wasmtime: Add support for func.ref and table.grow with funcrefs

`funcref`s are implemented as `NonNull<VMCallerCheckedAnyfunc>`.

This should be more efficient than using a `VMExternRef` that points at a
`VMCallerCheckedAnyfunc` because it gets rid of an indirection, dynamic
allocation, and some reference counting.

Note that the null function reference is *NOT* a null pointer; it is a
`VMCallerCheckedAnyfunc` that has a null `func_ptr` member.

Part of #929

crates/c-api/src/table.rs

@@ -42,7 +42,7 @@ pub extern "C" fn wasm_table_new(
 ) -> Option<Box<wasm_table_t>> {
     let init: Val = match init {
         Some(init) => init.r.into(),
-        None => Val::ExternRef(None),
+        None => Val::FuncRef(None),
     };
     let table = Table::new(&store.store, tt.ty().ty.clone(), init).ok()?;
     Some(Box::new(wasm_table_t {
@@ -60,8 +60,8 @@ pub extern "C" fn wasmtime_funcref_table_new(
     out: &mut *mut wasm_table_t,
 ) -> Option<Box<wasmtime_error_t>> {
     let init: Val = match init {
-        Some(val) => Val::FuncRef(val.func().borrow().clone()),
-        None => Val::ExternRef(None),
+        Some(val) => Val::FuncRef(Some(val.func().borrow().clone())),
+        None => Val::FuncRef(None),
     };
     handle_result(
         Table::new(&store.store, tt.ty().ty.clone(), init),
@@ -85,7 +85,7 @@ pub extern "C" fn wasm_table_type(t: &wasm_table_t) -> Box<wasm_tabletype_t> {
 pub extern "C" fn wasm_table_get(t: &wasm_table_t, index: wasm_table_size_t) -> *mut wasm_ref_t {
     match t.table().borrow().get(index) {
         Some(val) => into_funcref(val),
-        None => into_funcref(Val::ExternRef(None)),
+        None => into_funcref(Val::FuncRef(None)),
     }
 }
 
@@ -99,14 +99,14 @@ pub extern "C" fn wasmtime_funcref_table_get(
         Some(val) => {
             *ptr = match val {
                 // TODO: what do do about creating new `HostRef` handles here?
-                Val::FuncRef(f) => {
+                Val::FuncRef(None) => ptr::null_mut(),
+                Val::FuncRef(Some(f)) => {
                     let store = match t.table().as_ref().store() {
                         None => return false,
                         Some(store) => store,
                     };
                     Box::into_raw(Box::new(HostRef::new(&store, f).into()))
                 }
-                Val::ExternRef(None) => ptr::null_mut(),
                 _ => return false,
             };
         }
@@ -133,14 +133,14 @@ pub extern "C" fn wasmtime_funcref_table_set(
     val: Option<&wasm_func_t>,
 ) -> Option<Box<wasmtime_error_t>> {
     let val = match val {
-        Some(val) => Val::FuncRef(val.func().borrow().clone()),
-        None => Val::ExternRef(None),
+        Some(val) => Val::FuncRef(Some(val.func().borrow().clone())),
+        None => Val::FuncRef(None),
     };
     handle_result(t.table().borrow().set(index, val), |()| {})
 }
 
 fn into_funcref(val: Val) -> *mut wasm_ref_t {
-    if let Val::ExternRef(None) = val {
+    if let Val::FuncRef(None) = val {
         return ptr::null_mut();
     }
     let externref = match val.externref() {
@@ -155,7 +155,7 @@ unsafe fn from_funcref(r: *mut wasm_ref_t) -> Val {
     if !r.is_null() {
         Box::from_raw(r).r.into()
     } else {
-        Val::ExternRef(None)
+        Val::FuncRef(None)
     }
 }
 
@@ -182,8 +182,8 @@ pub extern "C" fn wasmtime_funcref_table_grow(
     prev_size: Option<&mut wasm_table_size_t>,
 ) -> Option<Box<wasmtime_error_t>> {
     let val = match init {
-        Some(val) => Val::FuncRef(val.func().borrow().clone()),
-        None => Val::ExternRef(None),
+        Some(val) => Val::FuncRef(Some(val.func().borrow().clone())),
+        None => Val::FuncRef(None),
     };
     handle_result(t.table().borrow().grow(delta, val), |prev| {
         if let Some(ptr) = prev_size {