Implement JsCast for all imported types

This commit implements the `JsCast` trait automatically for all imported types
in `#[wasm_bindgen] extern { ... }` blocks. The main change here was to generate
an `instanceof` shim for all imported types in case it's needed.

All imported types now also implement `AsRef<JsValue>` and `AsMut<JsValue>`

crates/backend/src/ast.rs

@@ -127,6 +127,7 @@ pub struct ImportType {
     pub name: Ident,
     pub attrs: Vec<syn::Attribute>,
     pub doc_comment: Option<String>,
+    pub instanceof_shim: String,
 }
 
 #[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq, Eq))]
@@ -408,7 +409,10 @@ impl ImportStatic {
 
 impl ImportType {
     fn shared(&self) -> shared::ImportType {
-        shared::ImportType {}
+        shared::ImportType {
+            name: self.name.to_string(),
+            instanceof_shim: self.instanceof_shim.clone(),
+        }
     }
 }
 

crates/backend/src/codegen.rs

@@ -521,10 +521,12 @@ impl ToTokens for ast::ImportType {
         };
         let const_name = format!("__wbg_generated_const_{}", name);
         let const_name = Ident::new(&const_name, Span::call_site());
+        let instanceof_shim = Ident::new(&self.instanceof_shim, Span::call_site());
         (quote! {
             #[allow(bad_style)]
             #(#attrs)*
             #[doc = #doc_comment]
+            #[repr(transparent)]
             #vis struct #name {
                 obj: ::wasm_bindgen::JsValue,
             }
@@ -533,7 +535,7 @@ impl ToTokens for ast::ImportType {
             const #const_name: () = {
                 use wasm_bindgen::convert::{IntoWasmAbi, FromWasmAbi, Stack};
                 use wasm_bindgen::convert::{OptionIntoWasmAbi, OptionFromWasmAbi};
-                use wasm_bindgen::convert::RefFromWasmAbi;
+                use wasm_bindgen::convert::{RefFromWasmAbi, GlobalStack};
                 use wasm_bindgen::describe::WasmDescribe;
                 use wasm_bindgen::{JsValue, JsCast};
                 use wasm_bindgen::__rt::core::mem::ManuallyDrop;
@@ -594,18 +596,63 @@ impl ToTokens for ast::ImportType {
                     }
                 }
 
+                // TODO: remove this on the next major version
                 impl From<JsValue> for #name {
                     fn from(obj: JsValue) -> #name {
                         #name { obj }
                     }
                 }
 
+                impl AsRef<JsValue> for #name {
+                    fn as_ref(&self) -> &JsValue { &self.obj }
+                }
+
+                impl AsMut<JsValue> for #name {
+                    fn as_mut(&mut self) -> &mut JsValue { &mut self.obj }
+                }
+
                 impl From<#name> for JsValue {
                     fn from(obj: #name) -> JsValue {
                         obj.obj
                     }
                 }
 
+                impl JsCast for #name {
+                    #[cfg(all(target_arch = "wasm32", not(target_os = "emscripten")))]
+                    fn instanceof(val: &JsValue) -> bool {
+                        #[link(wasm_import_module = "__wbindgen_placeholder__")]
+                        extern {
+                            fn #instanceof_shim(val: u32) -> u32;
+                        }
+                        unsafe {
+                            let idx = val.into_abi(&mut GlobalStack::new());
+                            #instanceof_shim(idx) != 0
+                        }
+                    }
+
+                    #[cfg(not(all(target_arch = "wasm32", not(target_os = "emscripten"))))]
+                    fn instanceof(val: &JsValue) -> bool {
+                        drop(val);
+                        panic!("cannot check instanceof on non-wasm targets");
+                    }
+
+                    fn unchecked_from_js(val: JsValue) -> Self {
+                        #name { obj: val }
+                    }
+
+                    fn unchecked_from_js_ref(val: &JsValue) -> &Self {
+                        // Should be safe because `#name` is a transparent
+                        // wrapper around `val`
+                        unsafe { &*(val as *const JsValue as *const #name) }
+                    }
+
+                    fn unchecked_from_js_mut(val: &mut JsValue) -> &mut Self {
+                        // Should be safe because `#name` is a transparent
+                        // wrapper around `val`
+                        unsafe { &mut *(val as *mut JsValue as *mut #name) }
+                    }
+                }
+
                 ()
             };
         }).to_tokens(tokens);

crates/cli-support/src/js/mod.rs

@@ -1755,7 +1755,14 @@ impl<'a, 'b> SubContext<'a, 'b> {
                     format!("failed to generate bindings for JS import `{}`", s.name)
                 })?;
             }
-            shared::ImportKind::Type(_) => {}
+            shared::ImportKind::Type(ref ty) => {
+                self.generate_import_type(import, ty).with_context(|_| {
+                    format!(
+                        "failed to generate bindings for JS import `{}`",
+                        ty.name,
+                    )
+                })?;
+            }
             shared::ImportKind::Enum(_) => {}
         }
         Ok(())
@@ -1936,6 +1943,27 @@ impl<'a, 'b> SubContext<'a, 'b> {
         Ok(())
     }
 
+    fn generate_import_type(
+        &mut self,
+        info: &shared::Import,
+        import: &shared::ImportType,
+    ) -> Result<(), Error> {
+        if !self.cx.wasm_import_needed(&import.instanceof_shim) {
+            return Ok(());
+        }
+        let name = self.import_name(info, &import.name)?;
+        self.cx.expose_get_object();
+        let body = format!("
+                function(idx) {{
+                    return getObject(idx) instanceof {} ? 1 : 0;
+                }}
+            ",
+            name,
+        );
+        self.cx.export(&import.instanceof_shim, &body, None);
+        Ok(())
+    }
+
     fn generate_enum(&mut self, enum_: &shared::Enum) {
         let mut variants = String::new();
 

crates/macro-support/src/parser.rs

@@ -524,11 +524,13 @@ impl ConvertToAst<()> for syn::ForeignItemType {
     type Target = ast::ImportKind;
 
     fn convert(self, (): ()) -> Result<Self::Target, Diagnostic> {
+        let shim = format!("__wbg_instanceof_{}_{}", self.ident, ShortHash(&self.ident));
         Ok(ast::ImportKind::Type(ast::ImportType {
             vis: self.vis,
-            name: self.ident,
             attrs: self.attrs,
             doc_comment: None,
+            instanceof_shim: shim,
+            name: self.ident,
         }))
     }
 }

crates/shared/src/lib.rs

@@ -3,7 +3,7 @@
 #[macro_use]
 extern crate serde_derive;
 
-pub const SCHEMA_VERSION: &str = "7";
+pub const SCHEMA_VERSION: &str = "8";
 
 #[derive(Deserialize)]
 pub struct ProgramOnlySchema {
@@ -81,7 +81,10 @@ pub struct ImportStatic {
 }
 
 #[derive(Deserialize, Serialize)]
-pub struct ImportType {}
+pub struct ImportType {
+    pub name: String,
+    pub instanceof_shim: String,
+}
 
 #[derive(Deserialize, Serialize)]
 pub struct ImportEnum {}

crates/webidl/src/lib.rs

@@ -246,6 +246,7 @@ impl<'src> WebidlParse<'src, ()> for weedle::InterfaceDefinition<'src> {
                 name: rust_ident(camel_case_ident(self.identifier.0).as_str()),
                 attrs: Vec::new(),
                 doc_comment,
+                instanceof_shim: format!("__widl_instanceof_{}", self.name),
             }),
         });