fix(runtime): for-of over a dynamically-typed string drives the String iterator (#4892)

[proggeramlug]

Summary

Fixes #4892. for…of (and spread / Array.from / array-destructuring) over a string value whose static type is unknown — v: any, or a value flowing out of a segmenter result / destructure — threw TypeError: (string).next is not a function. Statically-typed string for-of was unaffected.

Root cause

The dynamic path routes through the generic iterator protocol via js_get_iterator (lazy for-of, #4786). That helper resolves [Symbol.iterator] by reading the method off an object. For a string primitive there is no object slot, so js_object_get_symbol_property found nothing and js_get_iterator fell through to return val_f64 — handing the string itself back as the "iterator". The loop then called .next() on a plain string and threw.

Strings already have a correct String iterator object (string/iter_object.rs::string_values_iter, code-point iteration) installed on String.prototype[Symbol.iterator]; the dynamic path just never reached it.

Fix

One short-circuit in js_get_iterator (crates/perry-runtime/src/symbol.rs): when the value is a string primitive (heap STRING_TAG or inline SSO short string), build the real String iterator via string_values_iter, mirroring the array short-circuit at the top of the function. SSO strings are materialized to a *const StringHeader via the existing js_get_string_pointer_unified.

Verification

Against node --experimental-strip-types:

• f(v: any){ for (const c of v) n++ } → f("ab") = 2 ✅
• segmenter-destructured for (const {segment} of seg.segment("a👍b")) for (const ch of segment) ... = 3 ✅
• static-typed control still works ✅
• spread [...v], Array.from(v), const [a,b]=v over dynamic strings — all code-point correct ✅
• non-iterable primitive (37) still throws TypeError ✅; arrays still iterate ✅

End-to-end (the live ink blocker): with #4891 also on main, string-width@7+ now matches Node for mixed strings — stringWidth("a👍b") = 4 (was the hangulClusterWidth → for (const character of visibleSegment) throw). stringWidth of "abc"/"👍"/"👨‍👩‍👧" = 3/2/2, all matching Node.

cargo test -p perry-runtime (RUST_TEST_THREADS=1): 1007 passed; the only failure is the pre-existing TZ-dependent date::tests::test_full_year_setters_revive_invalid_date_only, which fails on pristine main too.

Related

• fix(regex): expand \p{RGI_Emoji} & friends (properties of strings) to supported emoji primitives (#4889) #4891 / regex: \p{RGI_Emoji} (property of strings, /v flag) rejected — Rust regex crate has no string-properties; blocks string-width/ink #4889 (previous string-width wall — \p{RGI_Emoji}, merged)
• fix(hir): statement-semantics (for-of iterator protocol, with, eval completion-values) test262 parity #4786 (lazy for-of iterator protocol — where the dynamic path was introduced)
• Compile ink (React-based TUI framework) end-to-end via perry.compilePackages #348 (ink end-to-end umbrella)