xref: /wasmtime-44.0.1/crates/wasmtime/src/runtime/vm/instance.rs (revision 3ecb338e)

//! An `Instance` contains all the runtime state used by execution of a
//! wasm module (except its callstack and register state). An
//! `InstanceHandle` is a reference-counting handle for an `Instance`.

use crate::prelude::*;
use crate::runtime::vm::const_expr::{ConstEvalContext, ConstExprEvaluator};
use crate::runtime::vm::export::Export;
use crate::runtime::vm::memory::{Memory, RuntimeMemoryCreator};
use crate::runtime::vm::table::{Table, TableElement, TableElementType};
use crate::runtime::vm::vmcontext::{
    VMBuiltinFunctionsArray, VMContext, VMFuncRef, VMFunctionImport, VMGlobalDefinition,
    VMGlobalImport, VMMemoryDefinition, VMMemoryImport, VMOpaqueContext, VMStoreContext,
    VMTableDefinition, VMTableImport, VMTagDefinition, VMTagImport,
};
use crate::runtime::vm::{
    GcStore, Imports, ModuleRuntimeInfo, SendSyncPtr, VMGcRef, VMGlobalKind, VMStore,
    VMStoreRawPtr, VmPtr, VmSafe, WasmFault,
};
use crate::store::{InstanceId, StoreId, StoreInstanceId, StoreOpaque};
use alloc::sync::Arc;
use core::alloc::Layout;
use core::marker;
use core::ops::Range;
use core::pin::Pin;
use core::ptr::NonNull;
#[cfg(target_has_atomic = "64")]
use core::sync::atomic::AtomicU64;
use core::{mem, ptr};
#[cfg(feature = "gc")]
use wasmtime_environ::ModuleInternedTypeIndex;
use wasmtime_environ::{
    DataIndex, DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex, DefinedTagIndex,
    ElemIndex, EntityIndex, EntityRef, EntitySet, FuncIndex, GlobalIndex, HostPtr, MemoryIndex,
    Module, PrimaryMap, PtrSize, TableIndex, TableInitialValue, TableSegmentElements, TagIndex,
    Trap, VMCONTEXT_MAGIC, VMOffsets, VMSharedTypeIndex, WasmHeapTopType,
    packed_option::ReservedValue,
};
#[cfg(feature = "wmemcheck")]
use wasmtime_wmemcheck::Wmemcheck;

mod allocator;
pub use allocator::*;

/// The pair of an instance and a raw pointer its associated store.
///
/// ### Safety
///
/// > **Note**: it's known that the documentation below is documenting an
/// > unsound pattern and we're in the process of fixing it, but it'll take
/// > some time to refactor. Notably `unpack_mut` is not sound because the
/// > returned store pointer can be used to accidentally alias the instance
/// > pointer returned as well.
///
/// Getting a borrow of a vmctx's store is one of the fundamental bits of unsafe
/// code in Wasmtime. No matter how we architect the runtime, some kind of
/// unsafe conversion from a raw vmctx pointer that Wasm is using into a Rust
/// struct must happen.
///
/// It is our responsibility to ensure that multiple (exclusive) borrows of the
/// vmctx's store never exist at the same time. The distinction between the
/// `Instance` type (which doesn't expose its underlying vmctx pointer or a way
/// to get a borrow of its associated store) and this type (which does) is
/// designed to help with that.
///
/// Going from a `*mut VMContext` to a `&mut StoreInner<T>` is naturally unsafe
/// due to the raw pointer usage, but additionally the `T` type parameter needs
/// to be the same `T` that was used to define the `dyn VMStore` trait object
/// that was stuffed into the vmctx.
///
/// ### Usage
///
/// Usage generally looks like:
///
/// 1. You get a raw `*mut VMContext` from Wasm
///
/// 2. You call `InstanceAndStore::from_vmctx` on that raw pointer
///
/// 3. You then call `InstanceAndStore::unpack_mut` (or another helper) to get
///    the underlying `Pin<&mut Instance>` and `&mut dyn VMStore` (or `&mut
///    StoreInner<T>`).
///
/// 4. You then use whatever `Instance` methods you need to, each of which take
///    a store argument as necessary.
///
/// In step (4) you no longer need to worry about double exclusive borrows of
/// the store, so long as you don't do (1-2) again. Note also that the borrow
/// checker prevents repeating step (3) if you never repeat (1-2). In general,
/// steps (1-3) should be done in a single, common, internally-unsafe,
/// plumbing-code bottleneck and the raw pointer should never be exposed to Rust
/// code that does (4) after the `InstanceAndStore` is created. Follow this
/// pattern, and everything using the resulting `Instance` and `Store` can be
/// safe code (at least, with regards to accessing the store itself).
///
/// As an illustrative example, the common plumbing code for our various
/// libcalls performs steps (1-3) before calling into each actual libcall
/// implementation function that does (4). The plumbing code hides the raw vmctx
/// pointer and never gives out access to it to the libcall implementation
/// functions, nor does an `Instance` expose its internal vmctx pointer, which
/// would allow unsafely repeating steps (1-2).
#[repr(transparent)]
pub struct InstanceAndStore {
    instance: Instance,
}

impl InstanceAndStore {
    /// Converts the provided `*mut VMContext` to an `InstanceAndStore`
    /// reference and calls the provided closure with it.
    ///
    /// This method will move the `vmctx` pointer backwards to point to the
    /// original `Instance` that precedes it. The closure is provided a
    /// temporary reference to the `InstanceAndStore` with a constrained
    /// lifetime to ensure that it doesn't accidentally escape.
    ///
    /// # Safety
    ///
    /// Callers must validate that the `vmctx` pointer is a valid allocation and
    /// that it's valid to acquire `&mut InstanceAndStore` at this time. For
    /// example this can't be called twice on the same `VMContext` to get two
    /// active mutable borrows to the same `InstanceAndStore`.
    ///
    /// See also the safety discussion in this type's documentation.
    #[inline]
    pub(crate) unsafe fn from_vmctx<R>(
        vmctx: NonNull<VMContext>,
        f: impl for<'a> FnOnce(&'a mut Self) -> R,
    ) -> R {
        const _: () = assert!(mem::size_of::<InstanceAndStore>() == mem::size_of::<Instance>());
        // SAFETY: The validity of this `byte_sub` relies on `vmctx` being a
        // valid allocation which is itself a contract of this function.
        let mut ptr = unsafe {
            vmctx
                .byte_sub(mem::size_of::<Instance>())
                .cast::<InstanceAndStore>()
        };

        // SAFETY: the ability to interpret `vmctx` as a safe pointer and
        // continue on is a contract of this function itself, so the safety here
        // is effectively up to callers.
        unsafe { f(ptr.as_mut()) }
    }

    /// Unpacks this `InstanceAndStore` into its underlying `Instance` and `dyn
    /// VMStore`.
    #[inline]
    pub(crate) fn unpack_mut(&mut self) -> (Pin<&mut Instance>, &mut dyn VMStore) {
        unsafe {
            let store = &mut *self.store_ptr();
            (Pin::new_unchecked(&mut self.instance), store)
        }
    }

    /// Gets a pointer to this instance's `Store` which was originally
    /// configured on creation.
    ///
    /// # Panics
    ///
    /// May panic if the originally configured store was `None`. That can happen
    /// for host functions so host functions can't be queried what their
    /// original `Store` was since it's just retained as null (since host
    /// functions are shared amongst threads and don't all share the same
    /// store).
    #[inline]
    fn store_ptr(&self) -> *mut dyn VMStore {
        self.instance.store.unwrap().0.as_ptr()
    }
}

/// A type that roughly corresponds to a WebAssembly instance, but is also used
/// for host-defined objects.
///
/// Instances here can correspond to actual instantiated modules, but it's also
/// used ubiquitously for host-defined objects. For example creating a
/// host-defined memory will have a `module` that looks like it exports a single
/// memory (and similar for other constructs).
///
/// This `Instance` type is used as a ubiquitous representation for WebAssembly
/// values, whether or not they were created on the host or through a module.
///
/// # Ownership
///
/// This structure is never allocated directly but is instead managed through
/// an `InstanceHandle`. This structure ends with a `VMContext` which has a
/// dynamic size corresponding to the `module` configured within. Memory
/// management of this structure is always done through `InstanceHandle` as the
/// sole owner of an instance.
///
/// # `Instance` and `Pin`
///
/// Given an instance it is accompanied with trailing memory for the
/// appropriate `VMContext`. The `Instance` also holds `runtime_info` and other
/// information pointing to relevant offsets for the `VMContext`. Thus it is
/// not sound to mutate `runtime_info` after an instance is created. More
/// generally it's also not safe to "swap" instances, for example given two
/// `&mut Instance` values it's not sound to swap them as then the `VMContext`
/// values are inaccurately described.
///
/// To encapsulate this guarantee this type is only ever mutated through Rust's
/// `Pin` type. All mutable methods here take `self: Pin<&mut Self>` which
/// statically disallows safe access to `&mut Instance`. There are assorted
/// "projection methods" to go from `Pin<&mut Instance>` to `&mut T` for
/// individual fields, for example `memories_mut`. More methods can be added as
/// necessary or methods may also be added to project multiple fields at a time
/// if necessary to. The precise ergonomics around getting mutable access to
/// some fields (but notably not `runtime_info`) is probably going to evolve
/// over time.
///
/// Note that is is not sound to basically ever pass around `&mut Instance`.
/// That should always instead be `Pin<&mut Instance>`. All usage of
/// `Pin::new_unchecked` should be here in this module in just a few `unsafe`
/// locations and it's recommended to use existing helpers if you can.
#[repr(C)] // ensure that the vmctx field is last.
pub struct Instance {
    /// The index, within a `Store` that this instance lives at
    id: InstanceId,

    /// The runtime info (corresponding to the "compiled module"
    /// abstraction in higher layers) that is retained and needed for
    /// lazy initialization. This provides access to the underlying
    /// Wasm module entities, the compiled JIT code, metadata about
    /// functions, lazy initialization state, etc.
    runtime_info: ModuleRuntimeInfo,

    /// WebAssembly linear memory data.
    ///
    /// This is where all runtime information about defined linear memories in
    /// this module lives.
    ///
    /// The `MemoryAllocationIndex` was given from our `InstanceAllocator` and
    /// must be given back to the instance allocator when deallocating each
    /// memory.
    memories: PrimaryMap<DefinedMemoryIndex, (MemoryAllocationIndex, Memory)>,

    /// WebAssembly table data.
    ///
    /// Like memories, this is only for defined tables in the module and
    /// contains all of their runtime state.
    ///
    /// The `TableAllocationIndex` was given from our `InstanceAllocator` and
    /// must be given back to the instance allocator when deallocating each
    /// table.
    tables: PrimaryMap<DefinedTableIndex, (TableAllocationIndex, Table)>,

    /// Stores the dropped passive element segments in this instantiation by index.
    /// If the index is present in the set, the segment has been dropped.
    dropped_elements: EntitySet<ElemIndex>,

    /// Stores the dropped passive data segments in this instantiation by index.
    /// If the index is present in the set, the segment has been dropped.
    dropped_data: EntitySet<DataIndex>,

    // TODO: add support for multiple memories; `wmemcheck_state` corresponds to
    // memory 0.
    #[cfg(feature = "wmemcheck")]
    pub(crate) wmemcheck_state: Option<Wmemcheck>,

    /// Self-pointer back to `Store<T>` and its functions. Not present for
    /// the brief time that `Store<T>` is itself being created. Also not
    /// present for some niche uses that are disconnected from stores (e.g.
    /// cross-thread stuff used in `InstancePre`)
    store: Option<VMStoreRawPtr>,

    /// Additional context used by compiled wasm code. This field is last, and
    /// represents a dynamically-sized array that extends beyond the nominal
    /// end of the struct (similar to a flexible array member).
    vmctx: OwnedVMContext<VMContext>,
}

impl Instance {
    /// Create an instance at the given memory address.
    ///
    /// It is assumed the memory was properly aligned and the
    /// allocation was `alloc_size` in bytes.
    ///
    /// # Safety
    ///
    /// The `req.imports` field must be appropriately sized/typed for the module
    /// being allocated according to `req.runtime_info`. Additionally `memories`
    /// and `tables` must have been allocated for `req.store`.
    unsafe fn new(
        req: InstanceAllocationRequest,
        memories: PrimaryMap<DefinedMemoryIndex, (MemoryAllocationIndex, Memory)>,
        tables: PrimaryMap<DefinedTableIndex, (TableAllocationIndex, Table)>,
        memory_tys: &PrimaryMap<MemoryIndex, wasmtime_environ::Memory>,
    ) -> InstanceHandle {
        let module = req.runtime_info.env_module();
        let dropped_elements = EntitySet::with_capacity(module.passive_elements.len());
        let dropped_data = EntitySet::with_capacity(module.passive_data_map.len());

        #[cfg(not(feature = "wmemcheck"))]
        let _ = memory_tys;

        let mut ret = OwnedInstance::new(Instance {
            id: req.id,
            runtime_info: req.runtime_info.clone(),
            memories,
            tables,
            dropped_elements,
            dropped_data,
            #[cfg(feature = "wmemcheck")]
            wmemcheck_state: {
                if req.wmemcheck {
                    let size = memory_tys
                        .iter()
                        .next()
                        .map(|memory| memory.1.limits.min)
                        .unwrap_or(0)
                        * 64
                        * 1024;
                    Some(Wmemcheck::new(size.try_into().unwrap()))
                } else {
                    None
                }
            },
            store: None,
            vmctx: OwnedVMContext::new(),
        });

        // SAFETY: this vmctx was allocated with the same layout above, so it
        // should be safe to initialize with the same values here.
        unsafe {
            ret.get_mut().initialize_vmctx(
                module,
                req.runtime_info.offsets(),
                req.store,
                req.imports,
            );
        }
        ret
    }

    /// Converts the provided `*mut VMContext` to an `Instance` pointer and
    /// returns it with the same lifetime as `self`.
    ///
    /// This function can be used when traversing a `VMContext` to reach into
    /// the context needed for imports, optionally.
    ///
    /// # Safety
    ///
    /// This function requires that the `vmctx` pointer is indeed valid and
    /// from the store that `self` belongs to.
    #[inline]
    unsafe fn sibling_vmctx<'a>(&'a self, vmctx: NonNull<VMContext>) -> &'a Instance {
        // SAFETY: it's a contract of this function itself that `vmctx` is a
        // valid pointer such that this pointer arithmetic is valid.
        let ptr = unsafe {
            vmctx
                .byte_sub(mem::size_of::<Instance>())
                .cast::<Instance>()
        };
        // SAFETY: it's a contract of this function itself that `vmctx` is a
        // valid pointer to dereference. Additionally the lifetime of the return
        // value is constrained to be the same as `self` to avoid granting a
        // too-long lifetime.
        unsafe { ptr.as_ref() }
    }

    /// Same as [`Self::sibling_vmctx`], but the mutable version.
    ///
    /// # Safety
    ///
    /// This function requires that the `vmctx` pointer is indeed valid and
    /// from the store that `self` belongs to.
    ///
    /// (Note that it is *NOT* required that `vmctx` be distinct from this
    /// instance's `vmctx`, or that usage of the resulting instance is limited
    /// to its defined items! The returned borrow has the same lifetime as
    /// `self`, which means that this instance cannot be used while the
    /// resulting instance is in use, and we therefore do not need to worry
    /// about mutable aliasing between this instance and the resulting
    /// instance.)
    #[inline]
    unsafe fn sibling_vmctx_mut<'a>(
        self: Pin<&'a mut Self>,
        vmctx: NonNull<VMContext>,
    ) -> Pin<&'a mut Instance> {
        // SAFETY: it's a contract of this function itself that `vmctx` is a
        // valid pointer such that this pointer arithmetic is valid.
        let mut ptr = unsafe {
            vmctx
                .byte_sub(mem::size_of::<Instance>())
                .cast::<Instance>()
        };

        // SAFETY: it's a contract of this function itself that `vmctx` is a
        // valid pointer to dereference. Additionally the lifetime of the return
        // value is constrained to be the same as `self` to avoid granting a
        // too-long lifetime. Finally mutable references to an instance are
        // always through `Pin`, so it's safe to create a pin-pointer here.
        unsafe { Pin::new_unchecked(ptr.as_mut()) }
    }

    pub(crate) fn env_module(&self) -> &Arc<wasmtime_environ::Module> {
        self.runtime_info.env_module()
    }

    #[cfg(feature = "gc")]
    pub(crate) fn runtime_module(&self) -> Option<&crate::Module> {
        match &self.runtime_info {
            ModuleRuntimeInfo::Module(m) => Some(m),
            ModuleRuntimeInfo::Bare(_) => None,
        }
    }

    /// Translate a module-level interned type index into an engine-level
    /// interned type index.
    #[cfg(feature = "gc")]
    pub fn engine_type_index(&self, module_index: ModuleInternedTypeIndex) -> VMSharedTypeIndex {
        self.runtime_info.engine_type_index(module_index)
    }

    #[inline]
    fn offsets(&self) -> &VMOffsets<HostPtr> {
        self.runtime_info.offsets()
    }

    /// Return the indexed `VMFunctionImport`.
    fn imported_function(&self, index: FuncIndex) -> &VMFunctionImport {
        unsafe { self.vmctx_plus_offset(self.offsets().vmctx_vmfunction_import(index)) }
    }

    /// Return the index `VMTableImport`.
    fn imported_table(&self, index: TableIndex) -> &VMTableImport {
        unsafe { self.vmctx_plus_offset(self.offsets().vmctx_vmtable_import(index)) }
    }

    /// Return the indexed `VMMemoryImport`.
    fn imported_memory(&self, index: MemoryIndex) -> &VMMemoryImport {
        unsafe { self.vmctx_plus_offset(self.offsets().vmctx_vmmemory_import(index)) }
    }

    /// Return the indexed `VMGlobalImport`.
    fn imported_global(&self, index: GlobalIndex) -> &VMGlobalImport {
        unsafe { self.vmctx_plus_offset(self.offsets().vmctx_vmglobal_import(index)) }
    }

    /// Return the indexed `VMTagImport`.
    fn imported_tag(&self, index: TagIndex) -> &VMTagImport {
        unsafe { self.vmctx_plus_offset(self.offsets().vmctx_vmtag_import(index)) }
    }

    /// Return the indexed `VMTagDefinition`.
    pub fn tag_ptr(&self, index: DefinedTagIndex) -> NonNull<VMTagDefinition> {
        unsafe { self.vmctx_plus_offset_raw(self.offsets().vmctx_vmtag_definition(index)) }
    }

    /// Return the indexed `VMTableDefinition`.
    pub fn table(&self, index: DefinedTableIndex) -> VMTableDefinition {
        unsafe { self.table_ptr(index).read() }
    }

    /// Updates the value for a defined table to `VMTableDefinition`.
    fn set_table(self: Pin<&mut Self>, index: DefinedTableIndex, table: VMTableDefinition) {
        unsafe {
            self.table_ptr(index).write(table);
        }
    }

    /// Return a pointer to the `index`'th table within this instance, stored
    /// in vmctx memory.
    pub fn table_ptr(&self, index: DefinedTableIndex) -> NonNull<VMTableDefinition> {
        unsafe { self.vmctx_plus_offset_raw(self.offsets().vmctx_vmtable_definition(index)) }
    }

    /// Get a locally defined or imported memory.
    pub(crate) fn get_memory(&self, index: MemoryIndex) -> VMMemoryDefinition {
        if let Some(defined_index) = self.env_module().defined_memory_index(index) {
            self.memory(defined_index)
        } else {
            let import = self.imported_memory(index);
            unsafe { VMMemoryDefinition::load(import.from.as_ptr()) }
        }
    }

    /// Return the indexed `VMMemoryDefinition`, loaded from vmctx memory
    /// already.
    #[inline]
    pub fn memory(&self, index: DefinedMemoryIndex) -> VMMemoryDefinition {
        unsafe { VMMemoryDefinition::load(self.memory_ptr(index).as_ptr()) }
    }

    /// Set the indexed memory to `VMMemoryDefinition`.
    fn set_memory(&self, index: DefinedMemoryIndex, mem: VMMemoryDefinition) {
        unsafe {
            self.memory_ptr(index).write(mem);
        }
    }

    /// Return the address of the specified memory at `index` within this vmctx.
    ///
    /// Note that the returned pointer resides in wasm-code-readable-memory in
    /// the vmctx.
    #[inline]
    pub fn memory_ptr(&self, index: DefinedMemoryIndex) -> NonNull<VMMemoryDefinition> {
        unsafe {
            self.vmctx_plus_offset::<VmPtr<_>>(self.offsets().vmctx_vmmemory_pointer(index))
                .as_non_null()
        }
    }

    /// Return the indexed `VMGlobalDefinition`.
    pub fn global_ptr(&self, index: DefinedGlobalIndex) -> NonNull<VMGlobalDefinition> {
        unsafe { self.vmctx_plus_offset_raw(self.offsets().vmctx_vmglobal_definition(index)) }
    }

    /// Get a raw pointer to the global at the given index regardless whether it
    /// is defined locally or imported from another module.
    ///
    /// Panics if the index is out of bound or is the reserved value.
    pub(crate) fn defined_or_imported_global_ptr(
        self: Pin<&mut Self>,
        index: GlobalIndex,
    ) -> NonNull<VMGlobalDefinition> {
        if let Some(index) = self.env_module().defined_global_index(index) {
            self.global_ptr(index)
        } else {
            self.imported_global(index).from.as_non_null()
        }
    }

    /// Get all globals within this instance.
    ///
    /// Returns both import and defined globals.
    ///
    /// Returns both exported and non-exported globals.
    ///
    /// Gives access to the full globals space.
    pub fn all_globals(
        &self,
        store: StoreId,
    ) -> impl ExactSizeIterator<Item = (GlobalIndex, crate::Global)> + '_ {
        let module = self.env_module();
        module
            .globals
            .keys()
            .map(move |idx| (idx, self.get_exported_global(store, idx)))
    }

    /// Get the globals defined in this instance (not imported).
    pub fn defined_globals(
        &self,
        store: StoreId,
    ) -> impl ExactSizeIterator<Item = (DefinedGlobalIndex, crate::Global)> + '_ {
        let module = self.env_module();
        self.all_globals(store)
            .skip(module.num_imported_globals)
            .map(move |(i, global)| (module.defined_global_index(i).unwrap(), global))
    }

    /// Return a pointer to the interrupts structure
    #[inline]
    pub fn vm_store_context(&self) -> NonNull<Option<VmPtr<VMStoreContext>>> {
        unsafe { self.vmctx_plus_offset_raw(self.offsets().ptr.vmctx_store_context()) }
    }

    /// Return a pointer to the global epoch counter used by this instance.
    #[cfg(target_has_atomic = "64")]
    pub fn epoch_ptr(self: Pin<&mut Self>) -> &mut Option<VmPtr<AtomicU64>> {
        let offset = self.offsets().ptr.vmctx_epoch_ptr();
        unsafe { self.vmctx_plus_offset_mut(offset) }
    }

    /// Return a pointer to the collector-specific heap data.
    pub fn gc_heap_data(self: Pin<&mut Self>) -> &mut Option<VmPtr<u8>> {
        let offset = self.offsets().ptr.vmctx_gc_heap_data();
        unsafe { self.vmctx_plus_offset_mut(offset) }
    }

    pub(crate) unsafe fn set_store(mut self: Pin<&mut Self>, store: Option<NonNull<dyn VMStore>>) {
        // FIXME: should be more targeted ideally with the `unsafe` than just
        // throwing this entire function in a large `unsafe` block.
        unsafe {
            *self.as_mut().store_mut() = store.map(VMStoreRawPtr);
            if let Some(mut store) = store {
                let store = store.as_mut();
                self.vm_store_context()
                    .write(Some(store.vm_store_context_ptr().into()));
                #[cfg(target_has_atomic = "64")]
                {
                    *self.as_mut().epoch_ptr() =
                        Some(NonNull::from(store.engine().epoch_counter()).into());
                }

                if self.env_module().needs_gc_heap {
                    self.as_mut().set_gc_heap(Some(store.gc_store().expect(
                        "if we need a GC heap, then `Instance::new_raw` should have already \
                     allocated it for us",
                    )));
                } else {
                    self.as_mut().set_gc_heap(None);
                }
            } else {
                self.vm_store_context().write(None);
                #[cfg(target_has_atomic = "64")]
                {
                    *self.as_mut().epoch_ptr() = None;
                }
                self.as_mut().set_gc_heap(None);
            }
        }
    }

    unsafe fn set_gc_heap(self: Pin<&mut Self>, gc_store: Option<&GcStore>) {
        if let Some(gc_store) = gc_store {
            *self.gc_heap_data() = Some(unsafe { gc_store.gc_heap.vmctx_gc_heap_data().into() });
        } else {
            *self.gc_heap_data() = None;
        }
    }

    /// Return a reference to the vmctx used by compiled wasm code.
    #[inline]
    pub fn vmctx(&self) -> NonNull<VMContext> {
        InstanceLayout::vmctx(self)
    }

    /// Lookup a function by index.
    ///
    /// # Panics
    ///
    /// Panics if `index` is out of bounds for this instance.
    ///
    /// # Safety
    ///
    /// The `store` parameter must be the store that owns this instance and the
    /// functions that this instance can reference.
    pub unsafe fn get_exported_func(
        self: Pin<&mut Self>,
        store: StoreId,
        index: FuncIndex,
    ) -> crate::Func {
        let func_ref = self.get_func_ref(index).unwrap();

        // SAFETY: the validity of `func_ref` is guaranteed by the validity of
        // `self`, and the contract that `store` must own `func_ref` is a
        // contract of this function itself.
        unsafe { crate::Func::from_vm_func_ref(store, func_ref) }
    }

    /// Lookup a table by index.
    ///
    /// # Panics
    ///
    /// Panics if `index` is out of bounds for this instance.
    pub fn get_exported_table(&self, store: StoreId, index: TableIndex) -> crate::Table {
        let (id, def_index) = if let Some(def_index) = self.env_module().defined_table_index(index)
        {
            (self.id, def_index)
        } else {
            let import = self.imported_table(index);
            // SAFETY: validity of this `Instance` guarantees validity of the
            // `vmctx` pointer being read here to find the transitive
            // `InstanceId` that the import is associated with.
            let id = unsafe { self.sibling_vmctx(import.vmctx.as_non_null()).id };
            (id, import.index)
        };
        crate::Table::from_raw(StoreInstanceId::new(store, id), def_index)
    }

    /// Lookup a memory by index.
    ///
    /// # Panics
    ///
    /// Panics if `index` is out-of-bounds for this instance.
    pub fn get_exported_memory(&self, store: StoreId, index: MemoryIndex) -> crate::Memory {
        let (id, def_index) = if let Some(def_index) = self.env_module().defined_memory_index(index)
        {
            (self.id, def_index)
        } else {
            let import = self.imported_memory(index);
            // SAFETY: validity of this `Instance` guarantees validity of the
            // `vmctx` pointer being read here to find the transitive
            // `InstanceId` that the import is associated with.
            let id = unsafe { self.sibling_vmctx(import.vmctx.as_non_null()).id };
            (id, import.index)
        };
        crate::Memory::from_raw(StoreInstanceId::new(store, id), def_index)
    }

    fn get_exported_global(&self, store: StoreId, index: GlobalIndex) -> crate::Global {
        // If this global is defined within this instance, then that's easy to
        // calculate the `Global`.
        if let Some(def_index) = self.env_module().defined_global_index(index) {
            let instance = StoreInstanceId::new(store, self.id);
            return crate::Global::from_core(instance, def_index);
        }

        // For imported globals it's required to match on the `kind` to
        // determine which `Global` constructor is going to be invoked.
        let import = self.imported_global(index);
        match import.kind {
            VMGlobalKind::Host(index) => crate::Global::from_host(store, index),
            VMGlobalKind::Instance(index) => {
                // SAFETY: validity of this `&Instance` means validity of its
                // imports meaning we can read the id of the vmctx within.
                let id = unsafe {
                    let vmctx = VMContext::from_opaque(import.vmctx.unwrap().as_non_null());
                    self.sibling_vmctx(vmctx).id
                };
                crate::Global::from_core(StoreInstanceId::new(store, id), index)
            }
            #[cfg(feature = "component-model")]
            VMGlobalKind::ComponentFlags(index) => {
                // SAFETY: validity of this `&Instance` means validity of its
                // imports meaning we can read the id of the vmctx within.
                let id = unsafe {
                    let vmctx = super::component::VMComponentContext::from_opaque(
                        import.vmctx.unwrap().as_non_null(),
                    );
                    super::component::ComponentInstance::vmctx_instance_id(vmctx)
                };
                crate::Global::from_component_flags(
                    crate::component::store::StoreComponentInstanceId::new(store, id),
                    index,
                )
            }
        }
    }

    /// Get an exported tag by index.
    ///
    /// # Panics
    ///
    /// Panics if the index is out-of-range.
    pub fn get_exported_tag(&self, store: StoreId, index: TagIndex) -> crate::Tag {
        let (id, def_index) = if let Some(def_index) = self.env_module().defined_tag_index(index) {
            (self.id, def_index)
        } else {
            let import = self.imported_tag(index);
            // SAFETY: validity of this `Instance` guarantees validity of the
            // `vmctx` pointer being read here to find the transitive
            // `InstanceId` that the import is associated with.
            let id = unsafe { self.sibling_vmctx(import.vmctx.as_non_null()).id };
            (id, import.index)
        };
        crate::Tag::from_raw(StoreInstanceId::new(store, id), def_index)
    }

    /// Return an iterator over the exports of this instance.
    ///
    /// Specifically, it provides access to the key-value pairs, where the keys
    /// are export names, and the values are export declarations which can be
    /// resolved `lookup_by_declaration`.
    pub fn exports(&self) -> wasmparser::collections::index_map::Iter<'_, String, EntityIndex> {
        self.env_module().exports.iter()
    }

    /// Grow memory by the specified amount of pages.
    ///
    /// Returns `None` if memory can't be grown by the specified amount
    /// of pages. Returns `Some` with the old size in bytes if growth was
    /// successful.
    pub(crate) fn memory_grow(
        mut self: Pin<&mut Self>,
        store: &mut dyn VMStore,
        idx: DefinedMemoryIndex,
        delta: u64,
    ) -> Result<Option<usize>, Error> {
        let memory = &mut self.as_mut().memories_mut()[idx].1;

        let result = unsafe { memory.grow(delta, Some(store)) };

        // Update the state used by a non-shared Wasm memory in case the base
        // pointer and/or the length changed.
        if memory.as_shared_memory().is_none() {
            let vmmemory = memory.vmmemory();
            self.set_memory(idx, vmmemory);
        }

        result
    }

    pub(crate) fn table_element_type(
        self: Pin<&mut Self>,
        table_index: TableIndex,
    ) -> TableElementType {
        self.get_table(table_index).element_type()
    }

    /// Grow table by the specified amount of elements, filling them with
    /// `init_value`.
    ///
    /// Returns `None` if table can't be grown by the specified amount of
    /// elements, or if `init_value` is the wrong type of table element.
    pub(crate) fn defined_table_grow(
        mut self: Pin<&mut Self>,
        store: &mut dyn VMStore,
        table_index: DefinedTableIndex,
        delta: u64,
        init_value: TableElement,
    ) -> Result<Option<usize>, Error> {
        let table = &mut self
            .as_mut()
            .tables_mut()
            .get_mut(table_index)
            .unwrap_or_else(|| panic!("no table for index {}", table_index.index()))
            .1;

        let result = unsafe { table.grow(delta, init_value, store) };

        // Keep the `VMContext` pointers used by compiled Wasm code up to
        // date.
        let element = table.vmtable();
        self.set_table(table_index, element);

        result
    }

    fn alloc_layout(offsets: &VMOffsets<HostPtr>) -> Layout {
        let size = mem::size_of::<Self>()
            .checked_add(usize::try_from(offsets.size_of_vmctx()).unwrap())
            .unwrap();
        let align = mem::align_of::<Self>();
        Layout::from_size_align(size, align).unwrap()
    }

    fn type_ids_array(&self) -> NonNull<VmPtr<VMSharedTypeIndex>> {
        unsafe { self.vmctx_plus_offset_raw(self.offsets().ptr.vmctx_type_ids_array()) }
    }

    /// Construct a new VMFuncRef for the given function
    /// (imported or defined in this module) and store into the given
    /// location. Used during lazy initialization.
    ///
    /// Note that our current lazy-init scheme actually calls this every
    /// time the funcref pointer is fetched; this turns out to be better
    /// than tracking state related to whether it's been initialized
    /// before, because resetting that state on (re)instantiation is
    /// very expensive if there are many funcrefs.
    ///
    /// # Safety
    ///
    /// This functions requires that `into` is a valid pointer.
    unsafe fn construct_func_ref(
        self: Pin<&mut Self>,
        index: FuncIndex,
        type_index: VMSharedTypeIndex,
        into: *mut VMFuncRef,
    ) {
        let func_ref = if let Some(def_index) = self.env_module().defined_func_index(index) {
            VMFuncRef {
                array_call: self
                    .runtime_info
                    .array_to_wasm_trampoline(def_index)
                    .expect("should have array-to-Wasm trampoline for escaping function")
                    .into(),
                wasm_call: Some(self.runtime_info.function(def_index).into()),
                vmctx: VMOpaqueContext::from_vmcontext(self.vmctx()).into(),
                type_index,
            }
        } else {
            let import = self.imported_function(index);
            VMFuncRef {
                array_call: import.array_call,
                wasm_call: Some(import.wasm_call),
                vmctx: import.vmctx,
                type_index,
            }
        };

        // SAFETY: the unsafe contract here is forwarded to callers of this
        // function.
        unsafe {
            ptr::write(into, func_ref);
        }
    }

    /// Get a `&VMFuncRef` for the given `FuncIndex`.
    ///
    /// Returns `None` if the index is the reserved index value.
    ///
    /// The returned reference is a stable reference that won't be moved and can
    /// be passed into JIT code.
    pub(crate) fn get_func_ref(
        self: Pin<&mut Self>,
        index: FuncIndex,
    ) -> Option<NonNull<VMFuncRef>> {
        if index == FuncIndex::reserved_value() {
            return None;
        }

        // For now, we eagerly initialize an funcref struct in-place
        // whenever asked for a reference to it. This is mostly
        // fine, because in practice each funcref is unlikely to be
        // requested more than a few times: once-ish for funcref
        // tables used for call_indirect (the usual compilation
        // strategy places each function in the table at most once),
        // and once or a few times when fetching exports via API.
        // Note that for any case driven by table accesses, the lazy
        // table init behaves like a higher-level cache layer that
        // protects this initialization from happening multiple
        // times, via that particular table at least.
        //
        // When `ref.func` becomes more commonly used or if we
        // otherwise see a use-case where this becomes a hotpath,
        // we can reconsider by using some state to track
        // "uninitialized" explicitly, for example by zeroing the
        // funcrefs (perhaps together with other
        // zeroed-at-instantiate-time state) or using a separate
        // is-initialized bitmap.
        //
        // We arrived at this design because zeroing memory is
        // expensive, so it's better for instantiation performance
        // if we don't have to track "is-initialized" state at
        // all!
        let func = &self.env_module().functions[index];
        let sig = func.signature.unwrap_engine_type_index();

        // SAFETY: the offset calculated here should be correct with
        // `self.offsets`
        let func_ref = unsafe {
            self.vmctx_plus_offset_raw::<VMFuncRef>(self.offsets().vmctx_func_ref(func.func_ref))
        };

        // SAFETY: the `func_ref` ptr should be valid as it's within our
        // `VMContext` area.
        unsafe {
            self.construct_func_ref(index, sig, func_ref.as_ptr());
        }

        Some(func_ref)
    }

    /// Get the passive elements segment at the given index.
    ///
    /// Returns an empty segment if the index is out of bounds or if the segment
    /// has been dropped.
    ///
    /// The `storage` parameter should always be `None`; it is a bit of a hack
    /// to work around lifetime issues.
    pub(crate) fn passive_element_segment<'a>(
        &self,
        storage: &'a mut Option<(Arc<wasmtime_environ::Module>, TableSegmentElements)>,
        elem_index: ElemIndex,
    ) -> &'a TableSegmentElements {
        debug_assert!(storage.is_none());
        *storage = Some((
            // TODO: this `clone()` shouldn't be necessary but is used for now to
            // inform `rustc` that the lifetime of the elements here are
            // disconnected from the lifetime of `self`.
            self.env_module().clone(),
            // NB: fall back to an expressions-based list of elements which
            // doesn't have static type information (as opposed to
            // `TableSegmentElements::Functions`) since we don't know what type
            // is needed in the caller's context. Let the type be inferred by
            // how they use the segment.
            TableSegmentElements::Expressions(Box::new([])),
        ));
        let (module, empty) = storage.as_ref().unwrap();

        match module.passive_elements_map.get(&elem_index) {
            Some(index) if !self.dropped_elements.contains(elem_index) => {
                &module.passive_elements[*index]
            }
            _ => empty,
        }
    }

    /// The `table.init` operation: initializes a portion of a table with a
    /// passive element.
    ///
    /// # Errors
    ///
    /// Returns a `Trap` error when the range within the table is out of bounds
    /// or the range within the passive element is out of bounds.
    pub(crate) fn table_init(
        self: Pin<&mut Self>,
        store: &mut StoreOpaque,
        table_index: TableIndex,
        elem_index: ElemIndex,
        dst: u64,
        src: u64,
        len: u64,
    ) -> Result<(), Trap> {
        let mut storage = None;
        let elements = self.passive_element_segment(&mut storage, elem_index);
        let mut const_evaluator = ConstExprEvaluator::default();
        Self::table_init_segment(
            store,
            self.id,
            &mut const_evaluator,
            table_index,
            elements,
            dst,
            src,
            len,
        )
    }

    pub(crate) fn table_init_segment(
        store: &mut StoreOpaque,
        elements_instance_id: InstanceId,
        const_evaluator: &mut ConstExprEvaluator,
        table_index: TableIndex,
        elements: &TableSegmentElements,
        dst: u64,
        src: u64,
        len: u64,
    ) -> Result<(), Trap> {
        // https://webassembly.github.io/bulk-memory-operations/core/exec/instructions.html#exec-table-init

        let elements_instance = store.instance_mut(elements_instance_id);
        let elements_module = elements_instance.env_module();
        let top = elements_module.tables[table_index].ref_type.heap_type.top();
        let (defined_table_index, mut table_instance) =
            elements_instance.defined_table_index_and_instance(table_index);
        let table_instance_id = table_instance.id;

        let src = usize::try_from(src).map_err(|_| Trap::TableOutOfBounds)?;
        let len = usize::try_from(len).map_err(|_| Trap::TableOutOfBounds)?;

        // In the initialization below we need to simultaneously have a mutable
        // borrow on the `Table` that we're initializing and the `StoreOpaque`
        // that it comes from. To solve this the tables are temporarily removed
        // from the instance at `id` to be re-inserted at the end of this
        // function via a `Drop` helper. The table and the store are then
        // accessed through the drop helper below.
        //
        // This will cause a runtime panic if the table is actually accessed
        // during the lifetime of the functions below, but that's a bug if that
        // happens which needs to be fixed anyway.
        let tables = mem::replace(table_instance.as_mut().tables_mut(), PrimaryMap::new());
        let mut replace = ReplaceTables {
            tables,
            id: table_instance_id,
            store,
        };

        struct ReplaceTables<'a> {
            tables: PrimaryMap<DefinedTableIndex, (TableAllocationIndex, Table)>,
            id: InstanceId,
            store: &'a mut StoreOpaque,
        }

        impl Drop for ReplaceTables<'_> {
            fn drop(&mut self) {
                mem::swap(
                    self.store.instance_mut(self.id).tables_mut(),
                    &mut self.tables,
                );
                debug_assert!(self.tables.is_empty());
            }
        }

        // Reborrow the table/store from `replace` for the below code.
        let table = &mut replace.tables[defined_table_index].1;
        let store = &mut *replace.store;

        match elements {
            TableSegmentElements::Functions(funcs) => {
                let mut instance = store.instance_mut(elements_instance_id);
                let elements = funcs
                    .get(src..)
                    .and_then(|s| s.get(..len))
                    .ok_or(Trap::TableOutOfBounds)?;
                table.init_func(
                    dst,
                    elements
                        .iter()
                        .map(|idx| instance.as_mut().get_func_ref(*idx)),
                )?;
            }
            TableSegmentElements::Expressions(exprs) => {
                let exprs = exprs
                    .get(src..)
                    .and_then(|s| s.get(..len))
                    .ok_or(Trap::TableOutOfBounds)?;
                let mut context = ConstEvalContext::new(elements_instance_id);
                match top {
                    WasmHeapTopType::Extern => table.init_gc_refs(
                        dst,
                        exprs.iter().map(|expr| unsafe {
                            let raw = const_evaluator
                                .eval(store, &mut context, expr)
                                .expect("const expr should be valid");
                            VMGcRef::from_raw_u32(raw.get_externref())
                        }),
                    )?,
                    WasmHeapTopType::Any | WasmHeapTopType::Exn => table.init_gc_refs(
                        dst,
                        exprs.iter().map(|expr| unsafe {
                            let raw = const_evaluator
                                .eval(store, &mut context, expr)
                                .expect("const expr should be valid");
                            VMGcRef::from_raw_u32(raw.get_anyref())
                        }),
                    )?,
                    WasmHeapTopType::Func => table.init_func(
                        dst,
                        exprs.iter().map(|expr| unsafe {
                            NonNull::new(
                                const_evaluator
                                    .eval(store, &mut context, expr)
                                    .expect("const expr should be valid")
                                    .get_funcref()
                                    .cast(),
                            )
                        }),
                    )?,
                    WasmHeapTopType::Cont => todo!(), // FIXME: #10248 stack switching support.
                }
            }
        }

        Ok(())
    }

    /// Drop an element.
    pub(crate) fn elem_drop(self: Pin<&mut Self>, elem_index: ElemIndex) {
        // https://webassembly.github.io/reference-types/core/exec/instructions.html#exec-elem-drop

        self.dropped_elements_mut().insert(elem_index);

        // Note that we don't check that we actually removed a segment because
        // dropping a non-passive segment is a no-op (not a trap).
    }

    /// Get a locally-defined memory.
    pub fn get_defined_memory_mut(self: Pin<&mut Self>, index: DefinedMemoryIndex) -> &mut Memory {
        &mut self.memories_mut()[index].1
    }

    /// Get a locally-defined memory.
    pub fn get_defined_memory(&self, index: DefinedMemoryIndex) -> &Memory {
        &self.memories[index].1
    }

    /// Do a `memory.copy`
    ///
    /// # Errors
    ///
    /// Returns a `Trap` error when the source or destination ranges are out of
    /// bounds.
    pub(crate) fn memory_copy(
        self: Pin<&mut Self>,
        dst_index: MemoryIndex,
        dst: u64,
        src_index: MemoryIndex,
        src: u64,
        len: u64,
    ) -> Result<(), Trap> {
        // https://webassembly.github.io/reference-types/core/exec/instructions.html#exec-memory-copy

        let src_mem = self.get_memory(src_index);
        let dst_mem = self.get_memory(dst_index);

        let src = self.validate_inbounds(src_mem.current_length(), src, len)?;
        let dst = self.validate_inbounds(dst_mem.current_length(), dst, len)?;
        let len = usize::try_from(len).unwrap();

        // Bounds and casts are checked above, by this point we know that
        // everything is safe.
        unsafe {
            let dst = dst_mem.base.as_ptr().add(dst);
            let src = src_mem.base.as_ptr().add(src);
            // FIXME audit whether this is safe in the presence of shared memory
            // (https://github.com/bytecodealliance/wasmtime/issues/4203).
            ptr::copy(src, dst, len);
        }

        Ok(())
    }

    fn validate_inbounds(&self, max: usize, ptr: u64, len: u64) -> Result<usize, Trap> {
        let oob = || Trap::MemoryOutOfBounds;
        let end = ptr
            .checked_add(len)
            .and_then(|i| usize::try_from(i).ok())
            .ok_or_else(oob)?;
        if end > max {
            Err(oob())
        } else {
            Ok(ptr.try_into().unwrap())
        }
    }

    /// Perform the `memory.fill` operation on a locally defined memory.
    ///
    /// # Errors
    ///
    /// Returns a `Trap` error if the memory range is out of bounds.
    pub(crate) fn memory_fill(
        self: Pin<&mut Self>,
        memory_index: DefinedMemoryIndex,
        dst: u64,
        val: u8,
        len: u64,
    ) -> Result<(), Trap> {
        let memory_index = self.env_module().memory_index(memory_index);
        let memory = self.get_memory(memory_index);
        let dst = self.validate_inbounds(memory.current_length(), dst, len)?;
        let len = usize::try_from(len).unwrap();

        // Bounds and casts are checked above, by this point we know that
        // everything is safe.
        unsafe {
            let dst = memory.base.as_ptr().add(dst);
            // FIXME audit whether this is safe in the presence of shared memory
            // (https://github.com/bytecodealliance/wasmtime/issues/4203).
            ptr::write_bytes(dst, val, len);
        }

        Ok(())
    }

    /// Get the internal storage range of a particular Wasm data segment.
    pub(crate) fn wasm_data_range(&self, index: DataIndex) -> Range<u32> {
        match self.env_module().passive_data_map.get(&index) {
            Some(range) if !self.dropped_data.contains(index) => range.clone(),
            _ => 0..0,
        }
    }

    /// Given an internal storage range of a Wasm data segment (or subset of a
    /// Wasm data segment), get the data's raw bytes.
    pub(crate) fn wasm_data(&self, range: Range<u32>) -> &[u8] {
        let start = usize::try_from(range.start).unwrap();
        let end = usize::try_from(range.end).unwrap();
        &self.runtime_info.wasm_data()[start..end]
    }

    /// Performs the `memory.init` operation.
    ///
    /// # Errors
    ///
    /// Returns a `Trap` error if the destination range is out of this module's
    /// memory's bounds or if the source range is outside the data segment's
    /// bounds.
    pub(crate) fn memory_init(
        self: Pin<&mut Self>,
        memory_index: MemoryIndex,
        data_index: DataIndex,
        dst: u64,
        src: u32,
        len: u32,
    ) -> Result<(), Trap> {
        let range = self.wasm_data_range(data_index);
        self.memory_init_segment(memory_index, range, dst, src, len)
    }

    pub(crate) fn memory_init_segment(
        self: Pin<&mut Self>,
        memory_index: MemoryIndex,
        range: Range<u32>,
        dst: u64,
        src: u32,
        len: u32,
    ) -> Result<(), Trap> {
        // https://webassembly.github.io/bulk-memory-operations/core/exec/instructions.html#exec-memory-init

        let memory = self.get_memory(memory_index);
        let data = self.wasm_data(range);
        let dst = self.validate_inbounds(memory.current_length(), dst, len.into())?;
        let src = self.validate_inbounds(data.len(), src.into(), len.into())?;
        let len = len as usize;

        unsafe {
            let src_start = data.as_ptr().add(src);
            let dst_start = memory.base.as_ptr().add(dst);
            // FIXME audit whether this is safe in the presence of shared memory
            // (https://github.com/bytecodealliance/wasmtime/issues/4203).
            ptr::copy_nonoverlapping(src_start, dst_start, len);
        }

        Ok(())
    }

    /// Drop the given data segment, truncating its length to zero.
    pub(crate) fn data_drop(self: Pin<&mut Self>, data_index: DataIndex) {
        self.dropped_data_mut().insert(data_index);

        // Note that we don't check that we actually removed a segment because
        // dropping a non-passive segment is a no-op (not a trap).
    }

    /// Get a table by index regardless of whether it is locally-defined
    /// or an imported, foreign table. Ensure that the given range of
    /// elements in the table is lazily initialized.  We define this
    /// operation all-in-one for safety, to ensure the lazy-init
    /// happens.
    ///
    /// Takes an `Iterator` for the index-range to lazy-initialize,
    /// for flexibility. This can be a range, single item, or empty
    /// sequence, for example. The iterator should return indices in
    /// increasing order, so that the break-at-out-of-bounds behavior
    /// works correctly.
    pub(crate) fn get_table_with_lazy_init(
        self: Pin<&mut Self>,
        table_index: TableIndex,
        range: impl Iterator<Item = u64>,
    ) -> &mut Table {
        let (idx, instance) = self.defined_table_index_and_instance(table_index);
        instance.get_defined_table_with_lazy_init(idx, range)
    }

    /// Gets the raw runtime table data structure owned by this instance
    /// given the provided `idx`.
    ///
    /// The `range` specified is eagerly initialized for funcref tables.
    pub fn get_defined_table_with_lazy_init(
        mut self: Pin<&mut Self>,
        idx: DefinedTableIndex,
        range: impl Iterator<Item = u64>,
    ) -> &mut Table {
        let elt_ty = self.tables[idx].1.element_type();

        if elt_ty == TableElementType::Func {
            for i in range {
                let value = match self.tables[idx].1.get(None, i) {
                    Some(value) => value,
                    None => {
                        // Out-of-bounds; caller will handle by likely
                        // throwing a trap. No work to do to lazy-init
                        // beyond the end.
                        break;
                    }
                };

                if !value.is_uninit() {
                    continue;
                }

                // The table element `i` is uninitialized and is now being
                // initialized. This must imply that a `precompiled` list of
                // function indices is available for this table. The precompiled
                // list is extracted and then it is consulted with `i` to
                // determine the function that is going to be initialized. Note
                // that `i` may be outside the limits of the static
                // initialization so it's a fallible `get` instead of an index.
                let module = self.env_module();
                let precomputed = match &module.table_initialization.initial_values[idx] {
                    TableInitialValue::Null { precomputed } => precomputed,
                    TableInitialValue::Expr(_) => unreachable!(),
                };
                // Panicking here helps catch bugs rather than silently truncating by accident.
                let func_index = precomputed.get(usize::try_from(i).unwrap()).cloned();
                let func_ref =
                    func_index.and_then(|func_index| self.as_mut().get_func_ref(func_index));
                self.as_mut().tables_mut()[idx]
                    .1
                    .set(i, TableElement::FuncRef(func_ref))
                    .expect("Table type should match and index should be in-bounds");
            }
        }

        self.get_defined_table(idx)
    }

    /// Get a table by index regardless of whether it is locally-defined or an
    /// imported, foreign table.
    pub(crate) fn get_table(self: Pin<&mut Self>, table_index: TableIndex) -> &mut Table {
        let (idx, instance) = self.defined_table_index_and_instance(table_index);
        instance.get_defined_table(idx)
    }

    /// Get a locally-defined table.
    pub(crate) fn get_defined_table(self: Pin<&mut Self>, index: DefinedTableIndex) -> &mut Table {
        &mut self.tables_mut()[index].1
    }

    pub(crate) fn defined_table_index_and_instance<'a>(
        self: Pin<&'a mut Self>,
        index: TableIndex,
    ) -> (DefinedTableIndex, Pin<&'a mut Instance>) {
        if let Some(defined_table_index) = self.env_module().defined_table_index(index) {
            (defined_table_index, self)
        } else {
            let import = self.imported_table(index);
            let index = import.index;
            let vmctx = import.vmctx.as_non_null();
            // SAFETY: the validity of `self` means that the reachable instances
            // should also all be owned by the same store and fully initialized,
            // so it's safe to laterally move from a mutable borrow of this
            // instance to a mutable borrow of a sibling instance.
            let foreign_instance = unsafe { self.sibling_vmctx_mut(vmctx) };
            (index, foreign_instance)
        }
    }

    /// Initialize the VMContext data associated with this Instance.
    ///
    /// The `VMContext` memory is assumed to be uninitialized; any field
    /// that we need in a certain state will be explicitly written by this
    /// function.
    unsafe fn initialize_vmctx(
        mut self: Pin<&mut Self>,
        module: &Module,
        offsets: &VMOffsets<HostPtr>,
        store: StorePtr,
        imports: Imports,
    ) {
        assert!(ptr::eq(module, self.env_module().as_ref()));

        // SAFETY: the type of the magic field is indeed `u32` and this function
        // is initializing its value.
        unsafe {
            self.vmctx_plus_offset_raw::<u32>(offsets.ptr.vmctx_magic())
                .write(VMCONTEXT_MAGIC);
        }

        // SAFETY: it's up to the caller to provide a valid store pointer here.
        unsafe {
            self.as_mut().set_store(store.as_raw());
        }

        // Initialize shared types
        //
        // SAFETY: validity of the vmctx means it should be safe to write to it
        // here.
        unsafe {
            let types = NonNull::from(self.runtime_info.type_ids());
            self.type_ids_array().write(types.cast().into());
        }

        // Initialize the built-in functions
        //
        // SAFETY: the type of the builtin functions field is indeed a pointer
        // and the pointer being filled in here, plus the vmctx is valid to
        // write to during initialization.
        unsafe {
            static BUILTINS: VMBuiltinFunctionsArray = VMBuiltinFunctionsArray::INIT;
            let ptr = BUILTINS.expose_provenance();
            self.vmctx_plus_offset_raw(offsets.ptr.vmctx_builtin_functions())
                .write(VmPtr::from(ptr));
        }

        // Initialize the imports
        //
        // SAFETY: the vmctx is safe to initialize during this function and
        // validity of each item itself is a contract the caller must uphold.
        debug_assert_eq!(imports.functions.len(), module.num_imported_funcs);
        unsafe {
            ptr::copy_nonoverlapping(
                imports.functions.as_ptr(),
                self.vmctx_plus_offset_raw(offsets.vmctx_imported_functions_begin())
                    .as_ptr(),
                imports.functions.len(),
            );
            debug_assert_eq!(imports.tables.len(), module.num_imported_tables);
            ptr::copy_nonoverlapping(
                imports.tables.as_ptr(),
                self.vmctx_plus_offset_raw(offsets.vmctx_imported_tables_begin())
                    .as_ptr(),
                imports.tables.len(),
            );
            debug_assert_eq!(imports.memories.len(), module.num_imported_memories);
            ptr::copy_nonoverlapping(
                imports.memories.as_ptr(),
                self.vmctx_plus_offset_raw(offsets.vmctx_imported_memories_begin())
                    .as_ptr(),
                imports.memories.len(),
            );
            debug_assert_eq!(imports.globals.len(), module.num_imported_globals);
            ptr::copy_nonoverlapping(
                imports.globals.as_ptr(),
                self.vmctx_plus_offset_raw(offsets.vmctx_imported_globals_begin())
                    .as_ptr(),
                imports.globals.len(),
            );
            debug_assert_eq!(imports.tags.len(), module.num_imported_tags);
            ptr::copy_nonoverlapping(
                imports.tags.as_ptr(),
                self.vmctx_plus_offset_raw(offsets.vmctx_imported_tags_begin())
                    .as_ptr(),
                imports.tags.len(),
            );
        }

        // N.B.: there is no need to initialize the funcrefs array because we
        // eagerly construct each element in it whenever asked for a reference
        // to that element. In other words, there is no state needed to track
        // the lazy-init, so we don't need to initialize any state now.

        // Initialize the defined tables
        //
        // SAFETY: it's safe to initialize these tables during initialization
        // here and the various types of pointers and such here should all be
        // valid.
        unsafe {
            let mut ptr = self.vmctx_plus_offset_raw(offsets.vmctx_tables_begin());
            let tables = self.as_mut().tables_mut();
            for i in 0..module.num_defined_tables() {
                ptr.write(tables[DefinedTableIndex::new(i)].1.vmtable());
                ptr = ptr.add(1);
            }
        }

        // Initialize the defined memories. This fills in both the
        // `defined_memories` table and the `owned_memories` table at the same
        // time. Entries in `defined_memories` hold a pointer to a definition
        // (all memories) whereas the `owned_memories` hold the actual
        // definitions of memories owned (not shared) in the module.
        //
        // SAFETY: it's safe to initialize these memories during initialization
        // here and the various types of pointers and such here should all be
        // valid.
        unsafe {
            let mut ptr = self.vmctx_plus_offset_raw(offsets.vmctx_memories_begin());
            let mut owned_ptr = self.vmctx_plus_offset_raw(offsets.vmctx_owned_memories_begin());
            let memories = self.as_mut().memories_mut();
            for i in 0..module.num_defined_memories() {
                let defined_memory_index = DefinedMemoryIndex::new(i);
                let memory_index = module.memory_index(defined_memory_index);
                if module.memories[memory_index].shared {
                    let def_ptr = memories[defined_memory_index]
                        .1
                        .as_shared_memory()
                        .unwrap()
                        .vmmemory_ptr();
                    ptr.write(VmPtr::from(def_ptr));
                } else {
                    owned_ptr.write(memories[defined_memory_index].1.vmmemory());
                    ptr.write(VmPtr::from(owned_ptr));
                    owned_ptr = owned_ptr.add(1);
                }
                ptr = ptr.add(1);
            }
        }

        // Zero-initialize the globals so that nothing is uninitialized memory
        // after this function returns. The globals are actually initialized
        // with their const expression initializers after the instance is fully
        // allocated.
        //
        // SAFETY: it's safe to initialize globals during initialization
        // here. Note that while the value being written is not valid for all
        // types of globals it's initializing the memory to zero instead of
        // being in an undefined state. So it's still unsafe to access globals
        // after this, but if it's read then it'd hopefully crash faster than
        // leaving this undefined.
        unsafe {
            for (index, _init) in module.global_initializers.iter() {
                self.global_ptr(index).write(VMGlobalDefinition::new());
            }
        }

        // Initialize the defined tags
        //
        // SAFETY: it's safe to initialize these tags during initialization
        // here and the various types of pointers and such here should all be
        // valid.
        unsafe {
            let mut ptr = self.vmctx_plus_offset_raw(offsets.vmctx_tags_begin());
            for i in 0..module.num_defined_tags() {
                let defined_index = DefinedTagIndex::new(i);
                let tag_index = module.tag_index(defined_index);
                let tag = module.tags[tag_index];
                ptr.write(VMTagDefinition::new(
                    tag.signature.unwrap_engine_type_index(),
                ));
                ptr = ptr.add(1);
            }
        }
    }

    /// Attempts to convert from the host `addr` specified to a WebAssembly
    /// based address recorded in `WasmFault`.
    ///
    /// This method will check all linear memories that this instance contains
    /// to see if any of them contain `addr`. If one does then `Some` is
    /// returned with metadata about the wasm fault. Otherwise `None` is
    /// returned and `addr` doesn't belong to this instance.
    pub fn wasm_fault(&self, addr: usize) -> Option<WasmFault> {
        let mut fault = None;
        for (_, (_, memory)) in self.memories.iter() {
            let accessible = memory.wasm_accessible();
            if accessible.start <= addr && addr < accessible.end {
                // All linear memories should be disjoint so assert that no
                // prior fault has been found.
                assert!(fault.is_none());
                fault = Some(WasmFault {
                    memory_size: memory.byte_size(),
                    wasm_address: u64::try_from(addr - accessible.start).unwrap(),
                });
            }
        }
        fault
    }

    /// Returns the id, within this instance's store, that it's assigned.
    pub fn id(&self) -> InstanceId {
        self.id
    }

    /// Get all memories within this instance.
    ///
    /// Returns both import and defined memories.
    ///
    /// Returns both exported and non-exported memories.
    ///
    /// Gives access to the full memories space.
    pub fn all_memories(
        &self,
        store: StoreId,
    ) -> impl ExactSizeIterator<Item = (MemoryIndex, crate::Memory)> + '_ {
        self.env_module()
            .memories
            .iter()
            .map(move |(i, _)| (i, self.get_exported_memory(store, i)))
    }

    /// Return the memories defined in this instance (not imported).
    pub fn defined_memories<'a>(
        &'a self,
        store: StoreId,
    ) -> impl ExactSizeIterator<Item = crate::Memory> + 'a {
        let num_imported = self.env_module().num_imported_memories;
        self.all_memories(store)
            .skip(num_imported)
            .map(|(_i, memory)| memory)
    }

    /// Lookup an item with the given index.
    ///
    /// # Panics
    ///
    /// Panics if `export` is not valid for this instance.
    ///
    /// # Safety
    ///
    /// This function requires that `store` is the correct store which owns this
    /// instance.
    pub unsafe fn get_export_by_index_mut(
        self: Pin<&mut Self>,
        store: StoreId,
        export: EntityIndex,
    ) -> Export {
        match export {
            // SAFETY: the contract of `store` owning the this instance is a
            // safety requirement of this function itself.
            EntityIndex::Function(i) => {
                Export::Function(unsafe { self.get_exported_func(store, i) })
            }
            EntityIndex::Global(i) => Export::Global(self.get_exported_global(store, i)),
            EntityIndex::Table(i) => Export::Table(self.get_exported_table(store, i)),
            EntityIndex::Memory(i) => Export::Memory {
                memory: self.get_exported_memory(store, i),
                shared: self.env_module().memories[i].shared,
            },
            EntityIndex::Tag(i) => Export::Tag(self.get_exported_tag(store, i)),
        }
    }

    fn store_mut(self: Pin<&mut Self>) -> &mut Option<VMStoreRawPtr> {
        // SAFETY: this is a pin-projection to get a mutable reference to an
        // internal field and is safe so long as the `&mut Self` temporarily
        // created is not overwritten, which it isn't here.
        unsafe { &mut self.get_unchecked_mut().store }
    }

    fn dropped_elements_mut(self: Pin<&mut Self>) -> &mut EntitySet<ElemIndex> {
        // SAFETY: see `store_mut` above.
        unsafe { &mut self.get_unchecked_mut().dropped_elements }
    }

    fn dropped_data_mut(self: Pin<&mut Self>) -> &mut EntitySet<DataIndex> {
        // SAFETY: see `store_mut` above.
        unsafe { &mut self.get_unchecked_mut().dropped_data }
    }

    fn memories_mut(
        self: Pin<&mut Self>,
    ) -> &mut PrimaryMap<DefinedMemoryIndex, (MemoryAllocationIndex, Memory)> {
        // SAFETY: see `store_mut` above.
        unsafe { &mut self.get_unchecked_mut().memories }
    }

    pub(crate) fn tables_mut(
        self: Pin<&mut Self>,
    ) -> &mut PrimaryMap<DefinedTableIndex, (TableAllocationIndex, Table)> {
        // SAFETY: see `store_mut` above.
        unsafe { &mut self.get_unchecked_mut().tables }
    }

    #[cfg(feature = "wmemcheck")]
    pub(super) fn wmemcheck_state_mut(self: Pin<&mut Self>) -> &mut Option<Wmemcheck> {
        // SAFETY: see `store_mut` above.
        unsafe { &mut self.get_unchecked_mut().wmemcheck_state }
    }
}

// SAFETY: `layout` should describe this accurately and `OwnedVMContext` is the
// last field of `ComponentInstance`.
unsafe impl InstanceLayout for Instance {
    const INIT_ZEROED: bool = false;
    type VMContext = VMContext;

    fn layout(&self) -> Layout {
        Self::alloc_layout(self.runtime_info.offsets())
    }

    fn owned_vmctx(&self) -> &OwnedVMContext<VMContext> {
        &self.vmctx
    }

    fn owned_vmctx_mut(&mut self) -> &mut OwnedVMContext<VMContext> {
        &mut self.vmctx
    }
}

pub type InstanceHandle = OwnedInstance<Instance>;

/// A handle holding an `Instance` of a WebAssembly module.
///
/// This structure is an owning handle of the `instance` contained internally.
/// When this value goes out of scope it will deallocate the `Instance` and all
/// memory associated with it.
///
/// Note that this lives within a `StoreOpaque` on a list of instances that a
/// store is keeping alive.
#[derive(Debug)]
#[repr(transparent)] // guarantee this is a zero-cost wrapper
pub struct OwnedInstance<T: InstanceLayout> {
    /// The raw pointer to the instance that was allocated.
    ///
    /// Note that this is not equivalent to `Box<Instance>` because the
    /// allocation here has a `VMContext` trailing after it. Thus the custom
    /// destructor to invoke the `dealloc` function with the appropriate
    /// layout.
    instance: SendSyncPtr<T>,
    _marker: marker::PhantomData<Box<(T, OwnedVMContext<T::VMContext>)>>,
}

/// Structure that must be placed at the end of a type implementing
/// `InstanceLayout`.
#[repr(align(16))] // match the alignment of VMContext
pub struct OwnedVMContext<T> {
    /// A pointer to the `vmctx` field at the end of the `structure`.
    ///
    /// If you're looking at this a reasonable question would be "why do we need
    /// a pointer to ourselves?" because after all the pointer's value is
    /// trivially derivable from any `&Instance` pointer. The rationale for this
    /// field's existence is subtle, but it's required for correctness. The
    /// short version is "this makes miri happy".
    ///
    /// The long version of why this field exists is that the rules that MIRI
    /// uses to ensure pointers are used correctly have various conditions on
    /// them depend on how pointers are used. More specifically if `*mut T` is
    /// derived from `&mut T`, then that invalidates all prior pointers drived
    /// from the `&mut T`. This means that while we liberally want to re-acquire
    /// a `*mut VMContext` throughout the implementation of `Instance` the
    /// trivial way, a function `fn vmctx(Pin<&mut Instance>) -> *mut VMContext`
    /// would effectively invalidate all prior `*mut VMContext` pointers
    /// acquired. The purpose of this field is to serve as a sort of
    /// source-of-truth for where `*mut VMContext` pointers come from.
    ///
    /// This field is initialized when the `Instance` is created with the
    /// original allocation's pointer. That means that the provenance of this
    /// pointer contains the entire allocation (both instance and `VMContext`).
    /// This provenance bit is then "carried through" where `fn vmctx` will base
    /// all returned pointers on this pointer itself. This provides the means of
    /// never invalidating this pointer throughout MIRI and additionally being
    /// able to still temporarily have `Pin<&mut Instance>` methods and such.
    ///
    /// It's important to note, though, that this is not here purely for MIRI.
    /// The careful construction of the `fn vmctx` method has ramifications on
    /// the LLVM IR generated, for example. A historical CVE on Wasmtime,
    /// GHSA-ch89-5g45-qwc7, was caused due to relying on undefined behavior. By
    /// deriving VMContext pointers from this pointer it specifically hints to
    /// LLVM that trickery is afoot and it properly informs `noalias` and such
    /// annotations and analysis. More-or-less this pointer is actually loaded
    /// in LLVM IR which helps defeat otherwise present aliasing optimizations,
    /// which we want, since writes to this should basically never be optimized
    /// out.
    ///
    /// As a final note it's worth pointing out that the machine code generated
    /// for accessing `fn vmctx` is still as one would expect. This member isn't
    /// actually ever loaded at runtime (or at least shouldn't be). Perhaps in
    /// the future if the memory consumption of this field is a problem we could
    /// shrink it slightly, but for now one extra pointer per wasm instance
    /// seems not too bad.
    vmctx_self_reference: SendSyncPtr<T>,

    /// This field ensures that going from `Pin<&mut T>` to `&mut T` is not a
    /// safe operation.
    _marker: core::marker::PhantomPinned,
}

impl<T> OwnedVMContext<T> {
    /// Creates a new blank vmctx to place at the end of an instance.
    pub fn new() -> OwnedVMContext<T> {
        OwnedVMContext {
            vmctx_self_reference: SendSyncPtr::new(NonNull::dangling()),
            _marker: core::marker::PhantomPinned,
        }
    }
}

/// Helper trait to plumb both core instances and component instances into
/// `OwnedInstance` below.
///
/// # Safety
///
/// This trait requires `layout` to correctly describe `Self` and appropriately
/// allocate space for `Self::VMContext` afterwards. Additionally the field
/// returned by `owned_vmctx()` must be the last field in the structure.
pub unsafe trait InstanceLayout {
    /// Whether or not to allocate this instance with `alloc_zeroed` or `alloc`.
    const INIT_ZEROED: bool;

    /// The trailing `VMContext` type at the end of this instance.
    type VMContext;

    /// The memory layout to use to allocate and deallocate this instance.
    fn layout(&self) -> Layout;

    fn owned_vmctx(&self) -> &OwnedVMContext<Self::VMContext>;
    fn owned_vmctx_mut(&mut self) -> &mut OwnedVMContext<Self::VMContext>;

    /// Returns the `vmctx_self_reference` set above.
    #[inline]
    fn vmctx(&self) -> NonNull<Self::VMContext> {
        // The definition of this method is subtle but intentional. The goal
        // here is that effectively this should return `&mut self.vmctx`, but
        // it's not quite so simple. Some more documentation is available on the
        // `vmctx_self_reference` field, but the general idea is that we're
        // creating a pointer to return with proper provenance. Provenance is
        // still in the works in Rust at the time of this writing but the load
        // of the `self.vmctx_self_reference` field is important here as it
        // affects how LLVM thinks about aliasing with respect to the returned
        // pointer.
        //
        // The intention of this method is to codegen to machine code as `&mut
        // self.vmctx`, however. While it doesn't show up like this in LLVM IR
        // (there's an actual load of the field) it does look like that by the
        // time the backend runs. (that's magic to me, the backend removing
        // loads...)
        let owned_vmctx = self.owned_vmctx();
        let owned_vmctx_raw = NonNull::from(owned_vmctx);
        // SAFETY: it's part of the contract of `InstanceLayout` and the usage
        // with `OwnedInstance` that this indeed points to the vmctx.
        let addr = unsafe { owned_vmctx_raw.add(1) };
        owned_vmctx
            .vmctx_self_reference
            .as_non_null()
            .with_addr(addr.addr())
    }

    /// Helper function to access various locations offset from our `*mut
    /// VMContext` object.
    ///
    /// Note that this method takes `&self` as an argument but returns
    /// `NonNull<T>` which is frequently used to mutate said memory. This is an
    /// intentional design decision where the safety of the modification of
    /// memory is placed as a burden onto the caller. The implementation of this
    /// method explicitly does not require `&mut self` to acquire mutable
    /// provenance to update the `VMContext` region. Instead all pointers into
    /// the `VMContext` area have provenance/permissions to write.
    ///
    /// Also note though that care must be taken to ensure that reads/writes of
    /// memory must only happen where appropriate, for example a non-atomic
    /// write (as most are) should never happen concurrently with another read
    /// or write. It's generally on the burden of the caller to adhere to this.
    ///
    /// Also of note is that most of the time the usage of this method falls
    /// into one of:
    ///
    /// * Something in the VMContext is being read or written. In that case use
    ///   `vmctx_plus_offset` or `vmctx_plus_offset_mut` if possible due to
    ///   that having a safer lifetime.
    ///
    /// * A pointer is being created to pass to other VM* data structures. In
    ///   that situation the lifetime of all VM data structures are typically
    ///   tied to the `Store<T>` which is what provides the guarantees around
    ///   concurrency/etc.
    ///
    /// There's quite a lot of unsafety riding on this method, especially
    /// related to the ascription `T` of the byte `offset`. It's hoped that in
    /// the future we're able to settle on an in theory safer design.
    ///
    /// # Safety
    ///
    /// This method is unsafe because the `offset` must be within bounds of the
    /// `VMContext` object trailing this instance. Additionally `T` must be a
    /// valid ascription of the value that resides at that location.
    unsafe fn vmctx_plus_offset_raw<T: VmSafe>(&self, offset: impl Into<u32>) -> NonNull<T> {
        // SAFETY: the safety requirements of `byte_add` are forwarded to this
        // method's caller.
        unsafe {
            self.vmctx()
                .byte_add(usize::try_from(offset.into()).unwrap())
                .cast()
        }
    }

    /// Helper above `vmctx_plus_offset_raw` which transfers the lifetime of
    /// `&self` to the returned reference `&T`.
    ///
    /// # Safety
    ///
    /// See the safety documentation of `vmctx_plus_offset_raw`.
    unsafe fn vmctx_plus_offset<T: VmSafe>(&self, offset: impl Into<u32>) -> &T {
        // SAFETY: this method has the same safety requirements as
        // `vmctx_plus_offset_raw`.
        unsafe { self.vmctx_plus_offset_raw(offset).as_ref() }
    }

    /// Helper above `vmctx_plus_offset_raw` which transfers the lifetime of
    /// `&mut self` to the returned reference `&mut T`.
    ///
    /// # Safety
    ///
    /// See the safety documentation of `vmctx_plus_offset_raw`.
    unsafe fn vmctx_plus_offset_mut<T: VmSafe>(
        self: Pin<&mut Self>,
        offset: impl Into<u32>,
    ) -> &mut T {
        // SAFETY: this method has the same safety requirements as
        // `vmctx_plus_offset_raw`.
        unsafe { self.vmctx_plus_offset_raw(offset).as_mut() }
    }
}

impl<T: InstanceLayout> OwnedInstance<T> {
    /// Allocates a new `OwnedInstance` and places `instance` inside of it.
    ///
    /// This will `instance`
    pub(super) fn new(mut instance: T) -> OwnedInstance<T> {
        let layout = instance.layout();
        debug_assert!(layout.size() >= size_of_val(&instance));
        debug_assert!(layout.align() >= align_of_val(&instance));

        // SAFETY: it's up to us to assert that `layout` has a non-zero size,
        // which is asserted here.
        let ptr = unsafe {
            assert!(layout.size() > 0);
            if T::INIT_ZEROED {
                alloc::alloc::alloc_zeroed(layout)
            } else {
                alloc::alloc::alloc(layout)
            }
        };
        if ptr.is_null() {
            alloc::alloc::handle_alloc_error(layout);
        }
        let instance_ptr = NonNull::new(ptr.cast::<T>()).unwrap();

        // SAFETY: it's part of the unsafe contract of `InstanceLayout` that the
        // `add` here is appropriate for the layout allocated.
        let vmctx_self_reference = unsafe { instance_ptr.add(1).cast() };
        instance.owned_vmctx_mut().vmctx_self_reference = vmctx_self_reference.into();

        // SAFETY: we allocated above and it's an unsafe contract of
        // `InstanceLayout` that the layout is suitable for writing the
        // instance.
        unsafe {
            instance_ptr.write(instance);
        }

        let ret = OwnedInstance {
            instance: SendSyncPtr::new(instance_ptr),
            _marker: marker::PhantomData,
        };

        // Double-check various vmctx calculations are correct.
        debug_assert_eq!(
            vmctx_self_reference.addr(),
            // SAFETY: `InstanceLayout` should guarantee it's safe to add 1 to
            // the last field to get a pointer to 1-byte-past-the-end of an
            // object, which should be valid.
            unsafe { NonNull::from(ret.get().owned_vmctx()).add(1).addr() }
        );
        debug_assert_eq!(vmctx_self_reference.addr(), ret.get().vmctx().addr());

        ret
    }

    /// Gets the raw underlying `&Instance` from this handle.
    pub fn get(&self) -> &T {
        // SAFETY: this is an owned instance handle that retains exclusive
        // ownership of the `Instance` inside. With `&self` given we know
        // this pointer is valid valid and the returned lifetime is connected
        // to `self` so that should also be valid.
        unsafe { self.instance.as_non_null().as_ref() }
    }

    /// Same as [`Self::get`] except for mutability.
    pub fn get_mut(&mut self) -> Pin<&mut T> {
        // SAFETY: The lifetime concerns here are the same as `get` above.
        // Otherwise `new_unchecked` is used here to uphold the contract that
        // instances are always pinned in memory.
        unsafe { Pin::new_unchecked(self.instance.as_non_null().as_mut()) }
    }
}

impl<T: InstanceLayout> Drop for OwnedInstance<T> {
    fn drop(&mut self) {
        unsafe {
            let layout = self.get().layout();
            ptr::drop_in_place(self.instance.as_ptr());
            alloc::alloc::dealloc(self.instance.as_ptr().cast(), layout);
        }
    }
}