wasmtime: Add externref Rust example

examples/externref.rs

@@ -0,0 +1,51 @@
+//! Small example of how to use `externref`s.
+
+// You can execute this example with `cargo run --example externref`
+
+use anyhow::Result;
+use wasmtime::*;
+
+fn main() -> Result<()> {
+    println!("Initializing...");
+    let mut config = Config::new();
+    config.wasm_reference_types(true);
+    let engine = Engine::new(&config);
+    let store = Store::new(&engine);
+
+    println!("Compiling module...");
+    let module = Module::from_file(&engine, "examples/externref.wat")?;
+
+    println!("Instantiating module...");
+    let imports = [];
+    let instance = Instance::new(&store, &module, &imports)?;
+
+    println!("Creating new `externref`...");
+    let externref = ExternRef::new("Hello, World!");
+    assert!(externref.data().is::<&'static str>());
+    assert_eq!(
+        *externref.data().downcast_ref::<&'static str>().unwrap(),
+        "Hello, World!"
+    );
+
+    println!("Touching `externref` table...");
+    let table = instance.get_table("table").unwrap();
+    table.set(3, Some(externref.clone()).into())?;
+    let elem = table.get(3).unwrap().unwrap_externref().unwrap();
+    assert!(elem.ptr_eq(&externref));
+
+    println!("Touching `externref` global...");
+    let global = instance.get_global("global").unwrap();
+    global.set(Some(externref.clone()).into())?;
+    let global_val = global.get().unwrap_externref().unwrap();
+    assert!(global_val.ptr_eq(&externref));
+
+    println!("Calling `externref` func...");
+    let func = instance.get_func("func").unwrap();
+    let func = func.get1::<Option<ExternRef>, Option<ExternRef>>()?;
+    let ret = func(Some(externref.clone()))?;
+    assert!(ret.is_some());
+    assert!(ret.unwrap().ptr_eq(&externref));
+
+    println!("Done.");
+    Ok(())
+}