Method prepare_call

crates/wasmtime/src/runtime/component/concurrent.rs:2590–2779

Prepare (but do not start) a guest->guest call. This is called from fused adapter code generated in `wasmtime_environ::fact::trampoline::Compiler`. `start` and `return_` are synthesized Wasm functions which move the parameters from the caller to the callee and the result from the callee to the caller, respectively. The adapter will call `Self::start_call` immediately after calling this function

(
        self,
        mut store: StoreContextMut<T>,
        start: NonNull<VMFuncRef>,
        return_: NonNull<VMFuncRef>,
        caller_instance: RuntimeComponentInstanceIndex,
        callee_in

    /// entities (and with the expected signatures for the function references
    /// -- see `wasmtime_environ::fact::trampoline::Compiler` for details).
    unsafe fn prepare_call<T: 'static>(
        self,
        mut store: StoreContextMut<T>,
        start: NonNull<VMFuncRef>,
        return_: NonNull<VMFuncRef>,
        caller_instance: RuntimeComponentInstanceIndex,
        callee_instance: RuntimeComponentInstanceIndex,
        task_return_type: TypeTupleIndex,
        callee_async: bool,
        memory: *mut VMMemoryDefinition,
        string_encoding: StringEncoding,
        caller_info: CallerInfo,
    ) -> Result<()> {
        if let (CallerInfo::Sync { .. }, true) = (&caller_info, callee_async) {
            // A task may only call an async-typed function via a sync lower if
            // it was created by a call to an async export.  Otherwise, we'll
            // trap.
            store.0.check_blocking()?;
        }

        enum ResultInfo {
            Heap { results: u32 },
            Stack { result_count: u32 },
        }

        let result_info = match &caller_info {
            CallerInfo::Async {
                has_result: true,
                params,
            } => ResultInfo::Heap {
                results: match params.last() {
                    Some(r) => r.get_u32(),
                    None => bail_bug!("retptr missing"),
                },
            },
            CallerInfo::Async {
                has_result: false, ..
            } => ResultInfo::Stack { result_count: 0 },
            CallerInfo::Sync {
                result_count,
                params,
            } if *result_count > u32::try_from(MAX_FLAT_RESULTS)? => ResultInfo::Heap {
                results: match params.last() {
                    Some(r) => r.get_u32(),
                    None => bail_bug!("arg ptr missing"),
                },
            },
            CallerInfo::Sync { result_count, .. } => ResultInfo::Stack {
                result_count: *result_count,
            },
        };

        let sync_caller = matches!(caller_info, CallerInfo::Sync { .. });

        // Create a new guest task for the call, closing over the `start` and
        // `return_` functions to lift the parameters and lower the result,
        // respectively.
        let start = SendSyncPtr::new(start);