[ASDAlexander77]

Follow-up 6 of .superpowers/own-raytrace-rulings.md: an object literal re-viewed as another interface read wrong offsets. It was wider than filed: every cast from one interface to another that it does not extend kept the value's own vtable, and the target interface read it by its own slot numbers. Every model, gc included:

interface Hit { thing: number; ray: number; dist: number; }
interface HasDist { dist: number; }
const d: HasDist = h;   // d.dist printed h.thing
interface A { f(): number; g(): number; }  interface B { g(): number; }
const b: B = a;         // b.g() called a.f()

A cast to an interface the value extends worked, because the base's members come first.

Fix

• MLIRGen (castInterfaceToInterface): for each slot of the target's vtable, find the slot of the source's that holds the same member. If that is every slot in place (an extends base, or the same layout), the cast is unchanged. Otherwise the ts.Cast carries the map (__vtable_map). Lowering builds the target's vtable from the value's own one. A slot is copied as it is, so the cast is rejected when a member is a method of one interface and a field of the other, or when a field has another type. A member the source lacks reads as absent (-1). tslang has always compiled tube = ball, where Ball has no length, and structural-typing.ts relies on that.
• Lowering: ts_remap_vtable(vtable, map, count) is generated into the module with internal linkage, so it needs no runtime-library symbol under any model, JIT or AOT. A vtable with the same content is built once and kept in a list for as long as the program runs. A cast in a loop therefore allocates nothing after the first time, and nothing has to free a vtable. The list is pushed with a compare-exchange. A null interface stays null.
• The cast stays a ts.Cast, so the rc and own passes see it exactly as before.

Tests

• tests/00interface_cast_unrelated.ts covers:

  • fields of an object literal, of a call's result, of an argument, through <T>, and of an element;
  • two fields in another order;
  • a class object;
  • a cast of a cast;
  • an absent optional member;
  • methods of a class object and of an object literal;
  • the second interface of a diamond extends;
  • 1000 casts in a loop.

  It runs under gc and in the rc/none corpus.

• Tooth: with the new path turned off, gc prints 1 (thing) where 3 (dist) is right, and 1 | 1 where 2 | 20 is right for the methods.

• Two compile errors, in interface-cast/method_field.ts and interface-cast/field_type.ts.

Not covered

• own: own rejects an interface-to-interface cast of an existing value ("'this value' takes a second reference") with or without this change, so the test doesn't run under own.
• Unions: I haven't checked an interface-to-interface cast inside a union (S | null → T | null). A null class object, null or undefined given to an interface is a null interface whose methods throw #665 handles a class inside a union.

Verification

• Windows: 5377/5377.
• Linux: 5359/5359, and the 25 interface-cast and structural-typing tests pass with MALLOC_PERTURB_=165 MALLOC_CHECK_=3.
• The default library compiles (lib.ts, lib.win32.ts) under gc, rc, none and own.

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