js: this-binding for callbacks + generic .call/.apply

[hiett]

What

Two related pieces of this-plumbing, targeting the largest remaining test262 blocker after wave 10 bound the built-in constructors.

Bucket 2 — generic .call/.apply on a built-in prototype method value (landed)

A static <Ctor>.prototype.<m>.call(recv, …args) / .apply(recv, [args]) on a built-in prototype method (Array/String/Object/Number/Boolean/Date/RegExp/Map/Set/Function) is rewritten to the ordinary method call recv.<m>(…args). In this receiver-first model the unbound method must run with recv as its this, so the rewrite reuses the entire tested method-dispatch path and forwards the receiver first. Previously func_call/func_apply dropped the receiver as the ignored thisArg (e.g. String.prototype.trim.call(x) trimmed undefined).

.apply is unrolled only when its argument array is a literal; otherwise it falls through.

Also gave the String receiver-coercion hook str_this its full spec meaning — ToString(RequireObjectCoercible(this)): undefined/null throw a TypeError, every non-string primitive/array/object is ToString-coerced, plain strings pass through. This flows to every string method already routing through str_this (upper/lower/charAt/split/trim family), so trim.call(0) → "0", .call(false) → "false", .call([1]) → "1", .call(undefined) throws.

Bucket 1 — thisArg on iteration callbacks (partial)

The thisArg of the Array iteration methods (map/filter/forEach/some/every/find/findIndex/findLast/findLastIndex) is now honored when the callback is an INLINE function(){…} expression that reads this — the callback closure's this capture is bound to the thisArg, the same mechanism object getters/setters already use. Arrows correctly ignore thisArg (lexical this).

Deferred (documented): a callback passed by NAME (arr.map(callbackfn, thisArg) — the shape most test262 this-arg files use) still ignores thisArg. A top-level function carries no this parameter (its this resolves to globalThis at lower time), so binding a dynamic receiver would require making this a call-time parameter of every plain function — a cross-cutting ABI change touching all call sites, apply_fn/call_cb, func_call/func_apply, and every iteration op. Out of scope for this wave.

Before → after (test262)

built-ins/{Array/prototype/{map,filter,forEach}, String/prototype/trim, Object/prototype/hasOwnProperty}:

202 → 241 pass (+39). Nearly all of the movement is bucket 2 (the String.prototype.trim.call(primitive) cluster + slice/hasOwnProperty .call). Bucket 1's inline-only subset moves ~0 test262 files (they use named callbacks) but is spec-correct and lands the requested unit behavior.

Tests

Full gleam test green (2781 passed; the only 3 failures are the pre-existing missing WASM .wat conformance fixtures, unrelated). Added 9 spec-driven tests under // ==== this-binding + call/apply (wave 12) ==== covering Array.prototype.slice.call/.apply, String.prototype.trim.call (string + coercion + RequireObjectCoercible throw), Object.prototype.hasOwnProperty.call, inline-callback map/forEach thisArg, and arrow-ignores-thisArg.

Files

• src/twocore/frontend/js/lower.gleam — .call/.apply rewrite + inline-callback thisArg binding + header docs
• src/twocore_rt_js_ffi.erl — str_this full ToString/RequireObjectCoercible; trim family routes through it (no -export changes)
• test/js_compiler_test.gleam — wave-12 tests