## Summary
- Fix regression where `new Bun.FFI.CString(ptr)` throws "function is
not a constructor"
- Pass the same function as both call and constructor callbacks for
CString
## Root Cause
PR #24910 replaced `jsc.createCallback` with `jsc.JSFunction.create` for
all FFI functions. However, `JSFunction.create` doesn't allow
constructor calls by default (it uses `callHostFunctionAsConstructor`
which throws). The old `createCallback` used `JSFFIFunction` which
allowed the same function to be called with `new`.
## Fix
Pass the same function as both the `implementation` and `constructor`
option to `JSFunction.create` for CString specifically. This allows `new
CString(ptr)` to work while keeping the refactoring from #24910.
Additionally, the `bun:ffi` module now replaces `Bun.FFI.CString` with
the proper JS CString class after loading, so users get the full class
with `.ptr`, `.byteOffset`, etc. properties.
## Test plan
- [x] Added regression test `test/regression/issue/25231.test.ts`
- [x] Test fails with `USE_SYSTEM_BUN=1` (v1.3.3), passes with fix
- [x] Verified reproduction case from issue works
Fixes#25231🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
