Change wasm-to-host trampolines to take the values_vec size (#4192)

* Change wasm-to-host trampolines to take the values_vec size

This commit changes the ABI of wasm-to-host trampolines, which are
only used right now for functions created with `Func::new`, to pass
along the size of the `values_vec` argument. Previously the trampoline
simply received `*mut ValRaw` and assumed that it was the appropriate
size. By receiving a size as well we can thread through `&mut [ValRaw]`
internally instead of `*mut ValRaw`.

The original motivation for this is that I'm planning to leverage these
trampolines for the component model for host-defined functions. Out of
an abundance of caution of making sure that everything lines up I wanted
to be able to write down asserts about the size received at runtime
compared to the size expected. This overall led me to the desire to
thread this size parameter through on the assumption that it would not
impact performance all that much.

I ran two benchmarks locally from the `call.rs` benchmark and got:

* `sync/no-hook/wasm-to-host - nop - unchecked` - no change
* `sync/no-hook/wasm-to-host - nop-params-and-results - unchecked` - 5%
  slower

This is what I roughly expected in that if nothing actually reads the
new parameter (e.g. no arguments) then threading through the parameter
is effectively otherwise free. Otherwise though accesses to the `ValRaw`
storage is now bounds-checked internally in Wasmtime instead of assuming
it's valid, leading to the 5% slowdown (~9.6ns to ~10.3ns). If this
becomes a peformance bottleneck for a particular use case then we should
be fine to remove the bounds checking here or otherwise only bounds
check in debug mode, otherwise I plan on leaving this as-is.

Of particular note this also changes the C API for `*_unchecked`
functions where the C callback now receives the size of the array as
well.

* Add docs

crates/c-api/include/wasmtime/func.h

@@ -101,6 +101,8 @@ WASM_API_EXTERN void wasmtime_func_new(
  *        array depends on the function type that the host function is created
  *        with, but it will be the maximum of the number of parameters and
  *        number of results.
+ * \param num_args_and_results the size of the `args_and_results` parameter in
+ *        units of #wasmtime_val_raw_t.
  *
  * This callback can optionally return a #wasm_trap_t indicating that a trap
  * should be raised in WebAssembly. It's expected that in this case the caller
@@ -121,7 +123,8 @@ WASM_API_EXTERN void wasmtime_func_new(
 typedef wasm_trap_t* (*wasmtime_func_unchecked_callback_t)(
     void *env,
     wasmtime_caller_t* caller,
-    wasmtime_val_raw_t *args_and_results);
+    wasmtime_val_raw_t *args_and_results,
+    size_t num_args_and_results);
 
 /**
  * \brief Creates a new host function in the same manner of #wasmtime_func_new,

crates/c-api/src/func.rs

@@ -209,8 +209,12 @@ pub type wasmtime_func_callback_t = extern "C" fn(
     usize,
 ) -> Option<Box<wasm_trap_t>>;
 
-pub type wasmtime_func_unchecked_callback_t =
-    extern "C" fn(*mut c_void, *mut wasmtime_caller_t, *mut ValRaw) -> Option<Box<wasm_trap_t>>;
+pub type wasmtime_func_unchecked_callback_t = extern "C" fn(
+    *mut c_void,
+    *mut wasmtime_caller_t,
+    *mut ValRaw,
+    usize,
+) -> Option<Box<wasm_trap_t>>;
 
 #[no_mangle]
 pub unsafe extern "C" fn wasmtime_func_new(
@@ -295,12 +299,12 @@ pub(crate) unsafe fn c_unchecked_callback_to_rust_fn(
     callback: wasmtime_func_unchecked_callback_t,
     data: *mut c_void,
     finalizer: Option<extern "C" fn(*mut std::ffi::c_void)>,
-) -> impl Fn(Caller<'_, crate::StoreData>, *mut ValRaw) -> Result<(), Trap> {
+) -> impl Fn(Caller<'_, crate::StoreData>, &mut [ValRaw]) -> Result<(), Trap> {
     let foreign = crate::ForeignData { data, finalizer };
     move |caller, values| {
         drop(&foreign); // move entire foreign into this closure
         let mut caller = wasmtime_caller_t { caller };
-        match callback(foreign.data, &mut caller, values) {
+        match callback(foreign.data, &mut caller, values.as_mut_ptr(), values.len()) {
             None => Ok(()),
             Some(trap) => Err(trap.trap),
         }