miniextendr_api/

into_r.rs

#![allow(rustdoc::private_intra_doc_links)]
//! Conversions from Rust types to R SEXP.
//!
//! This module provides the [`IntoR`] trait for converting Rust values to R SEXPs.
//!
//! # Submodules
//!
//! | Module | Contents |
//! |--------|----------|
//! | [`large_integers`] | `i64`, `u64`, `isize`, `usize` → REALSXP, plus string/bool/Option scalars |
//! | [`collections`] | `HashMap`, `BTreeMap`, `HashSet`, `BTreeSet` → named/unnamed lists |
//! | [`result`] | `Result<T, E>` → list with `ok`/`err` fields |
//! | [`altrep`] | `Altrep<T>` marker type, `Lazy<T>` alias, `IntoRAltrep` trait |
//!
//! # Choosing the right outbound conversion
//!
//! [`IntoR`] is the **lax** outbound path — it silently widens 64-bit integers
//! to R's `REALSXP` when the value doesn't fit `INTSXP`. The strict alternative
//! lives in [`crate::strict`] and is opted into via `#[miniextendr(strict)]`,
//! which routes through [`crate::strict::checked_into_sexp_i64`] and friends.
//! Failure mode of staying on the default lax path when you care about exact
//! representation: an `i64` value just above `i32::MAX` lands in R as an
//! inexact `f64` (and IDs/counters larger than `2^53` start colliding).
//!
//! For storage-directed conversions (e.g. force `Vec<i64>` into `INTSXP`
//! when values fit), reach for [`crate::into_r_as::IntoRAs`] instead.
//!
//! The inbound counterpart is [`crate::from_r::TryFromSexp`] (strict by
//! construction — returns `Result`); the inbound looser path is
//! [`crate::coerce::Coerce`] / [`crate::coerce::TryCoerce`]. There is
//! intentionally no `TryFromSexpStrict` trait.
//!
//! # Thread Safety
//!
//! The trait provides two methods:
//! - [`IntoR::into_sexp`] - checked version with debug thread assertions
//! - [`IntoR::into_sexp_unchecked`] - unchecked version for performance-critical paths
//!
//! Use `into_sexp_unchecked` when you're certain you're on the main thread:
//! - Inside ALTREP callbacks
//! - Inside standalone `#[miniextendr]` functions (they run on the main thread)
//! - Inside `extern "C-unwind"` functions called directly by R
//!
//! # The absence contract: what `None` becomes in R
//!
//! `None` does **not** map to one universal R value — it depends on the
//! shape of `T` in `Option<T>`:
//!
//! - Owned scalars (`Option<i32>`, `Option<f64>`, `Option<bool>`,
//!   `Option<String>`, and friends in [`large_integers`]) → the matching
//!   `NA_<type>_`. R has a native NA sentinel for each of these.
//! - `Option<&T>` where `T: Copy` (e.g. `Option<&i32>`) → `NULL`. A
//!   reference has nothing to copy on `None`, so there is no NA to
//!   produce. See the impl doc on `Option<&T>` in [`large_integers`].
//! - `Option<&str>` is the one exception to the previous rule: it returns
//!   `NA_character_`, matching `Option<String>`, because `str` is unsized
//!   and can't use the generic `Copy`-bounded blanket impl — it has its
//!   own hand-written impl instead.
//! - Containers (`Option<Vec<T>>`, `Option<HashMap<..>>`, `Option<HashSet<..>>`,
//!   `Option<BTreeMap<..>>`, `Option<BTreeSet<..>>`) → `NULL`. No container
//!   type has a native R NA sentinel either.
//! - The reading markers' `Option` (`Option<AsNumeric>`,
//!   `Option<AsNumericVec>`, `Option<AsCharacter>`, `Option<AsCharacterVec>`;
//!   see [`AsNumeric`](crate::convert::AsNumeric) and
//!   [`AsCharacter`](crate::convert::AsCharacter)) → `NULL`. `NA` already
//!   means `Some(marker(None))`, so `NULL` is what the marker's `Option`
//!   reads back as `None`.
//!
//! Changing a return type from `Option<i32>` to `Option<&i32>` or
//! `Option<Vec<i32>>` therefore silently flips the R-visible absence value
//! from `NA_integer_` to `NULL`, and switching a return from `Option<f64>`
//! to `Option<AsNumeric>` changes `NA` to `NULL`, with no compiler warning. See
//! [`result`] for the analogous (and differently-shaped) contract on
//! `Result::Err`.

use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
use std::hash::Hash;

use crate::SexpExt;
use crate::altrep_traits::{NA_INTEGER, NA_LOGICAL, NA_REAL};
use crate::gc_protect::ProtectScope;
use crate::list::ListBuilder;
use crate::strvec::StrVecBuilder;

/// Trait for converting Rust types to R SEXP values.
///
/// Outbound counterpart of [`crate::from_r::TryFromSexp`]. This is the **lax**
/// path for 64-bit integer types — values that overflow `i32` silently land
/// as `REALSXP`. For the strict alternative, see [`crate::strict`] and the
/// `#[miniextendr(strict)]` attribute.
///
/// # Required Method
///
/// Implementors must provide [`try_into_sexp`](IntoR::try_into_sexp) and
/// specify [`Error`](IntoR::Error). The other three methods have sensible
/// defaults.
///
/// # Examples
///
/// ```no_run
/// use miniextendr_api::into_r::IntoR;
///
/// let sexp = 42i32.into_sexp();
/// let sexp = "hello".to_string().into_sexp();
///
/// // Fallible path:
/// let result = "hello".try_into_sexp();
/// assert!(result.is_ok());
/// ```
#[diagnostic::on_unimplemented(
    note = "an `Option<Class>` return converts when spelled `Option<ExternalPtr<Class>>`, or with an explicit class wrap: `Option<WrapAsR6<Class>>` or `#[miniextendr(wrap = \"r6\")]` (any class system)"
)]
pub trait IntoR {
    /// The error type for fallible conversions.
    ///
    /// Use [`std::convert::Infallible`] for types that can never fail.
    /// Use [`IntoRError`](crate::into_r_error::IntoRError) for types
    /// that may fail (e.g. strings exceeding R's i32 length limit).
    type Error: std::fmt::Display;

    /// Try to convert this value to an R SEXP.
    ///
    /// This is the **required** method. All other methods delegate to it.
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error>;

    /// Try to convert to SEXP without thread safety checks.
    ///
    /// # Safety
    ///
    /// Must be called from R's main thread.
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error>
    where
        Self: Sized,
    {
        self.try_into_sexp()
    }

    /// Convert this value to an R SEXP, panicking on error.
    ///
    /// In debug builds, asserts that we're on R's main thread.
    fn into_sexp(self) -> crate::SEXP
    where
        Self: Sized,
    {
        match self.try_into_sexp() {
            Ok(sexp) => sexp,
            Err(e) => panic!("IntoR conversion failed: {e}"),
        }
    }

    /// Convert to SEXP without thread safety checks, panicking on error.
    ///
    /// # Safety
    ///
    /// Must be called from R's main thread. In debug builds, this still
    /// calls the checked version by default, but implementations may
    /// skip thread assertions for performance.
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP
    where
        Self: Sized,
    {
        // Default: just call the checked version
        self.into_sexp()
    }
}

impl IntoR for crate::SEXP {
    type Error = std::convert::Infallible;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self)
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self)
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        self
    }
}

impl IntoR for crate::worker::Sendable<crate::SEXP> {
    type Error = std::convert::Infallible;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.0)
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.0)
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        self.0
    }
}

impl From<crate::worker::Sendable<crate::SEXP>> for crate::SEXP {
    #[inline]
    fn from(s: crate::worker::Sendable<crate::SEXP>) -> Self {
        s.0
    }
}

impl IntoR for () {
    type Error = std::convert::Infallible;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(crate::SEXP::nil())
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        self.try_into_sexp()
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        crate::SEXP::nil()
    }
}

impl IntoR for std::convert::Infallible {
    type Error = std::convert::Infallible;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        match self {}
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        match self {}
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        match self {}
    }
}

/// Macro for scalar IntoR via SEXP::scalar_* methods.
macro_rules! impl_scalar_into_r {
    ($ty:ty, $checked:ident, $unchecked:ident) => {
        impl IntoR for $ty {
            type Error = std::convert::Infallible;
            #[inline]
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(crate::SEXP::$checked(self))
            }
            #[inline]
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            #[inline]
            fn into_sexp(self) -> crate::SEXP {
                crate::SEXP::$checked(self)
            }
            #[inline]
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                unsafe { crate::SEXP::$unchecked(self) }
            }
        }
    };
}

impl_scalar_into_r!(i32, scalar_integer, scalar_integer_unchecked);
impl_scalar_into_r!(f64, scalar_real, scalar_real_unchecked);
impl_scalar_into_r!(u8, scalar_raw, scalar_raw_unchecked);
impl_scalar_into_r!(crate::Rcomplex, scalar_complex, scalar_complex_unchecked);

/// Generate an infallible [`IntoR`] impl whose only real method is `into_sexp`.
///
/// Collapses the four-line boilerplate shell shared by every `Infallible`
/// `IntoR` impl in the optional-type modules: `try_into_sexp` delegates to
/// `into_sexp`, `try_into_sexp_unchecked` delegates to `try_into_sexp`, and
/// `into_sexp_unchecked` is left as the trait default. Only the `into_sexp`
/// body is supplied, as a closure-style `|recv| expr` binding the receiver
/// (a macro cannot reuse a `self` that arrives through a fragment, so the
/// caller names it). The generated methods are byte-identical to the
/// hand-written shell, so this is purely a dedup.
///
/// ```ignore
/// into_r_infallible!(Uuid, |this| this.to_string().into_sexp());
/// into_r_infallible!(Vec<Uuid>,
///     |this| this.into_iter().map(|u| u.to_string()).collect::<Vec<_>>().into_sexp());
/// ```
macro_rules! into_r_infallible {
    ($ty:ty, |$recv:ident| $body:expr) => {
        impl $crate::into_r::IntoR for $ty {
            type Error = ::std::convert::Infallible;
            #[inline]
            fn try_into_sexp(self) -> ::core::result::Result<$crate::SEXP, Self::Error> {
                ::core::result::Result::Ok(self.into_sexp())
            }
            #[inline]
            unsafe fn try_into_sexp_unchecked(
                self,
            ) -> ::core::result::Result<$crate::SEXP, Self::Error> {
                self.try_into_sexp()
            }
            #[inline]
            fn into_sexp(self) -> $crate::SEXP {
                let $recv = self;
                $body
            }
        }
    };
}
#[allow(unused_imports)]
pub(crate) use into_r_infallible;

impl IntoR for Option<crate::Rcomplex> {
    type Error = std::convert::Infallible;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        match self {
            Some(v) => v.into_sexp(),
            None => crate::SEXP::scalar_complex(crate::Rcomplex {
                r: NA_REAL,
                i: NA_REAL,
            }),
        }
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        match self {
            Some(v) => unsafe { v.into_sexp_unchecked() },
            None => unsafe {
                crate::SEXP::scalar_complex_unchecked(crate::Rcomplex {
                    r: NA_REAL,
                    i: NA_REAL,
                })
            },
        }
    }
}

/// Macro for infallible widening IntoR via Coerce.
macro_rules! impl_into_r_via_coerce {
    ($from:ty => $to:ty) => {
        impl IntoR for $from {
            type Error = std::convert::Infallible;
            #[inline]
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(crate::coerce::Coerce::<$to>::coerce(self).into_sexp())
            }
            #[inline]
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            #[inline]
            fn into_sexp(self) -> crate::SEXP {
                crate::coerce::Coerce::<$to>::coerce(self).into_sexp()
            }
            #[inline]
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                unsafe { crate::coerce::Coerce::<$to>::coerce(self).into_sexp_unchecked() }
            }
        }
    };
}

// Infallible widening to i32 (R's INTSXP)
impl_into_r_via_coerce!(i8 => i32);
impl_into_r_via_coerce!(i16 => i32);
impl_into_r_via_coerce!(u16 => i32);

// Infallible widening to f64 (R's REALSXP)
impl_into_r_via_coerce!(f32 => f64);
impl_into_r_via_coerce!(u32 => f64); // all u32 exactly representable in f64

mod large_integers;
pub(crate) use large_integers::{str_to_charsxp, str_to_charsxp_unchecked};

// region: Vector conversions

// Concrete IntoR impls for Vec<T> where T: RNativeType.
//
// These are written as concrete impls rather than a blanket
// `impl<T: RNativeType> IntoR for Vec<T>` to avoid a coherence conflict with the
// one `impl<T: IntoRVecElement> IntoR for Vec<T>` blanket (see `crate::newtype`):
// `Vec<T>` admits exactly one open-bound `IntoR` blanket slot, and that slot is
// the shared element-marker route used by `MatchArg` enums and `#[derive(IntoR)]`
// newtypes. A second `RNativeType`-bounded blanket would be E0119 — negative
// trait bounds are not stable, so coherence cannot prove the bounds disjoint.
// Concrete per-type impls sidestep the slot entirely (concrete-vs-blanket is
// allowed: the foreign R-native types provably do not implement the marker).
macro_rules! impl_into_r_vec_native {
    ($t:ty) => {
        impl IntoR for Vec<$t> {
            type Error = std::convert::Infallible;
            #[inline]
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { vec_to_sexp(&self) })
            }
            #[inline]
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            #[inline]
            fn into_sexp(self) -> crate::SEXP {
                unsafe { vec_to_sexp(&self) }
            }
            #[inline]
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                unsafe { vec_to_sexp_unchecked(&self) }
            }
        }
    };
}

impl_into_r_vec_native!(i32);
impl_into_r_vec_native!(f64);
impl_into_r_vec_native!(u8);
impl_into_r_vec_native!(crate::RLogical);
impl_into_r_vec_native!(crate::Rcomplex);

impl<T> IntoR for &[T]
where
    T: crate::RNativeType,
{
    type Error = std::convert::Infallible;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { vec_to_sexp(self) })
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        unsafe { vec_to_sexp(self) }
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { vec_to_sexp_unchecked(self) }
    }
}

// A boxed slice converts exactly like the owned vector. `into_vec()` is an
// O(1), allocation-free widen, so this one blanket subsumes every per-element
// `Box<[X]>` impl (native, String, Cow, bool, …): any `Vec<X>: IntoR` element
// gets `Box<[X]>` for free, inheriting the vector impl's error type and path.
// Coherence-equivalent to the prior `impl<T: RNativeType> IntoR for Box<[T]>`:
// both occupy the whole `Box<[T]>` slot (where-clauses don't narrow overlap),
// so concrete downstream `impl IntoR for Box<[Local]>` stays just as available.
impl<T> IntoR for Box<[T]>
where
    Vec<T>: IntoR,
{
    type Error = <Vec<T> as IntoR>::Error;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        self.into_vec().try_into_sexp()
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        unsafe { self.into_vec().try_into_sexp_unchecked() }
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        self.into_vec().into_sexp()
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { self.into_vec().into_sexp_unchecked() }
    }
}

// region: R vector allocation helpers
//
// These are the ONLY place in the codebase that should call Rf_allocVector
// for typed vectors and obtain a mutable data slice. All conversion code
// uses these helpers instead of raw FFI pointer arithmetic.

/// Allocate an R vector of type `T` with `n` elements and return `(SEXP, &mut [T])`.
///
/// The returned SEXP is **unprotected** — caller must protect via `Rf_protect`,
/// `OwnedProtect`, or `ProtectScope` before any further R allocation.
///
/// # Safety
///
/// Must be called from R's main thread.
#[inline]
pub(crate) unsafe fn alloc_r_vector<T: crate::RNativeType>(
    n: usize,
) -> (crate::SEXP, &'static mut [T]) {
    unsafe {
        let sexp = crate::sys::Rf_allocVector(T::SEXP_TYPE, n as crate::R_xlen_t);
        let slice = crate::from_r::r_slice_mut(T::dataptr_mut(sexp), n);
        (sexp, slice)
    }
}

/// Allocate an R vector (unchecked FFI variant).
///
/// # Safety
///
/// Must be called from R's main thread.
#[inline]
pub(crate) unsafe fn alloc_r_vector_unchecked<T: crate::RNativeType>(
    n: usize,
) -> (crate::SEXP, &'static mut [T]) {
    unsafe {
        let sexp = crate::sys::Rf_allocVector_unchecked(T::SEXP_TYPE, n as crate::R_xlen_t);
        let slice = crate::from_r::r_slice_mut(T::dataptr_mut(sexp), n);
        (sexp, slice)
    }
}

// endregion

/// Convert a slice to an R vector (checked) using `copy_from_slice`.
#[inline]
unsafe fn vec_to_sexp<T: crate::RNativeType>(slice: &[T]) -> crate::SEXP {
    unsafe {
        let (sexp, dst) = alloc_r_vector::<T>(slice.len());
        dst.copy_from_slice(slice);
        sexp
    }
}

/// Convert a slice to an R vector (unchecked) using `copy_from_slice`.
#[inline]
unsafe fn vec_to_sexp_unchecked<T: crate::RNativeType>(slice: &[T]) -> crate::SEXP {
    unsafe {
        let (sexp, dst) = alloc_r_vector_unchecked::<T>(slice.len());
        dst.copy_from_slice(slice);
        sexp
    }
}
// endregion

// region: Vec coercion for non-native types (i8, i16, u16 → i32; f32 → f64)

/// Macro for `Vec<T>` where `T` coerces to a native R type.
///
/// Allocates the R vector directly and coerces in-place — no intermediate Vec.
macro_rules! impl_vec_coerce_into_r {
    ($from:ty => $to:ty) => {
        impl IntoR for Vec<$from> {
            type Error = std::convert::Infallible;
            #[inline]
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            #[inline]
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            #[inline]
            fn into_sexp(self) -> crate::SEXP {
                unsafe {
                    let (sexp, dst) = alloc_r_vector::<$to>(self.len());
                    for (slot, val) in dst.iter_mut().zip(self.into_iter()) {
                        *slot = <$to>::from(val);
                    }
                    sexp
                }
            }
            #[inline]
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                unsafe {
                    let (sexp, dst) = alloc_r_vector_unchecked::<$to>(self.len());
                    for (slot, val) in dst.iter_mut().zip(self.into_iter()) {
                        *slot = <$to>::from(val);
                    }
                    sexp
                }
            }
        }

        impl IntoR for &[$from] {
            type Error = std::convert::Infallible;
            #[inline]
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            #[inline]
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            #[inline]
            fn into_sexp(self) -> crate::SEXP {
                unsafe {
                    let (sexp, dst) = alloc_r_vector::<$to>(self.len());
                    for (slot, &val) in dst.iter_mut().zip(self.iter()) {
                        *slot = <$to>::from(val);
                    }
                    sexp
                }
            }
            #[inline]
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                unsafe {
                    let (sexp, dst) = alloc_r_vector_unchecked::<$to>(self.len());
                    for (slot, &val) in dst.iter_mut().zip(self.iter()) {
                        *slot = <$to>::from(val);
                    }
                    sexp
                }
            }
        }
    };
}

// Sub-i32 integer types coerce to i32 (R's INTSXP)
impl_vec_coerce_into_r!(i8 => i32);
impl_vec_coerce_into_r!(i16 => i32);
impl_vec_coerce_into_r!(u16 => i32);

// f32 coerces to f64 (R's REALSXP)
impl_vec_coerce_into_r!(f32 => f64);

// i64/u64/isize/usize/u32: smart conversion (INTSXP when all fit, else REALSXP)
//
// Allocates the R vector directly and coerces in-place — no intermediate Vec.
// The slice impl carries the conversion; the `Vec` impl borrows into it (the
// elements are `Copy`), so `Vec<T>` and `&[T]` always agree.
macro_rules! impl_vec_smart_i64_into_r {
    ($t:ty, $fits_i32:expr) => {
        impl IntoR for &[$t] {
            type Error = std::convert::Infallible;
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            fn into_sexp(self) -> crate::SEXP {
                unsafe {
                    if self.iter().all(|&x| $fits_i32(x)) {
                        let (sexp, dst) = alloc_r_vector::<i32>(self.len());
                        for (slot, &val) in dst.iter_mut().zip(self.iter()) {
                            // fits_i32 guard verified range
                            *slot = val as i32;
                        }
                        sexp
                    } else {
                        let (sexp, dst) = alloc_r_vector::<f64>(self.len());
                        for (slot, &val) in dst.iter_mut().zip(self.iter()) {
                            // R has no 64-bit integer; f64 loses precision > 2^53
                            *slot = val as f64;
                        }
                        sexp
                    }
                }
            }
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                unsafe {
                    if self.iter().all(|&x| $fits_i32(x)) {
                        let (sexp, dst) = alloc_r_vector_unchecked::<i32>(self.len());
                        for (slot, &val) in dst.iter_mut().zip(self.iter()) {
                            // fits_i32 guard verified range
                            *slot = val as i32;
                        }
                        sexp
                    } else {
                        let (sexp, dst) = alloc_r_vector_unchecked::<f64>(self.len());
                        for (slot, &val) in dst.iter_mut().zip(self.iter()) {
                            // R has no 64-bit integer; f64 loses precision > 2^53
                            *slot = val as f64;
                        }
                        sexp
                    }
                }
            }
        }

        impl IntoR for Vec<$t> {
            type Error = std::convert::Infallible;
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            fn into_sexp(self) -> crate::SEXP {
                self.as_slice().into_sexp()
            }
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                unsafe { self.as_slice().into_sexp_unchecked() }
            }
        }
    };
}

// i32::MIN is NA_integer_ in R, so exclude it
impl_vec_smart_i64_into_r!(i64, |x: i64| x > i32::MIN as i64 && x <= i32::MAX as i64);
impl_vec_smart_i64_into_r!(u64, |x: u64| x <= i32::MAX as u64);
impl_vec_smart_i64_into_r!(isize, |x: isize| x > i32::MIN as isize
    && x <= i32::MAX as isize);
impl_vec_smart_i64_into_r!(usize, |x: usize| x <= i32::MAX as usize);
// Matches Vec<Option<u32>> (impl_vec_option_coerce_into_r!(u32 => i64)):
// INTSXP while everything fits, REALSXP otherwise.
impl_vec_smart_i64_into_r!(u32, |x: u32| x <= i32::MAX as u32);
// endregion

mod altrep;
mod collections;
mod owned_str;
mod result;

pub use altrep::*;
pub use result::*;

// region: Fixed-size array conversions

/// Convert a fixed-size array through its borrowed slice conversion.
///
/// Native values, smart integers, strings and logicals keep the slice's R
/// representation and error type, without allocating an intermediate `Vec`.
impl<T, E, const N: usize> IntoR for [T; N]
where
    for<'a> &'a [T]: IntoR<Error = E>,
    E: std::fmt::Display,
{
    type Error = E;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        self.as_slice().try_into_sexp()
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        unsafe { self.as_slice().try_into_sexp_unchecked() }
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        self.as_slice().into_sexp()
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { self.as_slice().into_sexp_unchecked() }
    }
}

/// Convert an optional array: `Some` uses the array conversion, `None` is NULL.
impl<T, const N: usize> IntoR for Option<[T; N]>
where
    [T; N]: IntoR,
{
    type Error = <[T; N] as IntoR>::Error;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        match self {
            Some(array) => array.try_into_sexp(),
            None => Ok(crate::SEXP::nil()),
        }
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        match self {
            Some(array) => unsafe { array.try_into_sexp_unchecked() },
            None => Ok(crate::SEXP::nil()),
        }
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        match self {
            Some(array) => unsafe { array.into_sexp_unchecked() },
            None => crate::SEXP::nil(),
        }
    }
}

#[cfg(test)]
mod array_error_tests {
    use super::IntoR;

    struct Fallible;

    impl IntoR for &[Fallible] {
        type Error = &'static str;

        fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
            Err("checked slice error")
        }

        unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
            Err("unchecked slice error")
        }
    }

    #[test]
    fn array_forwards_slice_errors_without_panicking() {
        assert_eq!(
            [Fallible].try_into_sexp().unwrap_err(),
            "checked slice error"
        );
        // The test slice conversion performs no R calls.
        assert_eq!(
            unsafe { [Fallible].try_into_sexp_unchecked() }.unwrap_err(),
            "unchecked slice error"
        );
        assert_eq!(
            Some([Fallible]).try_into_sexp().unwrap_err(),
            "checked slice error"
        );
        assert_eq!(
            unsafe { Some([Fallible]).try_into_sexp_unchecked() }.unwrap_err(),
            "unchecked slice error"
        );
    }
}
// endregion

// region: VecDeque conversions

use std::collections::VecDeque;

/// Convert `VecDeque<T>` to R vector where T: RNativeType.
impl<T> IntoR for VecDeque<T>
where
    T: crate::RNativeType,
{
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        let vec: Vec<T> = self.into_iter().collect();
        vec.into_sexp()
    }
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        let vec: Vec<T> = self.into_iter().collect();
        unsafe { vec.into_sexp_unchecked() }
    }
}
// endregion

// region: BinaryHeap conversions

use std::collections::BinaryHeap;

/// Convert `BinaryHeap<T>` to R vector where T: RNativeType + Ord.
///
/// The heap is drained into a vector (destroying the heap property).
/// Elements are returned in arbitrary order, not sorted.
impl<T> IntoR for BinaryHeap<T>
where
    T: crate::RNativeType + Ord,
{
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_vec().into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        self.into_vec().into_sexp()
    }
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { self.into_vec().into_sexp_unchecked() }
    }
}
// endregion

// region: Cow conversions

use std::borrow::Cow;

/// Convert `Cow<'_, [T]>` to R vector where T: RNativeType.
///
/// Always copies into a fresh R vector via the `&[T]` path. We deliberately do
/// *not* attempt speculative SEXP pointer recovery on the `Cow::Borrowed` arm:
/// a bare `&[T]` carries no provenance metadata, so a borrowed *sub-slice* of
/// an R vector (e.g. `&cow[2..5]`) is indistinguishable from a full borrow by
/// pointer + length alone. Probing `data_ptr − header` for such a sub-slice
/// reads provenance-free memory off the start of the R vector — the same hazard
/// removed from the Arrow path in #867 (see #880). The Arrow `Buffer` can prove
/// it is R-backed (`ptr_offset`/`capacity`); a `&[T]` cannot, so the only sound
/// choice is the always-correct copy. `Cow` remains zero-copy on the *input*
/// side through `TryFromRoot` and generated wrappers; raw `TryFromSexp`
/// returns owned data. `IntoR` always copies.
impl<T> IntoR for Cow<'_, [T]>
where
    T: crate::RNativeType + Clone,
{
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        self.as_ref().into_sexp()
    }
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { self.as_ref().into_sexp_unchecked() }
    }
}

/// Convert `Cow<'_, str>` to R character scalar.
impl IntoR for Cow<'_, str> {
    type Error = crate::into_r_error::IntoRError;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        self.as_ref().try_into_sexp()
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        self.as_ref().into_sexp()
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { self.as_ref().into_sexp_unchecked() }
    }
}
// endregion

// region: Box conversions (skipped - conflicts with IntoExternalPtr blanket impl)
//
// We can't add `impl<T: IntoR> IntoR for Box<T>` because it conflicts with
// the blanket impl `impl<T: IntoExternalPtr> IntoR for T`. If downstream
// crates implement `IntoExternalPtr for Box<SomeType>`, we'd have overlapping
// impls. Users can manually unbox with `*boxed_value` before conversion.
// endregion

// region: PathBuf / OsString conversions

use std::ffi::OsString;
use std::path::PathBuf;

/// Generate IntoR impls for types with `to_string_lossy()` (owned scalar, ref scalar,
/// Option, Vec, `Vec<Option>`). Used for PathBuf/&Path and OsString/&OsStr.
macro_rules! impl_lossy_string_into_r {
    (
        $(#[$owned_meta:meta])*
        owned: $owned_ty:ty;
        $(#[$ref_meta:meta])*
        ref: $ref_ty:ty;
        $(#[$option_meta:meta])*
        option: $opt_ty:ty;
        $(#[$vec_meta:meta])*
        vec: $vec_ty:ty;
        $(#[$vec_option_meta:meta])*
        vec_option: $vec_opt_ty:ty;
    ) => {
        $(#[$owned_meta])*
        impl IntoR for $owned_ty {
            type Error = crate::into_r_error::IntoRError;
            #[inline]
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                self.to_string_lossy().into_owned().try_into_sexp()
            }
            #[inline]
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            #[inline]
            fn into_sexp(self) -> crate::SEXP {
                self.to_string_lossy().into_owned().into_sexp()
            }
            #[inline]
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                unsafe { self.to_string_lossy().into_owned().into_sexp_unchecked() }
            }
        }

        $(#[$ref_meta])*
        impl IntoR for $ref_ty {
            type Error = crate::into_r_error::IntoRError;
            #[inline]
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                self.to_string_lossy().into_owned().try_into_sexp()
            }
            #[inline]
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            #[inline]
            fn into_sexp(self) -> crate::SEXP {
                self.to_string_lossy().into_owned().into_sexp()
            }
            #[inline]
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                unsafe { self.to_string_lossy().into_owned().into_sexp_unchecked() }
            }
        }

        $(#[$option_meta])*
        impl IntoR for Option<$owned_ty> {
            type Error = crate::into_r_error::IntoRError;
            #[inline]
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                self.map(|v| v.to_string_lossy().into_owned()).try_into_sexp()
            }
            #[inline]
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            #[inline]
            fn into_sexp(self) -> crate::SEXP {
                self.map(|v| v.to_string_lossy().into_owned()).into_sexp()
            }
            #[inline]
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                unsafe {
                    self.map(|v| v.to_string_lossy().into_owned())
                        .into_sexp_unchecked()
                }
            }
        }

        $(#[$vec_meta])*
        impl IntoR for Vec<$owned_ty> {
            type Error = crate::into_r_error::IntoRError;
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            fn into_sexp(self) -> crate::SEXP {
                let strings: Vec<String> = self
                    .into_iter()
                    .map(|v| v.to_string_lossy().into_owned())
                    .collect();
                strings.into_sexp()
            }
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                let strings: Vec<String> = self
                    .into_iter()
                    .map(|v| v.to_string_lossy().into_owned())
                    .collect();
                unsafe { strings.into_sexp_unchecked() }
            }
        }

        $(#[$vec_option_meta])*
        impl IntoR for Vec<Option<$owned_ty>> {
            type Error = crate::into_r_error::IntoRError;
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            fn into_sexp(self) -> crate::SEXP {
                let strings: Vec<Option<String>> = self
                    .into_iter()
                    .map(|opt| opt.map(|v| v.to_string_lossy().into_owned()))
                    .collect();
                strings.into_sexp()
            }
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                let strings: Vec<Option<String>> = self
                    .into_iter()
                    .map(|opt| opt.map(|v| v.to_string_lossy().into_owned()))
                    .collect();
                unsafe { strings.into_sexp_unchecked() }
            }
        }
    };
}

impl_lossy_string_into_r!(
    /// Convert `PathBuf` to R character scalar.
    ///
    /// On Unix, paths that are not valid UTF-8 will produce lossy output
    /// (invalid sequences replaced with U+FFFD).
    owned: PathBuf;
    /// Convert `&Path` to R character scalar.
    ref: &std::path::Path;
    /// Convert `Option<PathBuf>` to R: Some(path) -> character, None -> NA_character_.
    option: PathBuf;
    /// Convert `Vec<PathBuf>` to R character vector.
    vec: PathBuf;
    /// Convert `Vec<Option<PathBuf>>` to R character vector with NA support.
    vec_option: PathBuf;
);

impl_lossy_string_into_r!(
    /// Convert `OsString` to R character scalar.
    ///
    /// On Unix, strings that are not valid UTF-8 will produce lossy output
    /// (invalid sequences replaced with U+FFFD).
    owned: OsString;
    /// Convert `&OsStr` to R character scalar.
    ref: &std::ffi::OsStr;
    /// Convert `Option<OsString>` to R: Some(s) -> character, None -> NA_character_.
    option: OsString;
    /// Convert `Vec<OsString>` to R character vector.
    vec: OsString;
    /// Convert `Vec<Option<OsString>>` to R character vector with NA support.
    vec_option: OsString;
);
// endregion

// region: Set coercion for non-native types (i8, i16, u16 → i32)

/// Macro for `HashSet<T>`/`BTreeSet<T>` where `T` coerces to i32 (R's native integer type).
macro_rules! impl_set_coerce_into_r {
    ($from:ty) => {
        impl IntoR for HashSet<$from> {
            type Error = std::convert::Infallible;
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                self.try_into_sexp()
            }
            fn into_sexp(self) -> crate::SEXP {
                let vec: Vec<i32> = self.into_iter().map(|x| i32::from(x)).collect();
                vec.into_sexp()
            }
        }

        impl IntoR for BTreeSet<$from> {
            type Error = std::convert::Infallible;
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                self.try_into_sexp()
            }
            fn into_sexp(self) -> crate::SEXP {
                let vec: Vec<i32> = self.into_iter().map(|x| i32::from(x)).collect();
                vec.into_sexp()
            }
        }
    };
}

// Sub-i32 integer types in sets coerce to i32 (R's INTSXP)
impl_set_coerce_into_r!(i8);
impl_set_coerce_into_r!(i16);
impl_set_coerce_into_r!(u16);
// endregion

// region: Option<Collection> conversions
//
// These return NULL (R_NilValue) for None, and the converted collection for Some.
// This differs from Option<scalar> which returns NA for None.

/// Convert `Option<Vec<T>>` to R: `Some(vec)` → the R vector `Vec<T>`
/// produces, `None` → NULL, for every `T` with a `Vec<T>: IntoR` conversion.
impl<T> IntoR for Option<Vec<T>>
where
    Vec<T>: IntoR,
{
    type Error = <Vec<T> as IntoR>::Error;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        match self {
            Some(v) => v.try_into_sexp(),
            None => Ok(crate::SEXP::nil()),
        }
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        match self {
            Some(v) => unsafe { v.try_into_sexp_unchecked() },
            None => Ok(crate::SEXP::nil()),
        }
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        match self {
            Some(v) => v.into_sexp(),
            None => crate::SEXP::nil(),
        }
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        match self {
            Some(v) => unsafe { v.into_sexp_unchecked() },
            None => crate::SEXP::nil(),
        }
    }
}

/// Convert `Option<Box<[T]>>` to R exactly as `Option<Vec<T>>` (`into_vec()`
/// is an O(1) widen): `Some` → the R vector `Vec<T>` produces, `None` → NULL.
impl<T> IntoR for Option<Box<[T]>>
where
    Vec<T>: IntoR,
{
    type Error = <Vec<T> as IntoR>::Error;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        self.map(<[T]>::into_vec).try_into_sexp()
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        unsafe { self.map(<[T]>::into_vec).try_into_sexp_unchecked() }
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        self.map(<[T]>::into_vec).into_sexp()
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { self.map(<[T]>::into_vec).into_sexp_unchecked() }
    }
}

/// Convert `Option<&[T]>` to R as the slice converts: `Some` → the R vector
/// `&[T]` produces, `None` → NULL.
impl<'a, T> IntoR for Option<&'a [T]>
where
    &'a [T]: IntoR,
{
    type Error = <&'a [T] as IntoR>::Error;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        match self {
            Some(v) => v.try_into_sexp(),
            None => Ok(crate::SEXP::nil()),
        }
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        match self {
            Some(v) => unsafe { v.try_into_sexp_unchecked() },
            None => Ok(crate::SEXP::nil()),
        }
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        match self {
            Some(v) => v.into_sexp(),
            None => crate::SEXP::nil(),
        }
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        match self {
            Some(v) => unsafe { v.into_sexp_unchecked() },
            None => crate::SEXP::nil(),
        }
    }
}

/// Convert `Option<HashMap<String, V>>` to R: Some(map) -> named list, None -> NULL.
impl<V: IntoR> IntoR for Option<HashMap<String, V>> {
    type Error = crate::into_r_error::IntoRError;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        match self {
            Some(v) => v.into_sexp(),
            None => crate::SEXP::nil(),
        }
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        match self {
            Some(v) => unsafe { v.into_sexp_unchecked() },
            None => crate::SEXP::nil(),
        }
    }
}

/// Convert `Option<BTreeMap<String, V>>` to R: Some(map) -> named list, None -> NULL.
impl<V: IntoR> IntoR for Option<BTreeMap<String, V>> {
    type Error = crate::into_r_error::IntoRError;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        match self {
            Some(v) => v.into_sexp(),
            None => crate::SEXP::nil(),
        }
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        match self {
            Some(v) => unsafe { v.into_sexp_unchecked() },
            None => crate::SEXP::nil(),
        }
    }
}

/// Convert `Option<HashSet<T>>` to R: Some(set) -> vector, None -> NULL.
impl<T: crate::RNativeType + Eq + Hash> IntoR for Option<HashSet<T>> {
    type Error = std::convert::Infallible;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        match self {
            Some(v) => v.into_sexp(),
            None => crate::SEXP::nil(),
        }
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        match self {
            Some(v) => unsafe { v.into_sexp_unchecked() },
            None => crate::SEXP::nil(),
        }
    }
}

/// Convert `Option<BTreeSet<T>>` to R: Some(set) -> vector, None -> NULL.
impl<T: crate::RNativeType + Ord> IntoR for Option<BTreeSet<T>> {
    type Error = std::convert::Infallible;
    #[inline]
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    #[inline]
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    #[inline]
    fn into_sexp(self) -> crate::SEXP {
        match self {
            Some(v) => v.into_sexp(),
            None => crate::SEXP::nil(),
        }
    }
    #[inline]
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        match self {
            Some(v) => unsafe { v.into_sexp_unchecked() },
            None => crate::SEXP::nil(),
        }
    }
}

macro_rules! impl_option_collection_into_r {
    ($(#[$meta:meta])* $ty:ty) => {
        $(#[$meta])*
        impl IntoR for Option<$ty> {
            type Error = crate::into_r_error::IntoRError;
            #[inline]
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            #[inline]
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            #[inline]
            fn into_sexp(self) -> crate::SEXP {
                match self {
                    Some(v) => v.into_sexp(),
                    None => crate::SEXP::nil(),
                }
            }
            #[inline]
            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                match self {
                    Some(v) => unsafe { v.into_sexp_unchecked() },
                    None => crate::SEXP::nil(),
                }
            }
        }
    };
}

impl_option_collection_into_r!(
    /// Convert `Option<HashSet<String>>` to R: Some(set) -> character vector, None -> NULL.
    HashSet<String>
);
impl_option_collection_into_r!(
    /// Convert `Option<BTreeSet<String>>` to R: Some(set) -> character vector, None -> NULL.
    BTreeSet<String>
);

/// Helper: allocate STRSXP and fill from a string iterator (checked).
///
/// Routes the alloc + protect through [`StrVecBuilder`] over a local
/// [`ProtectScope`]; the CHARSXP policy (empty-string → `R_BlankString`) stays
/// here via [`str_to_charsxp`].
pub(crate) fn str_iter_to_strsxp<'a>(iter: impl ExactSizeIterator<Item = &'a str>) -> crate::SEXP {
    unsafe {
        let scope = ProtectScope::new();
        let builder = StrVecBuilder::new(&scope, iter.len());
        let vec = builder.as_sexp();
        for (i, s) in iter.enumerate() {
            vec.set_string_elt(i as isize, str_to_charsxp(s));
        }
        builder.into_sexp()
    }
}

/// Helper: allocate STRSXP and fill from a string iterator (unchecked).
pub(crate) unsafe fn str_iter_to_strsxp_unchecked<'a>(
    iter: impl ExactSizeIterator<Item = &'a str>,
) -> crate::SEXP {
    unsafe {
        let scope = ProtectScope::new();
        let builder = StrVecBuilder::new_unchecked(&scope, iter.len());
        let vec = builder.as_sexp();
        for (i, s) in iter.enumerate() {
            vec.set_string_elt_unchecked(i as isize, str_to_charsxp_unchecked(s));
        }
        builder.into_sexp()
    }
}

/// Helper: allocate STRSXP and fill from an optional-string iterator (checked).
///
/// `None` becomes `NA_character_`. Shared by the `Vec<Option<…str>>` impls.
pub(crate) fn opt_str_iter_to_strsxp<'a>(
    iter: impl ExactSizeIterator<Item = Option<&'a str>>,
) -> crate::SEXP {
    unsafe {
        let scope = ProtectScope::new();
        let builder = StrVecBuilder::new(&scope, iter.len());
        let vec = builder.as_sexp();
        for (i, opt_s) in iter.enumerate() {
            let charsxp = match opt_s {
                Some(s) => str_to_charsxp(s),
                None => crate::SEXP::na_string(),
            };
            vec.set_string_elt(i as isize, charsxp);
        }
        builder.into_sexp()
    }
}

/// Helper: allocate STRSXP and fill from an optional-string iterator (unchecked).
pub(crate) unsafe fn opt_str_iter_to_strsxp_unchecked<'a>(
    iter: impl ExactSizeIterator<Item = Option<&'a str>>,
) -> crate::SEXP {
    unsafe {
        let scope = ProtectScope::new();
        let builder = StrVecBuilder::new_unchecked(&scope, iter.len());
        let vec = builder.as_sexp();
        for (i, opt_s) in iter.enumerate() {
            let charsxp = match opt_s {
                Some(s) => str_to_charsxp_unchecked(s),
                None => crate::SEXP::na_string(),
            };
            vec.set_string_elt_unchecked(i as isize, charsxp);
        }
        builder.into_sexp()
    }
}

/// Convert `Vec<String>` to R character vector (STRSXP).
impl IntoR for Vec<String> {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        str_iter_to_strsxp(self.iter().map(|s| s.as_str()))
    }

    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { str_iter_to_strsxp_unchecked(self.iter().map(|s| s.as_str())) }
    }
}

/// Convert `&[String]` to R character vector (STRSXP).
impl IntoR for &[String] {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        str_iter_to_strsxp(self.iter().map(|s| s.as_str()))
    }

    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { str_iter_to_strsxp_unchecked(self.iter().map(|s| s.as_str())) }
    }
}

/// Convert &[&str] to R character vector (STRSXP).
impl IntoR for &[&str] {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        str_iter_to_strsxp(self.iter().copied())
    }

    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { str_iter_to_strsxp_unchecked(self.iter().copied()) }
    }
}

/// Convert `Vec<Cow<'_, str>>` to R character vector (STRSXP).
impl IntoR for Vec<std::borrow::Cow<'_, str>> {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        str_iter_to_strsxp(self.iter().map(|s| s.as_ref()))
    }
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { str_iter_to_strsxp_unchecked(self.iter().map(|s| s.as_ref())) }
    }
}

/// Convert `Vec<Option<Cow<'_, str>>>` to R character vector with NA support.
///
/// `None` values become `NA_character_` in R.
impl IntoR for Vec<Option<std::borrow::Cow<'_, str>>> {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        opt_str_iter_to_strsxp(self.iter().map(|o| o.as_ref().map(|c| c.as_ref())))
    }
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe {
            opt_str_iter_to_strsxp_unchecked(self.iter().map(|o| o.as_ref().map(|c| c.as_ref())))
        }
    }
}

// region: Vec<Option<borrowed string>>
/// Convert `Vec<Option<&str>>` to R character vector with NA support.
///
/// `None` values become `NA_character_` in R. Borrowed analogue of `Vec<Option<String>>`.
impl IntoR for Vec<Option<&str>> {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        opt_str_iter_to_strsxp(self.iter().copied())
    }
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { opt_str_iter_to_strsxp_unchecked(self.iter().copied()) }
    }
}
// endregion

/// Convert `Vec<&str>` to R character vector (STRSXP).
impl IntoR for Vec<&str> {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        self.as_slice().into_sexp()
    }

    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { self.as_slice().into_sexp_unchecked() }
    }
}
// endregion

// region: VECSXP-from-iterator helpers

/// Build a VECSXP from an exact-size iterator of child `SEXP`s (checked).
///
/// Routes through [`ListBuilder`]: the list is allocated and protected by a
/// local [`ProtectScope`] before iteration begins, then the iterator is consumed
/// inside the protected window so each child produced lazily (typically via a
/// `.map(|c| c.into_sexp())` adaptor at the callsite) is inserted immediately
/// via [`ListBuilder::set`], closing the GC gap.
///
/// The element type is `SEXP` (not a generic `IntoR`) deliberately: the caller
/// performs the conversion lazily in the iterator adaptor, which keeps the
/// trait solver from chasing the recursive `Vec<Option<Vec<T>>>` impl.
///
/// # Safety
///
/// Must be called from the R main thread.
#[inline]
unsafe fn vecsxp_from_iter<I>(iter: I) -> crate::SEXP
where
    I: ExactSizeIterator<Item = crate::SEXP>,
{
    unsafe {
        let scope = ProtectScope::new();
        let builder = ListBuilder::new(&scope, iter.len());
        for (i, child) in iter.enumerate() {
            builder.set(i as isize, child);
        }
        builder.into_sexp()
    }
}

/// `_unchecked` twin of [`vecsxp_from_iter`] — uses [`ListBuilder::set_unchecked`].
/// For ALTREP / `with_r_thread` / unwind contexts. The caller's adaptor must
/// produce children via `into_sexp_unchecked()`.
///
/// # Safety
///
/// Must be called from the R main thread, in a context where the checked-FFI
/// assertion is intentionally bypassed.
#[inline]
unsafe fn vecsxp_from_iter_unchecked<I>(iter: I) -> crate::SEXP
where
    I: ExactSizeIterator<Item = crate::SEXP>,
{
    unsafe {
        let scope = ProtectScope::new();
        let builder = ListBuilder::new_unchecked(&scope, iter.len());
        for (i, child) in iter.enumerate() {
            builder.set_unchecked(i as isize, child);
        }
        builder.into_sexp()
    }
}

// endregion

// region: Nested vector conversions (list of vectors)
//
// One generic impl per list-of-vectors shape, each bounded on the element
// vector's own conversion (`Vec<T>: IntoR`, or `&[T]: IntoR` for borrowed
// rows). Every row therefore lands exactly as the matching flat vector would:
// `Vec<Vec<usize>>` gives each row the smart INTSXP-or-REALSXP choice of
// `Vec<usize>`, `Vec<Vec<String>>` gives character vectors, and so on. The
// fallible paths propagate the row conversion's error type.

/// Fallible twin of [`vecsxp_from_iter`]: stops at the first row whose
/// conversion fails. The list stays protected by the local scope until it
/// returns, so the partially filled list is released on the error path.
///
/// # Safety
///
/// Must be called from the R main thread.
#[inline]
unsafe fn try_vecsxp_from_iter<I, E>(iter: I) -> Result<crate::SEXP, E>
where
    I: ExactSizeIterator<Item = Result<crate::SEXP, E>>,
{
    unsafe {
        let scope = ProtectScope::new();
        let builder = ListBuilder::new(&scope, iter.len());
        for (i, child) in iter.enumerate() {
            builder.set(i as isize, child?);
        }
        Ok(builder.into_sexp())
    }
}

/// `_unchecked` twin of [`try_vecsxp_from_iter`] — uses
/// [`ListBuilder::set_unchecked`]. The caller's adaptor must produce children
/// via `try_into_sexp_unchecked()`.
///
/// # Safety
///
/// Must be called from the R main thread, in a context where the checked-FFI
/// assertion is intentionally bypassed.
#[inline]
unsafe fn try_vecsxp_from_iter_unchecked<I, E>(iter: I) -> Result<crate::SEXP, E>
where
    I: ExactSizeIterator<Item = Result<crate::SEXP, E>>,
{
    unsafe {
        let scope = ProtectScope::new();
        let builder = ListBuilder::new_unchecked(&scope, iter.len());
        for (i, child) in iter.enumerate() {
            builder.set_unchecked(i as isize, child?);
        }
        Ok(builder.into_sexp())
    }
}

/// Convert `Vec<Vec<T>>` to an R list of vectors (VECSXP), for every `T` with
/// a `Vec<T>: IntoR` conversion. Each row is converted by that impl, so it has
/// the R type `Vec<T>` produces (for `usize` / `u64` / `i64` / `isize` / `u32`:
/// integer when every value fits, else double, decided per row).
impl<T> IntoR for Vec<Vec<T>>
where
    Vec<T>: IntoR,
{
    type Error = <Vec<T> as IntoR>::Error;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        unsafe { try_vecsxp_from_iter(self.into_iter().map(IntoR::try_into_sexp)) }
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        unsafe {
            try_vecsxp_from_iter_unchecked(self.into_iter().map(|c| c.try_into_sexp_unchecked()))
        }
    }
    fn into_sexp(self) -> crate::SEXP {
        unsafe { vecsxp_from_iter(self.into_iter().map(IntoR::into_sexp)) }
    }
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { vecsxp_from_iter_unchecked(self.into_iter().map(|c| c.into_sexp_unchecked())) }
    }
}

/// Convert `Vec<&[T]>` to an R list of vectors (VECSXP), for every `T` with a
/// `&[T]: IntoR` conversion.
///
/// Borrowed analogue of `Vec<Vec<T>>` — each slice is copied into a fresh R
/// vector of the type `&[T]` (and `Vec<T>`) produces.
impl<'a, T> IntoR for Vec<&'a [T]>
where
    &'a [T]: IntoR,
{
    type Error = <&'a [T] as IntoR>::Error;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        unsafe { try_vecsxp_from_iter(self.into_iter().map(IntoR::try_into_sexp)) }
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        unsafe {
            try_vecsxp_from_iter_unchecked(self.into_iter().map(|c| c.try_into_sexp_unchecked()))
        }
    }
    fn into_sexp(self) -> crate::SEXP {
        unsafe { vecsxp_from_iter(self.into_iter().map(IntoR::into_sexp)) }
    }
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { vecsxp_from_iter_unchecked(self.into_iter().map(|c| c.into_sexp_unchecked())) }
    }
}
// endregion

// region: NA-aware vector conversions

/// Macro for NA-aware `Vec<Option<T>> → R` vector conversions.
///
/// Uses `alloc_r_vector` to get a mutable slice, then fills it.
///
/// Unlike the sibling `Vec<Option<T>>` macros (`impl_vec_option_smart_i64_into_r!`
/// / `impl_vec_option_coerce_into_r!`, which route through `into_r_infallible!`
/// and inherit the default-unchecked policy), this one is written out by hand
/// because it carries a *real* `into_sexp_unchecked` path (`alloc_r_vector_unchecked`)
/// that the others have no analogue for. The divergence is deliberate.
macro_rules! impl_vec_option_into_r {
    ($t:ty, $na_value:expr) => {
        impl IntoR for Vec<Option<$t>> {
            type Error = std::convert::Infallible;
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            fn into_sexp(self) -> crate::SEXP {
                unsafe {
                    let (sexp, dst) = alloc_r_vector::<$t>(self.len());
                    for (slot, val) in dst.iter_mut().zip(self.into_iter()) {
                        *slot = val.unwrap_or($na_value);
                    }
                    sexp
                }
            }

            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                unsafe {
                    let (sexp, dst) = alloc_r_vector_unchecked::<$t>(self.len());
                    for (slot, val) in dst.iter_mut().zip(self.into_iter()) {
                        *slot = val.unwrap_or($na_value);
                    }
                    sexp
                }
            }
        }
    };
}

impl_vec_option_into_r!(f64, NA_REAL); // NA_real_
impl_vec_option_into_r!(i32, NA_INTEGER); // NA_integer_

/// Macro for NA-aware `Vec<Option<T>> → R` smart vector conversion.
/// Checks if all non-None values fit i32 → INTSXP, otherwise REALSXP.
///
/// Allocates the R vector directly and coerces in-place — no intermediate Vec.
macro_rules! impl_vec_option_smart_i64_into_r {
    ($t:ty, $fits_i32:expr) => {
        // Default-unchecked policy, single-sourced through `into_r_infallible!`
        // (no bespoke unchecked path — the checked allocation is the only one).
        into_r_infallible!(Vec<Option<$t>>, |this| unsafe {
            if this.iter().all(|opt| match opt {
                Some(x) => $fits_i32(*x),
                None => true,
            }) {
                let (sexp, dst) = alloc_r_vector::<i32>(this.len());
                for (slot, val) in dst.iter_mut().zip(this.into_iter()) {
                    *slot = match val {
                        Some(x) => x as i32,
                        None => NA_INTEGER,
                    };
                }
                sexp
            } else {
                let (sexp, dst) = alloc_r_vector::<f64>(this.len());
                for (slot, val) in dst.iter_mut().zip(this.into_iter()) {
                    *slot = match val {
                        Some(x) => x as f64,
                        None => NA_REAL,
                    };
                }
                sexp
            }
        });
    };
}

// i32::MIN is NA_integer_ in R, so exclude it
impl_vec_option_smart_i64_into_r!(i64, |x: i64| x > i32::MIN as i64 && x <= i32::MAX as i64);
impl_vec_option_smart_i64_into_r!(u64, |x: u64| x <= i32::MAX as u64);
impl_vec_option_smart_i64_into_r!(isize, |x: isize| x > i32::MIN as isize
    && x <= i32::MAX as isize);
impl_vec_option_smart_i64_into_r!(usize, |x: usize| x <= i32::MAX as usize);

/// Macro for `Vec<Option<T>>` where `T` coerces to a type with existing Option impl.
///
/// Delegates to the target type's `Vec<Option<$to>>` impl (which itself uses alloc_r_vector).
macro_rules! impl_vec_option_coerce_into_r {
    ($from:ty => $to:ty) => {
        // Default-unchecked policy, single-sourced through `into_r_infallible!`;
        // `into_sexp` delegates to the target `Vec<Option<$to>>` impl.
        into_r_infallible!(Vec<Option<$from>>, |this| {
            let coerced: Vec<Option<$to>> = this
                .into_iter()
                .map(|opt| opt.map(|x| <$to>::from(x)))
                .collect();
            coerced.into_sexp()
        });
    };
}

impl_vec_option_coerce_into_r!(i8 => i32);
impl_vec_option_coerce_into_r!(i16 => i32);
impl_vec_option_coerce_into_r!(u16 => i32);
impl_vec_option_coerce_into_r!(u32 => i64); // delegates to smart i64 path
impl_vec_option_coerce_into_r!(f32 => f64);

/// Helper: allocate LGLSXP and fill from an i32 iterator (checked).
///
/// Uses `alloc_r_vector` — logical vectors are `RLogical` (repr(transparent) i32).
fn logical_iter_to_lglsxp(n: usize, iter: impl Iterator<Item = i32>) -> crate::SEXP {
    unsafe {
        let (sexp, dst) = alloc_r_vector::<crate::RLogical>(n);
        // RLogical is repr(transparent) over i32, safe to write i32 values.
        let dst_i32: &mut [i32] = std::slice::from_raw_parts_mut(dst.as_mut_ptr().cast::<i32>(), n);
        for (slot, val) in dst_i32.iter_mut().zip(iter) {
            *slot = val;
        }
        sexp
    }
}

/// Helper: allocate LGLSXP and fill from an i32 iterator (unchecked).
unsafe fn logical_iter_to_lglsxp_unchecked(
    n: usize,
    iter: impl Iterator<Item = i32>,
) -> crate::SEXP {
    unsafe {
        let (sexp, dst) = alloc_r_vector_unchecked::<crate::RLogical>(n);
        let dst_i32: &mut [i32] = std::slice::from_raw_parts_mut(dst.as_mut_ptr().cast::<i32>(), n);
        for (slot, val) in dst_i32.iter_mut().zip(iter) {
            *slot = val;
        }
        sexp
    }
}

/// Convert `Vec<bool>` to R logical vector.
impl IntoR for Vec<bool> {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        let n = self.len();
        logical_iter_to_lglsxp(n, self.into_iter().map(i32::from))
    }

    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        let n = self.len();
        unsafe { logical_iter_to_lglsxp_unchecked(n, self.into_iter().map(i32::from)) }
    }
}

/// Convert `&[bool]` to R logical vector.
impl IntoR for &[bool] {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        let n = self.len();
        logical_iter_to_lglsxp(n, self.iter().map(|&v| i32::from(v)))
    }

    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        let n = self.len();
        unsafe { logical_iter_to_lglsxp_unchecked(n, self.iter().map(|&v| i32::from(v))) }
    }
}

/// Convert `Vec<Rboolean>` to R logical vector (no NA: Rboolean is TRUE/FALSE only).
impl IntoR for Vec<crate::Rboolean> {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        let n = self.len();
        logical_iter_to_lglsxp(n, self.into_iter().map(|b| b as i32))
    }

    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        let n = self.len();
        unsafe { logical_iter_to_lglsxp_unchecked(n, self.into_iter().map(|b| b as i32)) }
    }
}

/// Convert `&[Rboolean]` to R logical vector (no NA: Rboolean is TRUE/FALSE only).
impl IntoR for &[crate::Rboolean] {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        let n = self.len();
        logical_iter_to_lglsxp(n, self.iter().map(|&b| b as i32))
    }

    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        let n = self.len();
        unsafe { logical_iter_to_lglsxp_unchecked(n, self.iter().map(|&b| b as i32)) }
    }
}

macro_rules! impl_vec_option_logical_into_r {
    ($(#[$meta:meta])* $t:ty, $convert:expr) => {
        $(#[$meta])*
        impl IntoR for Vec<Option<$t>> {
            type Error = std::convert::Infallible;
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                Ok(unsafe { self.into_sexp_unchecked() })
            }
            fn into_sexp(self) -> crate::SEXP {
                let n = self.len();
                logical_iter_to_lglsxp(n, self.into_iter().map($convert))
            }

            unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
                let n = self.len();
                unsafe { logical_iter_to_lglsxp_unchecked(n, self.into_iter().map($convert)) }
            }
        }
    };
}

impl_vec_option_logical_into_r!(
    /// Convert `Vec<Option<bool>>` to R logical vector with NA support.
    bool,
    |v: Option<bool>| match v {
        Some(true) => 1,
        Some(false) => 0,
        None => NA_LOGICAL,
    }
);
impl_vec_option_logical_into_r!(
    /// Convert `Vec<Option<Rboolean>>` to R logical vector with NA support.
    crate::Rboolean,
    |v: Option<crate::Rboolean>| match v {
        Some(b) => b as i32,
        None => NA_LOGICAL,
    }
);
impl_vec_option_logical_into_r!(
    /// Convert `Vec<Option<RLogical>>` to R logical vector with NA support.
    crate::RLogical,
    |v: Option<crate::RLogical>| match v {
        Some(b) => b.to_i32(),
        None => NA_LOGICAL,
    }
);

/// Convert `Vec<Option<String>>` to R character vector with NA support.
///
/// `None` values become `NA_character_` in R.
impl IntoR for Vec<Option<String>> {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        Ok(unsafe { self.into_sexp_unchecked() })
    }
    fn into_sexp(self) -> crate::SEXP {
        opt_str_iter_to_strsxp(self.iter().map(|o| o.as_deref()))
    }

    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe { opt_str_iter_to_strsxp_unchecked(self.iter().map(|o| o.as_deref())) }
    }
}
// endregion

// region: Tuple to list conversions

// Reuse the protected streaming list builder for both tuple-row shapes.
// Failed rows leave NULL slots while later rows are checked; the partial list
// never escapes on failure. Each successful row is rooted before the next runs.
macro_rules! impl_tuple_vec_into_r {
    (($($T:ident),+), $row:ty) => {
        impl<$($T: IntoR),+> IntoR for Vec<$row> {
            type Error = crate::into_r_error::IntoRError;

            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                let mut errors = crate::from_r::BatchedErrors::default();
                let result = unsafe {
                    vecsxp_from_iter(self.into_iter().enumerate().map(|(index, row)| {
                        match row.try_into_sexp() {
                            Ok(child) => child,
                            Err(error) => {
                                errors.push(index, || error.to_string());
                                crate::SEXP::nil()
                            }
                        }
                    }))
                };
                if errors.is_empty() {
                    Ok(result)
                } else {
                    Err(crate::into_r_error::IntoRError::Inner(errors.element_message()))
                }
            }

            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                let mut errors = crate::from_r::BatchedErrors::default();
                let result = unsafe {
                    vecsxp_from_iter_unchecked(self.into_iter().enumerate().map(|(index, row)| {
                        match row.try_into_sexp_unchecked() {
                            Ok(child) => child,
                            Err(error) => {
                                errors.push(index, || error.to_string());
                                crate::SEXP::nil()
                            }
                        }
                    }))
                };
                if errors.is_empty() {
                    Ok(result)
                } else {
                    Err(crate::into_r_error::IntoRError::Inner(errors.element_message()))
                }
            }
        }
    };
}

/// Convert Rust tuples to unnamed R lists, collecting every failing field.
/// Field error types may differ; their messages share the existing batched
/// conversion format with 1-based positions, wrapped in `IntoRError::Inner`.
macro_rules! impl_tuple_into_r {
    ($n:literal, $name:literal; $($T:ident $R:ident $idx:tt),+) => {
        impl<$($T: IntoR),+> IntoR for ($($T,)+) {
            type Error = crate::into_r_error::IntoRError;

            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                unsafe {
                    let scope = ProtectScope::new();
                    let builder = ListBuilder::new(&scope, $n);
                    let mut errors = crate::from_r::BatchedErrors::default();
                    $(
                        match self.$idx.try_into_sexp() {
                            Ok(child) => builder.set($idx as isize, child),
                            Err(error) => errors.push($idx, || error.to_string()),
                        }
                    )+
                    if errors.is_empty() {
                        Ok(builder.into_sexp())
                    } else {
                        Err(crate::into_r_error::IntoRError::Inner(errors.element_message()))
                    }
                }
            }

            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                unsafe {
                    let scope = ProtectScope::new();
                    let builder = ListBuilder::new_unchecked(&scope, $n);
                    let mut errors = crate::from_r::BatchedErrors::default();
                    $(
                        match self.$idx.try_into_sexp_unchecked() {
                            Ok(child) => builder.set_unchecked($idx as isize, child),
                            Err(error) => errors.push($idx, || error.to_string()),
                        }
                    )+
                    if errors.is_empty() {
                        Ok(builder.into_sexp())
                    } else {
                        Err(crate::into_r_error::IntoRError::Inner(errors.element_message()))
                    }
                }
            }
        }

        impl<$($T: IntoR),+> IntoR for Option<($($T,)+)> {
            type Error = <($($T,)+) as IntoR>::Error;

            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                self.map_or_else(|| Ok(crate::SEXP::nil()), IntoR::try_into_sexp)
            }

            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                self.map_or_else(|| Ok(crate::SEXP::nil()), |value| unsafe { value.try_into_sexp_unchecked() })
            }
        }

        impl_tuple_vec_into_r!(($($T),+), ($($T,)+));
        impl_tuple_vec_into_r!(($($T),+), Option<($($T,)+)>);
    };
}

crate::tuple_macros::for_each_tuple!(impl_tuple_into_r);
// endregion

// region: ALTREP zero-copy extension trait

/// Extension trait for ALTREP conversions.
///
/// This trait provides ergonomic methods for converting Rust types to R ALTREP
/// vectors without copying data. The data stays in Rust memory (wrapped in an
/// ExternalPtr) and R accesses it via ALTREP callbacks.
///
/// # Performance Characteristics
///
/// | Operation | Regular (IntoR) | ALTREP (IntoRAltrep) |
/// |-----------|-----------------|------------------------|
/// | Creation | O(n) copy | O(1) wrap |
/// | Memory | Duplicated in R | Single copy in Rust |
/// | Element access | Direct pointer | Callback (~10ns overhead) |
/// | DATAPTR ops | O(1) | O(1) if Vec/Box, N/A if lazy |
///
/// # When to Use ALTREP
///
/// **Good candidates**:
/// - ✅ Large vectors (>1000 elements)
/// - ✅ Lazy/computed data (avoid eager materialization)
/// - ✅ External data sources (files, databases, APIs)
/// - ✅ Data that might not be fully accessed by R
///
/// **Not recommended**:
/// - ❌ Small vectors (<100 elements) - copy overhead is negligible
/// - ❌ Data R will immediately modify (triggers copy anyway)
/// - ❌ Temporary results (extra indirection not worth it)
///
/// # Example
///
/// ```rust,ignore
/// use miniextendr_api::{miniextendr, IntoRAltrep, IntoR, SEXP};
///
/// #[miniextendr]
/// fn large_dataset() -> SEXP {
///     let data: Vec<f64> = (0..1_000_000).map(|i| i as f64).collect();
///
///     // Zero-copy: wraps pointer instead of copying 1M elements
///     data.into_sexp_altrep()
/// }
///
/// #[miniextendr]
/// fn small_result() -> SEXP {
///     let data = vec![1, 2, 3, 4, 5];
///
///     // Regular copy is fine for small data
///     data.into_sexp()
/// }
/// ```
pub trait IntoRAltrep {
    /// Convert to R SEXP using ALTREP zero-copy representation.
    ///
    /// This is equivalent to `Altrep(self).into_sexp()` but more discoverable
    /// and explicit about the zero-copy intent.
    fn into_sexp_altrep(self) -> crate::SEXP;

    /// Convert to R SEXP using ALTREP, skipping debug thread assertions.
    ///
    /// # Safety
    ///
    /// Caller must ensure they are on R's main thread.
    unsafe fn into_sexp_altrep_unchecked(self) -> crate::SEXP
    where
        Self: Sized,
    {
        self.into_sexp_altrep()
    }

    /// Create an `Altrep<Self>` wrapper.
    ///
    /// This returns the wrapper explicitly, allowing you to store it or
    /// further process it before conversion.
    fn into_altrep(self) -> Altrep<Self>
    where
        Self: Sized,
    {
        Altrep(self)
    }
}

impl<T> IntoRAltrep for T
where
    T: crate::altrep::RegisterAltrep + crate::externalptr::TypedExternal,
{
    fn into_sexp_altrep(self) -> crate::SEXP {
        Altrep(self).into_sexp()
    }

    unsafe fn into_sexp_altrep_unchecked(self) -> crate::SEXP {
        unsafe { Altrep(self).into_sexp_unchecked() }
    }
}
// endregion

// region: Additional collection type conversions for DataFrameRow support

/// Convert `Vec<Box<[T]>>` to an R list of vectors, for every `T` with a
/// `Vec<T>: IntoR` conversion. Each boxed slice becomes the R vector its
/// `Vec<T>` would (`into_vec()` is an O(1) widen).
impl<T> IntoR for Vec<Box<[T]>>
where
    Vec<T>: IntoR,
{
    type Error = <Vec<T> as IntoR>::Error;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        unsafe { try_vecsxp_from_iter(self.into_iter().map(|b| b.into_vec().try_into_sexp())) }
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        unsafe {
            try_vecsxp_from_iter_unchecked(
                self.into_iter()
                    .map(|b| b.into_vec().try_into_sexp_unchecked()),
            )
        }
    }
    fn into_sexp(self) -> crate::SEXP {
        unsafe { vecsxp_from_iter(self.into_iter().map(|b| b.into_vec().into_sexp())) }
    }
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe {
            vecsxp_from_iter_unchecked(self.into_iter().map(|b| b.into_vec().into_sexp_unchecked()))
        }
    }
}

/// Convert `Vec<[T; N]>` to an R list of vectors, for every `T` with a
/// `Vec<T>: IntoR` conversion. Each array becomes the R vector its `Vec<T>`
/// would.
impl<T, const N: usize> IntoR for Vec<[T; N]>
where
    Vec<T>: IntoR,
{
    type Error = <Vec<T> as IntoR>::Error;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        unsafe { try_vecsxp_from_iter(self.into_iter().map(|a| Vec::from(a).try_into_sexp())) }
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        unsafe {
            try_vecsxp_from_iter_unchecked(
                self.into_iter()
                    .map(|a| Vec::from(a).try_into_sexp_unchecked()),
            )
        }
    }
    fn into_sexp(self) -> crate::SEXP {
        unsafe { vecsxp_from_iter(self.into_iter().map(|a| Vec::from(a).into_sexp())) }
    }
    unsafe fn into_sexp_unchecked(self) -> crate::SEXP {
        unsafe {
            vecsxp_from_iter_unchecked(self.into_iter().map(|a| Vec::from(a).into_sexp_unchecked()))
        }
    }
}

/// Helper: convert a Vec of IntoR items to an R list (VECSXP).
fn vec_of_into_r_to_list<T: IntoR>(items: Vec<T>) -> crate::SEXP {
    unsafe { vecsxp_from_iter(items.into_iter().map(|item| item.into_sexp())) }
}

// region: Vec<Option<Collection>> conversions

/// Helper: convert `Vec<Option<C: IntoR>>` to a VECSXP, with `None` mapping to
/// `R_NilValue` (NULL) and `Some(v)` mapping to whatever `v.into_sexp()` produces.
fn vec_option_of_into_r_to_list<T: IntoR>(items: Vec<Option<T>>) -> crate::SEXP {
    unsafe {
        vecsxp_from_iter(items.into_iter().map(|item| match item {
            Some(v) => v.into_sexp(),
            None => crate::SEXP::nil(),
        }))
    }
}

/// Convert `Vec<Option<Vec<T>>>` to an R list where `None` → NULL and
/// `Some(v)` → the R vector `Vec<T>` produces, for every `T` with a
/// `Vec<T>: IntoR` conversion.
impl<T> IntoR for Vec<Option<Vec<T>>>
where
    Vec<T>: IntoR,
{
    type Error = <Vec<T> as IntoR>::Error;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        unsafe {
            try_vecsxp_from_iter(self.into_iter().map(|item| match item {
                Some(v) => v.try_into_sexp(),
                None => Ok(crate::SEXP::nil()),
            }))
        }
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        unsafe {
            try_vecsxp_from_iter_unchecked(self.into_iter().map(|item| match item {
                Some(v) => v.try_into_sexp_unchecked(),
                None => Ok(crate::SEXP::nil()),
            }))
        }
    }
    fn into_sexp(self) -> crate::SEXP {
        vec_option_of_into_r_to_list(self)
    }
}

/// Convert `Vec<Option<Box<[T]>>>` exactly as `Vec<Option<Vec<T>>>` (each
/// `into_vec()` is an O(1) widen). The enum `DataFrameRow` companion stores an
/// opaque `Box<[T]>` variant field in this shape.
impl<T> IntoR for Vec<Option<Box<[T]>>>
where
    Vec<T>: IntoR,
{
    type Error = <Vec<T> as IntoR>::Error;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        unsafe {
            try_vecsxp_from_iter(self.into_iter().map(|item| match item {
                Some(b) => b.into_vec().try_into_sexp(),
                None => Ok(crate::SEXP::nil()),
            }))
        }
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        unsafe {
            try_vecsxp_from_iter_unchecked(self.into_iter().map(|item| match item {
                Some(b) => b.into_vec().try_into_sexp_unchecked(),
                None => Ok(crate::SEXP::nil()),
            }))
        }
    }
    fn into_sexp(self) -> crate::SEXP {
        vec_option_of_into_r_to_list::<Vec<T>>(
            self.into_iter().map(|b| b.map(<[T]>::into_vec)).collect(),
        )
    }
}

/// Convert `Vec<Option<HashSet<T>>>` to R list where `None` → NULL, `Some(s)` → unordered vector.
impl<T: crate::RNativeType + Eq + Hash> IntoR for Vec<Option<HashSet<T>>>
where
    HashSet<T>: IntoR,
{
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        self.try_into_sexp()
    }
    fn into_sexp(self) -> crate::SEXP {
        vec_option_of_into_r_to_list(self)
    }
}

// Convert `Vec<Option<HashSet<String>>>` to R list where `None` → NULL, `Some(s)` → character vector.
into_r_infallible!(Vec<Option<HashSet<String>>>, |this| {
    vec_option_of_into_r_to_list(this)
});

/// Convert `Vec<Option<BTreeSet<T>>>` to R list where `None` → NULL, `Some(s)` → sorted vector.
impl<T: crate::RNativeType + Ord> IntoR for Vec<Option<BTreeSet<T>>>
where
    BTreeSet<T>: IntoR,
{
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        self.try_into_sexp()
    }
    fn into_sexp(self) -> crate::SEXP {
        vec_option_of_into_r_to_list(self)
    }
}

// Convert `Vec<Option<BTreeSet<String>>>` to R list where `None` → NULL, `Some(s)` → sorted character vector.
into_r_infallible!(Vec<Option<BTreeSet<String>>>, |this| {
    vec_option_of_into_r_to_list(this)
});

/// Convert `Vec<Option<HashMap<String, V>>>` to R list where `None` → NULL, `Some(m)` → named list.
impl<V: IntoR> IntoR for Vec<Option<HashMap<String, V>>> {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        self.try_into_sexp()
    }
    fn into_sexp(self) -> crate::SEXP {
        vec_option_of_into_r_to_list(self)
    }
}

/// Convert `Vec<Option<BTreeMap<String, V>>>` to R list where `None` → NULL, `Some(m)` → named list.
impl<V: IntoR> IntoR for Vec<Option<BTreeMap<String, V>>> {
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        self.try_into_sexp()
    }
    fn into_sexp(self) -> crate::SEXP {
        vec_option_of_into_r_to_list(self)
    }
}

/// Convert `Vec<Option<&[T]>>` to an R list where `None` → NULL and
/// `Some(s)` → the R vector `&[T]` produces, for every `T` with a
/// `&[T]: IntoR` conversion.
///
/// Borrowed analogue of `Vec<Option<Vec<T>>>` — each slice is copied into a fresh R vector.
impl<'a, T> IntoR for Vec<Option<&'a [T]>>
where
    &'a [T]: IntoR,
{
    type Error = <&'a [T] as IntoR>::Error;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        unsafe {
            try_vecsxp_from_iter(self.into_iter().map(|item| match item {
                Some(v) => v.try_into_sexp(),
                None => Ok(crate::SEXP::nil()),
            }))
        }
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        unsafe {
            try_vecsxp_from_iter_unchecked(self.into_iter().map(|item| match item {
                Some(v) => v.try_into_sexp_unchecked(),
                None => Ok(crate::SEXP::nil()),
            }))
        }
    }
    fn into_sexp(self) -> crate::SEXP {
        vec_option_of_into_r_to_list(self)
    }
}

// endregion

/// Convert `Vec<HashSet<T>>` to R list of vectors (for RNativeType elements).
/// Each HashSet becomes an R vector (unordered).
impl<T: crate::RNativeType> IntoR for Vec<std::collections::HashSet<T>>
where
    Vec<T>: IntoR,
{
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        self.try_into_sexp()
    }
    fn into_sexp(self) -> crate::SEXP {
        let converted: Vec<Vec<T>> = self.into_iter().map(|s| s.into_iter().collect()).collect();
        vec_of_into_r_to_list(converted)
    }
}

/// Convert `Vec<BTreeSet<T>>` to R list of vectors (for RNativeType elements).
/// Each BTreeSet becomes an R vector (sorted).
impl<T: crate::RNativeType> IntoR for Vec<std::collections::BTreeSet<T>>
where
    Vec<T>: IntoR,
{
    type Error = std::convert::Infallible;
    fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
        Ok(self.into_sexp())
    }
    unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
        self.try_into_sexp()
    }
    fn into_sexp(self) -> crate::SEXP {
        let converted: Vec<Vec<T>> = self.into_iter().map(|s| s.into_iter().collect()).collect();
        vec_of_into_r_to_list(converted)
    }
}

// Convert `Vec<HashSet<String>>` to R list of character vectors.
into_r_infallible!(Vec<std::collections::HashSet<String>>, |this| {
    let converted: Vec<Vec<String>> = this.into_iter().map(|s| s.into_iter().collect()).collect();
    vec_of_into_r_to_list(converted)
});

// Convert `Vec<BTreeSet<String>>` to R list of character vectors.
into_r_infallible!(Vec<std::collections::BTreeSet<String>>, |this| {
    let converted: Vec<Vec<String>> = this.into_iter().map(|s| s.into_iter().collect()).collect();
    vec_of_into_r_to_list(converted)
});

macro_rules! impl_vec_map_into_r {
    ($(#[$meta:meta])* $map_ty:ident) => {
        $(#[$meta])*
        impl<V: IntoR> IntoR for Vec<$map_ty<String, V>> {
            type Error = std::convert::Infallible;
            fn try_into_sexp(self) -> Result<crate::SEXP, Self::Error> {
                Ok(self.into_sexp())
            }
            unsafe fn try_into_sexp_unchecked(self) -> Result<crate::SEXP, Self::Error> {
                self.try_into_sexp()
            }
            fn into_sexp(self) -> crate::SEXP {
                vec_of_maps_to_list(self)
            }
        }
    };
}

impl_vec_map_into_r!(
    /// Convert `Vec<HashMap<String, V>>` to R list of named lists.
    HashMap
);
impl_vec_map_into_r!(
    /// Convert `Vec<BTreeMap<String, V>>` to R list of named lists.
    BTreeMap
);

/// Helper to convert a Vec of map-like types to an R list of named lists.
fn vec_of_maps_to_list<T: IntoR>(vec: Vec<T>) -> crate::SEXP {
    unsafe { vecsxp_from_iter(vec.into_iter().map(|c| c.into_sexp())) }
}
// endregion

// region: R connections — IntoR impls (issue #175, #176)

#[cfg(feature = "connections")]
mod connections_into_r {
    use crate::SEXP;
    use crate::connection::{RNullConnection, RStderr, RStdin, RStdout};
    use crate::into_r::IntoR;

    // Evaluate a no-arg base function and return the resulting SEXP
    // (unprotected). An R error panics with R's message.
    //
    // # Safety
    // Must be called from the R main thread.
    unsafe fn eval_base_noarg(name: &str) -> SEXP {
        unsafe { crate::expression::RCall::new(name).eval_base() }
            .unwrap_or_else(|e| panic!("failed to evaluate {name}(): {e}"))
    }

    impl IntoR for RStdin {
        type Error = std::convert::Infallible;

        fn try_into_sexp(self) -> Result<SEXP, Self::Error> {
            Ok(self.into_sexp())
        }

        fn into_sexp(self) -> SEXP {
            unsafe { eval_base_noarg("stdin") }
        }
    }

    impl IntoR for RStdout {
        type Error = std::convert::Infallible;

        fn try_into_sexp(self) -> Result<SEXP, Self::Error> {
            Ok(self.into_sexp())
        }

        fn into_sexp(self) -> SEXP {
            unsafe { eval_base_noarg("stdout") }
        }
    }

    impl IntoR for RStderr {
        type Error = std::convert::Infallible;

        fn try_into_sexp(self) -> Result<SEXP, Self::Error> {
            Ok(self.into_sexp())
        }

        fn into_sexp(self) -> SEXP {
            unsafe { eval_base_noarg("stderr") }
        }
    }

    /// Returns the held SEXP and disarms the `Drop` guard (no double-close).
    impl IntoR for RNullConnection {
        type Error = std::convert::Infallible;

        fn try_into_sexp(self) -> Result<SEXP, Self::Error> {
            Ok(self.into_sexp())
        }

        fn into_sexp(self) -> SEXP {
            let sexp = self.sexp();
            // Transfer ownership to R: release from precious list (R's connection
            // table keeps the connection alive), then forget self to skip Drop.
            unsafe { crate::sys::R_ReleaseObject(sexp) };
            std::mem::forget(self);
            sexp
        }
    }
}

// endregion

// region: txtProgressBar — IntoR (issue #177)

#[cfg(feature = "connections")]
mod txt_progress_bar_into_r {
    use crate::SEXP;
    use crate::into_r::IntoR;
    use crate::txt_progress_bar::RTxtProgressBar;

    /// Transfer ownership of the `RTxtProgressBar` back to R.
    ///
    /// Releases the SEXP from the precious list and forgets `self` (so `Drop`
    /// is a no-op). R's environment keeps the bar alive after this call.
    impl IntoR for RTxtProgressBar {
        type Error = std::convert::Infallible;

        fn try_into_sexp(self) -> Result<SEXP, Self::Error> {
            Ok(self.into_sexp())
        }

        fn into_sexp(self) -> SEXP {
            let sexp = self.sexp();
            // Release from precious list; R manages lifetime from here on.
            unsafe { crate::sys::R_ReleaseObject(sexp) };
            std::mem::forget(self); // Disarm Drop guard — no double-release.
            sexp
        }
    }
}

// endregion