformance at this boundary may be claimed only when the full claimed PBS source surface is implemented at `IRBackend` level.

## 9. Explicit Deferrals

The following are intentionally out of scope for this document:

- `IRBackend -> IRVM` lowering,
- artifact encoding and binary layout,
- runtime lifecycle behavior,
- verifier and loader checks.

## 10. Non-Goals

- Freezing one optimizer strategy.
- Freezing one compiler architecture.
- Repeating runtime or bytecode authority.

## 11. Exit Criteria

This document is healthy when:

1. lowering preconditions are explicit,
2. preserved `IRBackend` obligations are explicit,
3. deterministic rejection policy is explicit and test-backed,
4. and scope boundaries with general/backend acceptance specs are explicit.