Vitte

Coroutine Lowering (172 Foundation)

Date: May 22, 2026

Objective

Transform async-style control flow into a state-machine-oriented MIR skeleton.

Implemented Foundation

  • HIR->MIR pass detects await-like expressions in statements.
  • For await-bearing statements, lowering inserts coroutine control-flow skeleton:
  • suspend block
  • resume block
  • continuation edge to normal control flow

This establishes a minimal resumable execution model in MIR.

Control-Flow Contract

  • Suspend and resume are represented as explicit MIR terminator behavior.
  • Resume target is deterministic and linked to continuation block.
  • Existing non-await statements keep standard lowering behavior.

Safety and Correctness Scope

Foundation phase includes:

  • explicit suspension/resumption edges
  • predictable continuation points

Deferred to advanced phase:

  • full generator frame layout
  • precise captured-local mapping
  • drop-order and cancellation guarantees
  • panic/unwind semantics through suspended frames

Backend Impact

  • Backend sees explicit coroutine-like CFG edges in MIR.
  • No full ABI/runtime coroutine frame guarantee is claimed yet; this is a structural lowering base.