Support returning custom types in imports (#350)

Closes #320

crates/cli-support/src/js/rust2js.rs

@@ -298,6 +298,26 @@ impl<'a, 'b> Rust2Js<'a, 'b> {
             );
             return Ok(());
         }
+
+        if let Some(class) = ty.rust_struct() {
+            if ty.is_by_ref() {
+                bail!("cannot invoke JS functions returning custom ref types yet")
+            }
+            // Insert an assertion to the type of the returned value as
+            // otherwise this will cause memory unsafety on the Rust side of
+            // things.
+            self.ret_expr = format!("\
+                const val = JS;
+                if (!(val instanceof {0})) {{
+                    throw new Error('expected value of type {0}');
+                }}
+                const ret = val.ptr;
+                val.ptr = 0;
+                return ret;\
+            ", class);
+            return Ok(())
+        }
+
         self.ret_expr = match *ty {
             Descriptor::Boolean => "return JS ? 1 : 0;".to_string(),
             Descriptor::Char => "return JS.codePointAt(0);".to_string(),

tests/all/imports.rs

@@ -607,3 +607,64 @@ fn rust_keyword2() {
         )
         .test();
 }
+
+#[test]
+fn custom_type() {
+    project()
+        .debug(false)
+        .file("src/lib.rs", r#"
+            #![feature(proc_macro, wasm_custom_section, wasm_import_module)]
+
+            extern crate wasm_bindgen;
+
+            use wasm_bindgen::prelude::*;
+
+            #[wasm_bindgen(module = "./test")]
+            extern {
+                fn foo(f: Foo) -> Foo;
+                fn bad2() -> Foo;
+            }
+
+            #[wasm_bindgen]
+            pub struct Foo(());
+
+            #[wasm_bindgen]
+            impl Foo {
+                pub fn touch(&self) {
+                    panic!()
+                }
+            }
+
+            #[wasm_bindgen]
+            pub fn run() {
+                foo(Foo(()));
+            }
+
+            #[wasm_bindgen]
+            pub fn bad() {
+                bad2();
+            }
+        "#)
+        .file("test.ts", r#"
+            import * as assert from "assert";
+            import { run, Foo, bad } from "./out";
+
+            let VAL: any = null;
+
+            export function foo(f: Foo): Foo {
+                VAL = f;
+                return f;
+            }
+
+            export function bad2(): number {
+                return 2;
+            }
+
+            export function test() {
+                run();
+                assert.throws(() => VAL.touch(), /Attempt to use a moved value/);
+                assert.throws(bad, /expected value of type Foo/);
+            }
+        "#)
+        .test();
+}