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GC to Inferred Ownership migration

Draton no longer uses a tracing GC runtime for safe code. Memory management is now decided at compile time by the ownership pass described in inferred-ownership-spec.md, and codegen lowers safe heap allocation to malloc with generated last-use free calls.

What changed across the six phases​

  1. The ownership rules were specified for value categories, moves, borrows, last-use analysis, escapes, aliasing, closures, cycles, diagnostics, and raw escape hatches.
  2. Ownership state and use annotations were added to the typed AST, plus ownership-specific diagnostics and a dedicated ownership pass entrypoint.
  3. OwnershipChecker grew into the real ownership engine: value classification, function summary inference, flow-sensitive binding analysis, closure capture handling, cycle checks, and use_effect/free-point annotations.
  4. Codegen stopped emitting GC allocation, write barriers, safepoints, and root management for safe values, and started lowering safe ownership to malloc and generated free.
  5. The GC runtime subsystem was removed from crates/draton-runtime, leaving panic, scheduler, channels, builtins, and IO in place.
  6. Ownership integration coverage, corpus regression guards, @gc_config deprecation, and migration closeout docs were added.

Removed​

  • the GC runtime subsystem, roughly 2900 lines under crates/draton-runtime/src/gc/
  • shadow-stack and llvm.gcroot integration
  • safepoint globals and slow paths
  • GC write barriers
  • GC allocation entrypoints and tests such as crates/draton-runtime/tests/gc_tests.rs

Added​

  • OwnershipChecker as the post-typecheck ownership analysis pass
  • use_effect annotations on typed expressions for codegen consumption
  • free-point insertion based on last-use analysis
  • @acyclic support for user-asserted acyclic ownership shapes
  • higher-order effect contracts such as (String) -> borrow and (String) -> move

Before and after performance characteristics​

Before:

  • safe heap memory used a tracing generational collector
  • codegen had to emit runtime allocation hooks, write barriers, and safepoint-related scaffolding
  • latency and memory reclamation depended on collector activity

After:

  • safe heap ownership is resolved at compile time
  • last-use frees happen on ownership edges selected by the compiler
  • safe-code allocation lowers directly to malloc
  • there is no GC pause behavior for safe Draton values
  • conservative compile-time rejection replaces runtime tracing when aliasing, escapes, or cycles cannot be proven safe

Compatibility note​

@gc_config is deprecated and safe to remove from existing code. The parser and typechecker still accept it for compatibility, but it has no effect because Draton now uses Inferred Ownership instead of a GC runtime.