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_CALL (defs.inc) already dispatches through an interface — but NOT as the data-ptr + vtable-ptr fat pointer this plan was written against. An interface value in pxx is ONE WORD: the instance pointer. The IMT is recovered per call from the instance's RTTI by PXXIntfIMTOf(inst, ifaceId), which is what lets the value stay a single pointer. AN_INTF_FROM_CLASS, which this line also named, was the fat-pointer BUILD node and was retired on 2026-09-09 — it had constructed nothing for months.

That matters for dyn Trait and is not a wording fix: the pxx mechanism gets its method table from the RECEIVER'S IDENTITY, so it needs the receiver to have one (an RTTI blob reachable from the value). A dyn Trait over a plain struct or a primitive has no such blob, so the reuse is NOT "same shape, generalize the binding" — it is either give those values an identity, or carry a real fat pointer for the Rust case alongside the one-word Pascal one. Cost that before planning against it. 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_CALL, one-word values, IMT via PXXIntfIMTOf) unaffected. Whether this GENERALIZES that binding or adds a second one beside it is open — see "What it does": the pxx mechanism needs the receiver to carry an identity, and a dyn Trait over a primitive does not.

Log

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