luau_vm/vm/

mod.rs

mod entry;
mod execute;

pub use entry::{PreCallResult, VmCallFrame};
pub use execute::VmExecution;

use core::cmp::Ordering;
use core::ptr;

use luau_common::{ByteSlice, flags};

use crate::Table;
use crate::call::{CallRuntime, ThreadStack};
use crate::debug::DebugRuntime;
use crate::gc::GcBarrier;
use crate::gc::GcObject;
use crate::handle::RawHandle;
use crate::handle::sealed::Sealed;
use crate::metamethod::{MetamethodRuntime, TmEvent};
use crate::native::NativeCallContext;
use crate::number::str_to_num;
use crate::state::ThreadState;
use crate::string::{MAX_STRING_SIZE, StringRuntime, TString};
use crate::table::TableRuntime;
use crate::thread::Thread;
use crate::types::{
    LUA_TBOOLEAN, LUA_TCLASS, LUA_TINTEGER, LUA_TLIGHTUSERDATA, LUA_TNIL, LUA_TNUMBER, LUA_TOBJECT,
    LUA_TSTRING, LUA_TTABLE, LUA_TUSERDATA, LUA_TVECTOR, LUA_VECTOR_SIZE,
};
use crate::value::{RAW_TVALUE_NIL, TValue, TValueCursor, nil_object};
use crate::{VmErrorResult, VmResult};

const MAX_TAG_LOOP: i32 = 100;

/// Unstable raw-value conversion capability.
///
/// # Safety
///
/// Values and result slots must be live, correctly tagged records belonging
/// to this thread's VM. Conversion can allocate, mutate stack values, raise an
/// error, and collect unrooted objects.
#[allow(
    clippy::missing_safety_doc,
    reason = "all methods share the capability-level safety contract"
)]
pub trait VmConversions: Sealed {
    /// `luaV_tonumber`
    unsafe fn to_number_internal(&self, value: TValue, result: TValue) -> Option<TValue>;

    /// `luaV_tovector`
    unsafe fn to_vector_internal(
        &self,
        value: TValue,
    ) -> Option<[f32; crate::types::LUA_VECTOR_SIZE]>;

    /// `luaV_tostring`
    unsafe fn to_string_internal(&self, value: TValue) -> VmErrorResult<i32>;
}

/// Unstable semantic VM operation capability.
///
/// # Safety
///
/// All values, tables, constants, and cursors must be live and belong to this
/// thread's VM. Stack shapes and indices must match each operation; operations
/// may invoke metamethods, relocate the stack, allocate, collect, or unwind.
#[allow(
    clippy::missing_safety_doc,
    reason = "all methods share the capability-level safety contract"
)]
pub trait VmOperations: Sealed {
    /// `luaV_prepareFORN`
    unsafe fn prepare_forn(
        &self,
        limit: TValueCursor,
        step: TValueCursor,
        index: TValueCursor,
    ) -> VmResult;

    /// `luaV_callTM`
    unsafe fn call_tm(&self, n_params: i32, result: i32) -> VmResult;

    /// `luaV_lessthan`
    unsafe fn less_than_internal(&self, left: TValue, right: TValue) -> VmResult<i32>;

    /// `luaV_lessequal`
    unsafe fn less_equal(&self, left: TValue, right: TValue) -> VmResult<i32>;

    /// `luaV_equalval`
    unsafe fn equal_value(&self, left: TValue, right: TValue) -> VmResult<i32>;

    /// `luaV_doarithimpl`
    unsafe fn do_arith_impl(
        &self,
        result: TValueCursor,
        left: TValue,
        right: TValue,
        op: TmEvent,
    ) -> VmResult;

    /// `luaV_dolen`
    unsafe fn do_len(&self, result: TValueCursor, value: TValue) -> VmResult;

    /// `luaV_gettable`
    unsafe fn get_table_internal(
        &self,
        table: TValue,
        key: TValue,
        value: TValueCursor,
    ) -> VmResult;

    /// `luaV_settable`
    unsafe fn set_table_internal(&self, table: TValue, key: TValue, value: TValue) -> VmResult;

    /// `luaV_concat`
    unsafe fn concat_internal(&self, total: i32, last: i32) -> VmResult;

    /// `luaV_getimport`
    unsafe fn get_import(
        &self,
        env: Table,
        constants: TValueCursor,
        target: TValueCursor,
        id: u32,
        propagate_nil: bool,
    ) -> VmResult;
}

/// `luaV_strcmp`
pub fn string_compare(left: TString, right: TString) -> i32 {
    if left == right {
        return 0;
    }

    let left_bytes = unsafe { left.as_bytes() };
    let right_bytes = unsafe { right.as_bytes() };

    let left_first = left_bytes.first().copied().unwrap_or(0);
    let right_first = right_bytes.first().copied().unwrap_or(0);
    if left_first != right_first {
        return i32::from(left_first) - i32::from(right_first);
    }

    match left_bytes.cmp(right_bytes) {
        Ordering::Less => -1,
        Ordering::Equal => 0,
        Ordering::Greater => 1,
    }
}

/// `get_compTM`
/// Finds a shared comparison metamethod for two metatables.
///
/// # Safety
///
/// `thread` must be live, both metatables must be live tables in that thread's
/// VM, and their metamethod caches must be coherent with table mutation.
pub unsafe fn get_comp_tm(
    thread: &Thread,
    left: Option<Table>,
    right: Option<Table>,
    event: TmEvent,
) -> Option<TValue> {
    let left = left?;
    let tm_left = unsafe { thread.fast_tm(Some(left), event) }?;

    if right == Some(left) {
        return Some(tm_left);
    }

    let tm_right = unsafe { thread.fast_tm(right, event) }?;
    tm_left.raw_equal(tm_right).then_some(tm_left)
}

impl Thread {
    /// `callTMres`
    unsafe fn call_tm_result(
        &self,
        result: TValueCursor,
        function: TValue,
        p1: TValue,
        p2: TValue,
    ) -> VmResult<TValueCursor> {
        unsafe {
            let result_offset = self.save_stack(result);
            let top = self.stack_top();
            let top_offset = self.save_stack(top);

            top.value_unchecked().set_obj(function);
            top.add(1).value_unchecked().set_obj(p1);
            top.add(2).value_unchecked().set_obj(p2);
            self.check_stack_internal(3)?;

            let top = self.restore_stack(top_offset);
            self.set_stack_top(top.add(3));
            self.call_internal(top, 1)?;

            let result = self.restore_stack(result_offset);
            let top = self.stack_top().sub(1);
            self.set_stack_top(top);
            result.value_unchecked().set_obj(top.value_unchecked());
            Ok(result)
        }
    }

    /// `callTM`
    unsafe fn call_tm3(&self, function: TValue, p1: TValue, p2: TValue, p3: TValue) -> VmResult {
        unsafe {
            let top = self.stack_top();
            let top_offset = self.save_stack(top);

            top.value_unchecked().set_obj(function);
            top.add(1).value_unchecked().set_obj(p1);
            top.add(2).value_unchecked().set_obj(p2);
            top.add(3).value_unchecked().set_obj(p3);
            self.check_stack_internal(4)?;

            let top = self.restore_stack(top_offset);
            self.set_stack_top(top.add(4));
            self.call_internal(top, 0)
        }
    }

    /// `callorderTM`
    unsafe fn call_order_tm(
        &self,
        left: TValue,
        right: TValue,
        event: TmEvent,
        error: bool,
    ) -> VmResult<i32> {
        unsafe {
            let Some(tm_left) = self.get_tm_by_obj(left, event) else {
                if error {
                    return self.order_error(left, right, event).map_err(Into::into);
                }
                return Ok(-1);
            };

            let Some(tm_right) = self.get_tm_by_obj(right, event) else {
                if error {
                    return self.order_error(left, right, event).map_err(Into::into);
                }
                return Ok(-1);
            };

            if !tm_left.raw_equal(tm_right) {
                if error {
                    return self.order_error(left, right, event).map_err(Into::into);
                }
                return Ok(-1);
            }

            let result = self.call_tm_result(self.stack_top(), tm_left, left, right)?;
            Ok(i32::from(!result.value_unchecked().is_false()))
        }
    }

    /// `callbinTM`
    unsafe fn call_bin_tm(
        &self,
        left: TValue,
        right: TValue,
        result: TValueCursor,
        event: TmEvent,
    ) -> VmResult<bool> {
        unsafe {
            let tm = self
                .get_tm_by_obj(left, event)
                .or_else(|| self.get_tm_by_obj(right, event));

            let Some(tm) = tm else {
                return Ok(false);
            };

            self.call_tm_result(result, tm, left, right)?;
            Ok(true)
        }
    }

    /// `tostringslot`
    unsafe fn coerce_string_slot(&self, object: TValue) -> VmResult<bool> {
        Ok(object.is_string() || unsafe { self.to_string_internal(object)? != 0 })
    }
}

unsafe fn set_vector_result(result: TValueCursor, value: [f32; LUA_VECTOR_SIZE]) {
    #[cfg(not(feature = "vector4"))]
    unsafe {
        result
            .value_unchecked()
            .set_vector([value[0], value[1], value[2]]);
    }

    #[cfg(feature = "vector4")]
    unsafe {
        result
            .value_unchecked()
            .set_vector([value[0], value[1], value[2], value[3]]);
    }
}

impl Thread {
    /// `luaV_callTM`
    unsafe fn call_tm_operation(&self, n_params: i32, result: i32) -> VmResult {
        unsafe {
            self.increment_native_call_depth();
            if self.as_ptr().as_ref().unwrap_unchecked().native_call_depth
                >= crate::thread::LUAI_MAX_NATIVE_CALLS
            {
                self.check_c_stack()?;
            }

            self.check_stack_internal(crate::thread::LUA_MIN_STACK as i32)?;

            let top = self.stack_top();
            let function = top.sub(n_params as usize + 1);
            let closure = function.value_unchecked().closure_value();
            let call_info = self.incr_ci()?.call_info_unchecked();

            call_info.init_call(
                function,
                top.add(crate::thread::LUA_MIN_STACK),
                i32::from(result >= 0),
                closure.proto(),
            );

            debug_assert!(call_info.top() <= self.stack_last());

            debug_assert!(function.value_unchecked().is_function());
            debug_assert!(closure.is_native());

            self.set_stack_base(function.add(1));
            debug_assert!(self.stack_top() == self.stack_base().add(n_params as usize));

            let function = closure.native_data().function.unwrap_unchecked();
            let produced = function(NativeCallContext::new(self))?;

            let parent_cursor = self.current_call_info_cursor().sub(1);
            let parent = parent_cursor.call_info_unchecked();

            if result >= 0 {
                let destination = parent.base().add(result as usize);
                if produced > 0 {
                    let produced_value = self.stack_top().sub(produced);
                    destination
                        .value_unchecked()
                        .set_obj(produced_value.value_unchecked());
                } else {
                    destination.value_unchecked().set_nil();
                }
            }

            self.decrement_native_call_depth();
            self.restore_call_frame(parent_cursor, parent.top());
        }
        Ok(())
    }

    /// `luaV_prepareFORN`
    unsafe fn prepare_forn_operation(
        &self,
        limit: TValueCursor,
        step: TValueCursor,
        index: TValueCursor,
    ) -> VmResult {
        unsafe {
            if !index.value_unchecked().is_number()
                && self
                    .to_number_internal(index.value_unchecked(), index.value_unchecked())
                    .is_none()
            {
                return self
                    .for_error(index.value_unchecked(), "initial value")
                    .map_err(Into::into);
            }

            if !limit.value_unchecked().is_number()
                && self
                    .to_number_internal(limit.value_unchecked(), limit.value_unchecked())
                    .is_none()
            {
                return self
                    .for_error(limit.value_unchecked(), "limit")
                    .map_err(Into::into);
            }

            if !step.value_unchecked().is_number()
                && self
                    .to_number_internal(step.value_unchecked(), step.value_unchecked())
                    .is_none()
            {
                return self
                    .for_error(step.value_unchecked(), "step")
                    .map_err(Into::into);
            }
        }
        Ok(())
    }
}
impl VmConversions for Thread {
    /// `luaV_tonumber`
    unsafe fn to_number_internal(&self, value: TValue, result: TValue) -> Option<TValue> {
        if value.is_number() {
            return Some(value);
        }

        if value.is_string() {
            let string = value.string_value();
            if let Some(number) = str_to_num(unsafe { string.as_bytes() }) {
                result.set_number(number);
                return Some(result);
            }
        }

        None
    }

    /// `luaV_tovector`
    unsafe fn to_vector_internal(&self, value: TValue) -> Option<[f32; LUA_VECTOR_SIZE]> {
        if value.is_vector() {
            Some(value.vector_value())
        } else {
            None
        }
    }

    /// `luaV_tostring`
    unsafe fn to_string_internal(&self, value: TValue) -> VmErrorResult<i32> {
        if !value.is_number() {
            return Ok(0);
        }

        let mut digits = [0u8; crate::number::LUAI_MAXNUM2STR];
        let len = crate::number::num_to_str(&mut digits, value.number_value());
        let string = unsafe { self.intern_string(digits[..len].as_bstr())? };
        value.set_string_value(string);
        Ok(1)
    }
}

impl VmOperations for Thread {
    unsafe fn prepare_forn(
        &self,
        limit: TValueCursor,
        step: TValueCursor,
        index: TValueCursor,
    ) -> VmResult {
        unsafe { self.prepare_forn_operation(limit, step, index) }
    }

    unsafe fn call_tm(&self, n_params: i32, result: i32) -> VmResult {
        unsafe { self.call_tm_operation(n_params, result) }
    }

    /// `luaV_lessthan`
    unsafe fn less_than_internal(&self, left: TValue, right: TValue) -> VmResult<i32> {
        if left.tt() != right.tt() {
            unsafe {
                self.order_error(left, right, TmEvent::Lt)
                    .map_err(Into::into)
            }
        } else if left.is_number() {
            Ok(i32::from(crate::number::num_lt(
                left.number_value(),
                right.number_value(),
            )))
        } else if left.is_string() {
            Ok(i32::from(
                string_compare(left.string_value(), right.string_value()) < 0,
            ))
        } else {
            unsafe { self.call_order_tm(left, right, TmEvent::Lt, true) }
        }
    }

    /// `luaV_lessequal`
    unsafe fn less_equal(&self, left: TValue, right: TValue) -> VmResult<i32> {
        unsafe {
            if left.tt() != right.tt() {
                self.order_error(left, right, TmEvent::Le)
                    .map_err(Into::into)
            } else if left.is_number() {
                Ok(i32::from(crate::number::num_le(
                    left.number_value(),
                    right.number_value(),
                )))
            } else if left.is_string() {
                Ok(i32::from(
                    string_compare(left.string_value(), right.string_value()) <= 0,
                ))
            } else {
                let result = self.call_order_tm(left, right, TmEvent::Le, false)?;
                if result != -1 {
                    Ok(result)
                } else {
                    let fallback = self.call_order_tm(right, left, TmEvent::Lt, false)?;
                    if fallback == -1 {
                        return self
                            .order_error(left, right, TmEvent::Le)
                            .map_err(Into::into);
                    }

                    Ok(i32::from(fallback == 0))
                }
            }
        }
    }

    /// `luaV_equalval`
    unsafe fn equal_value(&self, left: TValue, right: TValue) -> VmResult<i32> {
        debug_assert_eq!(left.tt(), right.tt());

        unsafe {
            let tm = match left.tt() {
                x if x == LUA_TNIL => return Ok(1),
                x if x == LUA_TNUMBER => {
                    return Ok(i32::from(crate::number::num_eq(
                        left.number_value(),
                        right.number_value(),
                    )));
                }
                x if x == LUA_TINTEGER => {
                    return Ok(i32::from(crate::number::int_eq(
                        left.integer_value(),
                        right.integer_value(),
                    )));
                }
                x if x == LUA_TVECTOR => {
                    return Ok(i32::from(crate::number::vec_eq(
                        &left.vector_value(),
                        &right.vector_value(),
                    )));
                }
                x if x == LUA_TBOOLEAN => {
                    return Ok(i32::from(left.boolean_value() == right.boolean_value()));
                }
                x if x == LUA_TLIGHTUSERDATA => {
                    return Ok(i32::from(
                        left.pointer_value() == right.pointer_value()
                            && left.light_userdata_tag() == right.light_userdata_tag(),
                    ));
                }
                x if x == LUA_TUSERDATA => {
                    let left_userdata = left.userdata_value();
                    let right_userdata = right.userdata_value();
                    let tm = get_comp_tm(
                        self,
                        left_userdata.metatable(),
                        right_userdata.metatable(),
                        TmEvent::Eq,
                    );

                    if tm.is_none() {
                        return Ok(i32::from(left_userdata == right_userdata));
                    }

                    tm.unwrap_unchecked()
                }
                x if x == LUA_TCLASS => {
                    return Ok(i32::from(left.class_value() == right.class_value()));
                }
                x if x == LUA_TOBJECT => {
                    let left_object = left.object_value();
                    let right_object = right.object_value();

                    if left_object.class() != right_object.class() {
                        return Ok(0);
                    }

                    let Some(tm) = self.get_tm_by_obj(left, TmEvent::Eq) else {
                        return Ok(i32::from(left_object == right_object));
                    };

                    tm
                }
                x if x == LUA_TTABLE => {
                    let left_table = left.table_value();
                    let right_table = right.table_value();
                    let tm = get_comp_tm(
                        self,
                        left_table.metatable(),
                        right_table.metatable(),
                        TmEvent::Eq,
                    );

                    if tm.is_none() {
                        return Ok(i32::from(left_table == right_table));
                    }

                    tm.unwrap_unchecked()
                }
                _ => return Ok(i32::from(left.gc_value() == right.gc_value())),
            };

            let result = self.call_tm_result(self.stack_top(), tm, left, right)?;
            Ok(i32::from(!result.value_unchecked().is_false()))
        }
    }

    /// `luaV_doarithimpl`
    unsafe fn do_arith_impl(
        &self,
        result: TValueCursor,
        left: TValue,
        right: TValue,
        op: TmEvent,
    ) -> VmResult {
        let mut left_number_storage = RAW_TVALUE_NIL;
        let mut right_number_storage = RAW_TVALUE_NIL;
        unsafe {
            let (left_number, right_number) = (
                TValue::from_mut(&mut left_number_storage),
                TValue::from_mut(&mut right_number_storage),
            );

            let left_vector = left.is_vector().then(|| left.vector_value());
            let right_vector = right.is_vector().then(|| right.vector_value());

            if let (Some(left_vector), Some(right_vector)) = (left_vector, right_vector) {
                match op {
                    TmEvent::Add | TmEvent::Sub | TmEvent::Mul | TmEvent::Div | TmEvent::IDiv => {
                        let mut value = [0.0; LUA_VECTOR_SIZE];
                        for index in 0..LUA_VECTOR_SIZE {
                            value[index] = match op {
                                TmEvent::Add => left_vector[index] + right_vector[index],
                                TmEvent::Sub => left_vector[index] - right_vector[index],
                                TmEvent::Mul => left_vector[index] * right_vector[index],
                                TmEvent::Div => left_vector[index] / right_vector[index],
                                TmEvent::IDiv => crate::number::num_idiv(
                                    left_vector[index] as f64,
                                    right_vector[index] as f64,
                                ) as f32,
                                _ => unreachable!(),
                            };
                        }
                        set_vector_result(result, value);
                        return Ok(());
                    }
                    TmEvent::Unm => {
                        let mut value = [0.0; LUA_VECTOR_SIZE];
                        for index in 0..LUA_VECTOR_SIZE {
                            value[index] = -left_vector[index];
                        }
                        set_vector_result(result, value);
                        return Ok(());
                    }
                    _ => {}
                }
            } else if let Some(left_vector) = left_vector {
                if let Some(right_number) = self
                    .to_number_internal(right, right_number)
                    .map(|n| n.number_value() as f32)
                {
                    match op {
                        TmEvent::Mul | TmEvent::Div | TmEvent::IDiv => {
                            let mut value = [0.0; LUA_VECTOR_SIZE];
                            for index in 0..LUA_VECTOR_SIZE {
                                value[index] = match op {
                                    TmEvent::Mul => left_vector[index] * right_number,
                                    TmEvent::Div => left_vector[index] / right_number,
                                    TmEvent::IDiv => crate::number::num_idiv(
                                        left_vector[index] as f64,
                                        right_number as f64,
                                    ) as f32,
                                    _ => unreachable!(),
                                };
                            }
                            set_vector_result(result, value);
                            return Ok(());
                        }
                        _ => {}
                    }
                }
            } else if let Some(right_vector) = right_vector
                && let Some(left_number) = self
                    .to_number_internal(left, left_number)
                    .map(|n| n.number_value() as f32)
            {
                match op {
                    TmEvent::Mul | TmEvent::Div | TmEvent::IDiv => {
                        let mut value = [0.0; LUA_VECTOR_SIZE];
                        for index in 0..LUA_VECTOR_SIZE {
                            value[index] = match op {
                                TmEvent::Mul => left_number * right_vector[index],
                                TmEvent::Div => left_number / right_vector[index],
                                TmEvent::IDiv => crate::number::num_idiv(
                                    left_number as f64,
                                    right_vector[index] as f64,
                                ) as f32,
                                _ => unreachable!(),
                            };
                        }
                        set_vector_result(result, value);
                        return Ok(());
                    }
                    _ => {}
                }
            }

            if let (Some(left_number), Some(right_number)) = (
                self.to_number_internal(left, left_number)
                    .map(|n| n.number_value()),
                self.to_number_internal(right, right_number)
                    .map(|n| n.number_value()),
            ) {
                result.value_unchecked().set_number(match op {
                    TmEvent::Add => crate::number::num_add(left_number, right_number),
                    TmEvent::Sub => crate::number::num_sub(left_number, right_number),
                    TmEvent::Mul => crate::number::num_mul(left_number, right_number),
                    TmEvent::Div => crate::number::num_div(left_number, right_number),
                    TmEvent::IDiv => crate::number::num_idiv(left_number, right_number),
                    TmEvent::Mod => crate::number::num_mod(left_number, right_number),
                    TmEvent::Pow => crate::number::num_pow(left_number, right_number),
                    TmEvent::Unm => crate::number::num_unm(left_number),
                    _ => unreachable!("invalid arithmetic metamethod {:?}", op),
                });
                return Ok(());
            }

            if !self.call_bin_tm(left, right, result, op)? {
                return self.arith_error(left, right, op).map_err(Into::into);
            }
        }
        Ok(())
    }

    /// `luaV_dolen`
    unsafe fn do_len(&self, result: TValueCursor, value: TValue) -> VmResult {
        unsafe {
            let tm = match value.tt() {
                x if x == LUA_TTABLE => {
                    let table = value.table_value();
                    match self.fast_tm(table.metatable(), TmEvent::Len) {
                        Some(tm) => tm,
                        None => {
                            result.value_unchecked().set_number(table.getn() as f64);
                            return Ok(());
                        }
                    }
                }
                x if x == LUA_TSTRING => {
                    result.value_unchecked().set_number(
                        value
                            .string_value()
                            .as_ptr()
                            .as_ref()
                            .unwrap_unchecked()
                            .len as f64,
                    );
                    return Ok(());
                }
                _ => match self.get_tm_by_obj(value, TmEvent::Len) {
                    Some(tm) => tm,
                    None => return self.type_error(value, "get length of").map_err(Into::into),
                },
            };

            let result = self.call_tm_result(result, tm, value, nil_object())?;
            if !result.value_unchecked().is_number() {
                return crate::run_error!(self, "'__len' must return a number").map_err(Into::into);
            }
        }
        Ok(())
    }

    /// `luaV_gettable`
    unsafe fn get_table_internal(
        &self,
        table: TValue,
        key: TValue,
        value: TValueCursor,
    ) -> VmResult {
        let mut table = table;

        unsafe {
            for _ in 0..MAX_TAG_LOOP {
                let tm;
                if table.is_table() {
                    let hash = table.table_value();
                    let result = hash.get(key);

                    if result != nil_object() {
                        self.set_cached_slot(hash.value_slot_unchecked(result));
                    }

                    if !result.is_nil() || {
                        tm = self.fast_tm(hash.metatable(), TmEvent::Index);
                        tm.is_none()
                    } {
                        value.value_unchecked().set_obj(result);
                        return Ok(());
                    }
                } else if flags::DebugLuauUserDefinedClassesRuntime.get() && table.is_object() {
                    let instance = table.object_value();
                    let Some(member) = instance.lookup_member(key) else {
                        return self.missing_member_error(table, key).map_err(Into::into);
                    };
                    value.value_unchecked().set_obj(member);
                    return Ok(());
                } else if flags::DebugLuauUserDefinedClassesRuntime.get() && table.is_class() {
                    let class = table.class_value();
                    let Some(static_member) = class.lookup_static_member(key) else {
                        return self.missing_member_error(table, key).map_err(Into::into);
                    };
                    value.value_unchecked().set_obj(static_member);
                    return Ok(());
                } else {
                    let Some(candidate) = self.get_tm_by_obj(table, TmEvent::Index) else {
                        return self.index_error(table, key).map_err(Into::into);
                    };
                    tm = Some(candidate);
                }

                let tm = tm.unwrap_unchecked();
                if tm.is_function() {
                    self.call_tm_result(value, tm, table, key)?;
                    return Ok(());
                }

                table = tm;
            }

            crate::run_error!(self, "'__index' chain too long; possible loop").map_err(Into::into)
        }
    }

    /// `luaV_settable`
    unsafe fn set_table_internal(&self, table: TValue, key: TValue, value: TValue) -> VmResult {
        let mut temp_storage = RAW_TVALUE_NIL;
        let temp = unsafe { TValue::from_mut(&mut temp_storage) };
        let mut table = table;

        unsafe {
            for _ in 0..MAX_TAG_LOOP {
                let tm;
                if table.is_table() {
                    let hash = table.table_value();
                    let old_value = hash.get(key);

                    if !old_value.is_nil() || {
                        tm = self.fast_tm(hash.metatable(), TmEvent::NewIndex);
                        tm.is_none()
                    } {
                        if hash.as_ptr().as_ref().unwrap_unchecked().readonly != 0 {
                            return self.readonly_error().map_err(Into::into);
                        }

                        let new_value = self.set_slot(hash, old_value, key)?;
                        self.set_cached_slot(hash.value_slot_unchecked(new_value));
                        new_value.set_obj(value);

                        if value.is_collectable() {
                            let table_object: GcObject = hash.into();
                            let child = value.gc_value();
                            if table_object.is_black() && child.is_white() {
                                self.barrier_table(hash, child);
                            }
                        }
                        return Ok(());
                    }
                } else if flags::DebugLuauUserDefinedClassesRuntime.get() && table.is_object() {
                    let instance = table.object_value();
                    let class = instance.class();
                    let Some(offset) = class.member_offset(key) else {
                        return self.missing_member_error(table, key).map_err(Into::into);
                    };
                    debug_assert!(instance.offset_in_bounds(offset));
                    if offset
                        >= class
                            .as_ptr()
                            .as_ref()
                            .unwrap_unchecked()
                            .number_of_instance_members
                    {
                        return self.index_error(table, key).map_err(Into::into);
                    }

                    let member = instance.lookup_member_at_offset(offset);
                    member.set_obj(value);

                    if value.is_collectable() {
                        let object: GcObject = instance.into();
                        let child = value.gc_value();
                        if object.is_black() && child.is_white() {
                            self.barrier_forward(object, child);
                        }
                    }
                    return Ok(());
                } else {
                    let Some(candidate) = self.get_tm_by_obj(table, TmEvent::NewIndex) else {
                        return self.index_error(table, key).map_err(Into::into);
                    };
                    tm = Some(candidate);
                }

                let tm = tm.unwrap_unchecked();
                if tm.is_function() {
                    self.call_tm3(tm, table, key, value)?;
                    return Ok(());
                }

                temp.set_obj(tm);
                table = temp;
            }

            crate::run_error!(self, "'__newindex' chain too long; possible loop")
                .map_err(Into::into)
        }
    }

    /// `luaV_concat`
    unsafe fn concat_internal(&self, mut total: i32, mut last: i32) -> VmResult {
        unsafe {
            while total > 1 {
                let top = self.stack_base().add(last as usize + 1);
                let mut count = 2;

                if !(top.sub(2).value_unchecked().is_string()
                    || top.sub(2).value_unchecked().is_number())
                    || !self.coerce_string_slot(top.sub(1).value_unchecked())?
                {
                    if !self.call_bin_tm(
                        top.sub(2).value_unchecked(),
                        top.sub(1).value_unchecked(),
                        top.sub(2),
                        TmEvent::Concat,
                    )? {
                        return self
                            .concat_error(
                                top.sub(2).value_unchecked(),
                                top.sub(1).value_unchecked(),
                            )
                            .map_err(Into::into);
                    }
                } else if top
                    .sub(1)
                    .value_unchecked()
                    .string_value()
                    .as_ptr()
                    .as_ref()
                    .unwrap_unchecked()
                    .len
                    == 0
                {
                    let _ = self.coerce_string_slot(top.sub(2).value_unchecked())?;
                } else {
                    let mut total_len = top
                        .sub(1)
                        .value_unchecked()
                        .string_value()
                        .as_ptr()
                        .as_ref()
                        .unwrap_unchecked()
                        .len as usize;

                    count = 1;
                    while count < total {
                        let slot = top.sub(count as usize + 1);
                        if !self.coerce_string_slot(slot.value_unchecked())? {
                            break;
                        }

                        let len = slot
                            .value_unchecked()
                            .string_value()
                            .as_ptr()
                            .as_ref()
                            .unwrap_unchecked()
                            .len as usize;
                        if len > MAX_STRING_SIZE - total_len {
                            return crate::run_error!(self, "string length overflow")
                                .map_err(Into::into);
                        }
                        total_len += len;
                        count += 1;
                    }

                    let mut local_buf = [0u8; crate::thread::LUA_BUFFER_SIZE];
                    let (buffer, heap_string) = if total_len < crate::thread::LUA_BUFFER_SIZE {
                        (local_buf.as_mut_ptr(), None)
                    } else {
                        let string = self.buffer_start(total_len)?;
                        (string.data_mut_ptr(), Some(string))
                    };

                    let mut write = 0usize;
                    for index in (1..=count).rev() {
                        let string = top.sub(index as usize).value_unchecked().string_value();
                        let bytes = string.as_bytes();
                        ptr::copy_nonoverlapping(bytes.as_ptr(), buffer.add(write), bytes.len());
                        write += bytes.len();
                    }

                    let dest = top.sub(count as usize);
                    let result = if let Some(string) = heap_string {
                        self.buffer_finish(string)?
                    } else {
                        self.intern_string(
                            core::slice::from_raw_parts(buffer, total_len).as_bstr(),
                        )?
                    };

                    dest.value_unchecked().set_string_value(result);
                }

                total -= count - 1;
                last -= count - 1;
            }
        }
        Ok(())
    }

    /// `luaV_getimport`
    unsafe fn get_import(
        &self,
        env: Table,
        constants: TValueCursor,
        target: TValueCursor,
        id: u32,
        propagate_nil: bool,
    ) -> VmResult {
        let count = (id >> 30) as i32;
        debug_assert!(count > 0);

        let id0 = ((id >> 20) & 1023) as usize;
        let id1 = ((id >> 10) & 1023) as usize;
        let id2 = (id & 1023) as usize;

        let mut global_storage = RAW_TVALUE_NIL;
        unsafe {
            let result_offset = self.save_stack(target);
            let global = TValue::from_mut(&mut global_storage);
            global.set_table_value(env);
            self.get_table_internal(global, constants.add(id0).value_unchecked(), target)?;

            if count < 2 {
                return Ok(());
            }

            let target = self.restore_stack(result_offset);
            if !propagate_nil || !target.value_unchecked().is_nil() {
                self.get_table_internal(
                    target.value_unchecked(),
                    constants.add(id1).value_unchecked(),
                    target,
                )?;
            }

            if count < 3 {
                return Ok(());
            }

            let target = self.restore_stack(result_offset);
            if !propagate_nil || !target.value_unchecked().is_nil() {
                self.get_table_internal(
                    target.value_unchecked(),
                    constants.add(id2).value_unchecked(),
                    target,
                )?;
            }
        }
        Ok(())
    }
}