Macro Expansion Pipeline (170 Foundation)

Date: May 22, 2026

Objective

Integrate macro expansion without destabilizing the frontend.

Target order:

source -> lexer -> macro expand -> parser -> AST

Current Foundation

• Frontend performs a pre-expansion lexing baseline and a post-expansion lexing pass.
• Macro expansion emits:
• transformed source text
• trace expansion
• diagnostics
• recursion-limit validity flag
• Frontend output stores:
• macro trace
• macro expanded flag
• diagnostics mapped to frontend source spans

Span and Diagnostics Contract

• Macro diagnostics carry line, column, width.
• Frontend translates these into standard diagnostic spans.
• Diagnostics are merged with lexer/parser diagnostics in a unified output stream.

Recursion Safety Contract

• Expansion pass computes a recursion depth signal.
• If depth exceeds configured limit, expansion is marked invalid and a fatal diagnosis is emitted.
• Frontend validity includes macro validity in addition to parser/AST validity.

Known Gaps (Next Steps)

• Token-level hygiene is currently foundational (string-level contract first).
• Expansion trace is linear and human-readable; structured machine trace can be added later.
• Recursion detection is heuristic; full macro call-graph limits can replace it in advanced phase.