Guarantee static_method_of uses the right this (#1795)

This came up during #1760 where `Promise.resolve` must be invoked with
`this` as the `Promise` object, but we were erroneously importing it in
such a way that it didn't have a shim and `this` was `undefined`.

Author: alexcrichton

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

@@ -2269,6 +2269,11 @@ impl<'a> Context<'a> {
                 }
             },
 
+            AuxImport::ValueWithThis(class, name) => {
+                let class = self.import_name(class)?;
+                Ok(format!("{}.{}({})", class, name, variadic_args(&args)?))
+            }
+
             AuxImport::Instanceof(js) => {
                 assert!(webidl_ty.kind == ast::WebidlFunctionKind::Static);
                 assert!(!variadic);

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

@@ -278,6 +278,10 @@ pub enum AuxImport {
     /// imported
     Value(AuxValue),
 
+    /// A static method on a class is being imported, and the `this` of the
+    /// function call is expected to always be the class.
+    ValueWithThis(JsImport, String),
+
     /// This import is expected to be a function that takes an `anyref` and
     /// returns a `bool`. It's expected that it tests if the argument is an
     /// instance of (using `instanceof`) the name specified.
@@ -950,8 +954,7 @@ impl<'a> Context<'a> {
         match op.kind {
             decode::OperationKind::Regular => {
                 if op.is_static {
-                    class.fields.push(function.name.to_string());
-                    Ok((AuxImport::Value(AuxValue::Bare(class)), false))
+                    Ok((AuxImport::ValueWithThis(class, function.name.to_string()), false))
                 } else if structural {
                     Ok((
                         AuxImport::StructuralMethod(function.name.to_string()),

crates/cli-support/src/webidl/standard.rs

@@ -574,6 +574,9 @@ fn check_standard_import(import: &AuxImport) -> Result<(), Error> {
         | AuxImport::Value(AuxValue::ClassSetter(js, name)) => {
             format!("field access of `{}` for {}", name, desc_js(js))
         }
+        AuxImport::ValueWithThis(js, method) => {
+            format!("method `{}.{}`", desc_js(js), method)
+        }
         AuxImport::Instanceof(js) => format!("instance of check of {}", desc_js(js)),
         AuxImport::Static(js) => format!("static js value {}", desc_js(js)),
         AuxImport::StructuralMethod(name) => format!("structural method `{}`", name),

guide/src/reference/attributes/on-js-imports/static_method_of.md

@@ -23,3 +23,6 @@ let instant = Date::now();
 
 This is similar to the `js_namespace` attribute, but the usage from within Rust
 is different since the method also becomes a static method of the imported type.
+Additionally this attribute also specifies that the `this` parameter when
+invoking the method is expected to be the JS class, e.g. always invoked as
+`Date.now()` instead of `const x = Date.now; x()`.

tests/wasm/imports.js

@@ -107,3 +107,11 @@ exports.import_inside_private_module = function() {};
 exports.should_call_undefined_functions = () => false;
 
 exports.STATIC_STRING = 'x';
+
+class StaticMethodCheck {
+  static static_method_of_right_this() {
+    assert.ok(this === StaticMethodCheck);
+  }
+}
+
+exports.StaticMethodCheck = StaticMethodCheck;

tests/wasm/imports.rs

@@ -52,6 +52,9 @@ extern "C" {
     fn assert_dead_import_not_generated();
     fn should_call_undefined_functions() -> bool;
 
+    type StaticMethodCheck;
+    #[wasm_bindgen(static_method_of = StaticMethodCheck)]
+    fn static_method_of_right_this();
 
     static STATIC_STRING: String;
 }
@@ -240,3 +243,8 @@ fn undefined_function_is_ok() {
 fn static_string_ok() {
     assert_eq!(*STATIC_STRING, "x");
 }
+
+#[wasm_bindgen_test]
+fn static_method_of_has_right_this() {
+    StaticMethodCheck::static_method_of_right_this();
+}