ArrayRuntime adapters

[RealColdFry]

The runtime-adapters-kernel branch adds a typed interface for swapping tslua's emit per category, propagating into lualib (which tslua compiles from TS source in the same pass). Kernel wires one primitive: ArrayRuntime.length. See examples/array-length-adapter/ for a working demo.

Before extending the interface, I want to pressure-test it against two cases @pilaoda explored upstream:

1. Proxy arrays. Host-tracked length where #arr returns the wrong value; the rewrite needs to reach into lualib so __TS__ArrayPush etc. use the right length too.

   • Support recompile lualib with custom plugin? TypeScriptToLua/TypeScriptToLua#1502
   • New option recompileLuaLib: Recompile the Lua standard library with custom plugins. TypeScriptToLua/TypeScriptToLua#1656

2. Metatable-tagged arrays for Array.isArray. Tagging literals at construction makes isArray exact and enables the class-based Array sketched there. Needs __TS__ArrayMap etc. to emit tagged arrays for their internal result = [].

   • Explore marking arrays with a metatable TypeScriptToLua/TypeScriptToLua#535

Current interface:

interface ArrayRuntime {
    length(arr: readonly unknown[]): number;
    create<T>(...items: T[]): T[];
    isArray(value: unknown): value is unknown[];
    spread<T>(arr: T[]): LuaMultiReturn<T[]>;
    setLength(arr: unknown[], n: number): void;
}

Open questions

1. TS-authored primitives. A class-based runtime could be written entirely in TS and transpile down, if the class encapsulates its raw representation. Missing: linking .d.ts declarations to .ts implementations; .lua is the only resolved path today.

2. Push fast path routing through length. tslua's push single-arg fast path emits arr[#arr + 1] = val. On a proxy host, # is wrong, so the fast path is broken. Should the fast path route its # through ArrayRuntime.length (so a user who overrides only length gets a working push for free), or should push be its own primitive (overridden explicitly when the user wants different append semantics)? The first feels cleaner; push becomes a primitive only when the append contract itself differs, not when length happens to.

If you've hit a case that doesn't fit, that's the most useful signal.

Other links

• Add custom Array type to lualib TypeScriptToLua/TypeScriptToLua#262
• Feature request: an option to disable +1 array indexing fix on ts code made explicitly for lua. TypeScriptToLua/TypeScriptToLua#1204
• Detecting types derived from array TypeScriptToLua/TypeScriptToLua#289