Method array_call_trampoline

crates/wasmtime/src/runtime/func.rs:2360–2428

Entrypoint of WebAssembly back into the host. This is the standard wasmtime "array call signature" which then delegates internally to the host. This assumes that `callee_vmctx` is a `VMArrayCallHostFuncContext` which was built above with `HostFuncState ` internally. This internal function is then used to dispatch based on the arguments. Details handled by this wrapper are: Host panics are han

(
        callee_vmctx: NonNull<VMOpaqueContext>,
        caller_vmctx: NonNull<VMContext>,
        args: NonNull<ValRaw>,
        args_len: usize,
    )

    /// * A GC LIFO scope is maintained around the execution of `F`.
    /// * Call hooks for entering/leaving the host are maintained.
    unsafe extern "C" fn array_call_trampoline<T, F>(
        callee_vmctx: NonNull<VMOpaqueContext>,
        caller_vmctx: NonNull<VMContext>,
        args: NonNull<ValRaw>,
        args_len: usize,
    ) -> bool
    where
        F: Fn(Caller<'_, T>, &mut [MaybeUninit<ValRaw>]) -> Result<()> + 'static,
        T: 'static,
    {
        let run = |store: &mut dyn crate::vm::VMStore, instance: InstanceId| {
            // SAFETY: correct usage of this trampoline requires correct
            // ascription of `T`, so it's the caller's responsibility to line
            // this up.
            let mut store = unsafe { store.unchecked_context_mut() };

            // Handle the entry call hook, with a corresponding exit call hook
            // below.
            store.0.call_hook(CallHook::CallingHost)?;

            // SAFETY: this function itself requires that the `vmctx` is
            // valid to use here.
            let state = unsafe {
                let vmctx = VMArrayCallHostFuncContext::from_opaque(callee_vmctx);
                vmctx.as_ref().host_state()
            };

            // Double-check ourselves in debug mode, but we control the
            // `Any` here so an unsafe downcast should also work.
            //
            // SAFETY: this function is only usable with `F`.
            let state = unsafe {
                debug_assert!(state.is::<HostFuncState<F>>());
                &*(state as *const _ as *const HostFuncState<F>)
            };

            let (gc_lifo_scope, ret) = {
                let gc_lifo_scope = store.0.gc_roots().enter_lifo_scope();

                let mut args = NonNull::slice_from_raw_parts(args.cast(), args_len);
                // SAFETY: it's a contract of this function itself that the values
                // provided are valid to view as a slice.
                let args = unsafe { args.as_mut() };

                let ret = (state.func)(
                    Caller {
                        caller: Instance::from_wasmtime(instance, store.0),
                        store: store.as_context_mut(),
                    },
                    args,
                );

                (gc_lifo_scope, ret)
            };

            store.0.exit_gc_lifo_scope(gc_lifo_scope);

            // Note that if this returns a trap then `ret` is discarded