Rust frontend — dyn Trait dispatch for arbitrary types

• Type: feature — Track A
• Status: backlog
• Owner: —
• Opened: 2026-07-03
• Umbrella: [[feature-rust-frontend]] — sub-ticket 4/12. Depends on [[feature-rust-frontend-skeleton]].

What it does

AN_INTF_FROM_CLASS/AN_INTF_CALL (defs.inc ~186-189) already build a data-ptr + vtable-ptr fat pointer for interface dispatch — same shape dyn Trait needs. The gap: current interfaces assume a class hierarchy underneath (COM/CORBA-style — only classes implement interfaces). Rust dyn Trait can wrap any type: structs, primitives, no inheritance required.

Needs a trait-impl table keyed by (concrete type, trait) independent of class hierarchy, so impl Display for MyStruct (a plain struct, not a class) can still produce a dyn Display fat pointer.

Scope

• Reuse the fat-pointer representation as-is (data ptr + vtable ptr) — only the lookup/binding model changes, not the runtime shape.
• shakmaty-syzygy uses dyn 20x in 4k LOC — confirms this isn't a rare path if/when dependency source is ever tackled, though v1 scope (per umbrella) is app code only.

Acceptance

• impl Trait for PlainStruct (no class involved) produces a working dyn Trait value; virtual call through it dispatches to the right impl.
• Existing class-based interface dispatch (AN_INTF_FROM_CLASS current callers) unaffected — this generalizes the binding, doesn't replace it.

Log

• 2026-07-03 — split from [[feature-rust-frontend]] umbrella at ticket-craft time. No code written yet.