Add back in resources destructors to Rust exports (bytecodealliance#888)

* Add back in resources destructors to Rust exports

This was accidentally left out of my refactoring from bytecodealliance#871 meaning that
exported Rust resources never actually got their destructors run because
the `export!` macro forgot to drop things.

Closes bytecodealliance#887

* Get test passing for Go

crates/rust/src/interface.rs

@@ -133,6 +133,7 @@ impl InterfaceGenerator<'_> {
     ) -> Result<String> {
         let mut traits = BTreeMap::new();
         let mut funcs_to_export = Vec::new();
+        let mut resources_to_drop = Vec::new();
 
         traits.insert(None, ("Guest".to_string(), Vec::new()));
 
@@ -142,6 +143,7 @@ impl InterfaceGenerator<'_> {
                     TypeDefKind::Resource => {}
                     _ => continue,
                 }
+                resources_to_drop.push(name);
                 let camel = name.to_upper_camel_case();
                 traits.insert(Some(*id), (format!("Guest{camel}"), Vec::new()));
             }
@@ -296,6 +298,29 @@ macro_rules! {macro_name} {{
             };
             self.generate_raw_cabi_export(func, &ty, "$($path_to_types)*");
         }
+        let export_prefix = self.gen.opts.export_prefix.as_deref().unwrap_or("");
+        for name in resources_to_drop {
+            let module = match self.identifier {
+                Identifier::Interface(_, key) => self.resolve.name_world_key(key),
+                Identifier::World(_) => unreachable!(),
+            };
+            let camel = name.to_upper_camel_case();
+            uwriteln!(
+                self.src,
+                r#"
+                const _: () = {{
+                    #[doc(hidden)]
+                    #[export_name = "{export_prefix}{module}#[dtor]{name}"]
+                    #[allow(non_snake_case)]
+                    unsafe extern "C" fn dtor(rep: *mut u8) {{
+                        $($path_to_types)*::{camel}::dtor::<
+                            <$ty as $($path_to_types)*::Guest>::{camel}
+                        >(rep)
+                    }}
+                }};
+                "#
+            );
+        }
         uwriteln!(self.src, "}};);");
         uwriteln!(self.src, "}}");
         uwriteln!(self.src, "#[doc(hidden)]");
@@ -2054,6 +2079,12 @@ impl {camel} {{
         }}
     }}
 
+    #[doc(hidden)]
+    pub unsafe fn dtor<T: 'static>(handle: *mut u8) {{
+        Self::type_guard::<T>();
+        let _ = {box_path}::from_raw(handle as *mut _{camel}Rep<T>);
+    }}
+
     fn as_ptr<T: Guest{camel}>(&self) -> *mut _{camel}Rep<T> {{
        {camel}::type_guard::<T>();
        unsafe {{ T::_resource_rep(self.handle()).cast() }}

tests/runtime/resources.rs

@@ -92,9 +92,16 @@ fn run_test(exports: Exports, store: &mut Store<crate::Wasi<MyImports>>) -> Resu
     let z_add = exports.call_add(&mut *store, z_instance_1, z_instance_2)?;
     assert_eq!(z.call_get_a(&mut *store, z_add)?, 30);
 
+    let dropped_zs_start = z.call_num_dropped(&mut *store)?;
+
     ResourceAny::resource_drop(x_instance, &mut *store)?;
     ResourceAny::resource_drop(z_instance_1, &mut *store)?;
     ResourceAny::resource_drop(z_instance_2, &mut *store)?;
 
+    let dropped_zs_end = z.call_num_dropped(&mut *store)?;
+    if dropped_zs_start != 0 {
+        assert_eq!(dropped_zs_end, dropped_zs_start + 2);
+    }
+
     Ok(())
 }

tests/runtime/resources/wasm.c

@@ -57,10 +57,17 @@ void exports_x_destructor(exports_x_t* x) {
     free(x);
 }
 
+static uint32_t NUM_Z_DROPPED = 0;
+
 void exports_z_destructor(exports_z_t* z) {
+    NUM_Z_DROPPED += 1;
     free(z);
 }
 
+uint32_t exports_static_z_num_dropped() {
+    return NUM_Z_DROPPED + 1;
+}
+
 exports_own_kebab_case_t exports_constructor_kebab_case(uint32_t a) {
     exports_kebab_case_t* kc_instance = (exports_kebab_case_t*)malloc(sizeof(exports_kebab_case_t));
     kc_instance->a = a;

tests/runtime/resources/wasm.go

@@ -52,6 +52,10 @@ func (z *MyZ) MethodZGetA() int32 {
 	return z.a
 }
 
+func (e ExportsImpl) StaticZNumDropped() uint32 {
+        return 0
+}
+
 func (e ExportsImpl) Add(z ExportsZ, b ExportsZ) ExportsZ {
 	return &MyZ{a: z.MethodZGetA() + b.MethodZGetA()}
 }

tests/runtime/resources/wasm.rs

@@ -79,13 +79,27 @@ impl exports::exports::GuestX for ComponentX {
     }
 }
 
+static mut NUM_DROPPED_ZS: u32 = 0;
+
 impl exports::exports::GuestZ for ComponentZ {
     fn new(a: i32) -> Self {
         Self { val: a }
     }
     fn get_a(&self) -> i32 {
         self.val
     }
+
+    fn num_dropped() -> u32 {
+        unsafe { NUM_DROPPED_ZS + 1 }
+    }
+}
+
+impl Drop for ComponentZ {
+    fn drop(&mut self) {
+        unsafe {
+            NUM_DROPPED_ZS += 1;
+        }
+    }
 }
 
 impl exports::exports::GuestKebabCase for ComponentKebabCase {

tests/runtime/resources/world.wit

@@ -21,6 +21,8 @@ world resources {
     resource z {
       constructor(a: s32);
       get-a: func() -> s32;
+
+      num-dropped: static func() -> u32;
     }
 
     add: func(a: borrow<z>, b: borrow<z>) -> own<z>;