feat: add example hello

__wasm/wasmtime/examples/hello.rs

@@ -0,0 +1,47 @@
+use std::error::Error;
+use wasmtime::*;
+
+// https://docs.wasmtime.dev/lang-rust.html
+fn main() -> Result<(), Box<dyn Error>> {
+    // An engine stores and configures global compilation settings like
+    // optimization level, enabled wasm features, etc.
+    let engine = Engine::default();
+
+    // We start off by creating a `Module` which represents a compiled form
+    // of our input wasm module. In this case it'll be JIT-compiled after
+    // we parse the text format.
+    let module = Module::new(&engine, r#"
+    (module
+      (func (export "answer") (result i32)
+         i32.const 42
+      )
+    )
+    "#)?;
+
+    // A `Store` is what will own instances, functions, globals, etc. All wasm
+    // items are stored within a `Store`, and it's what we'll always be using to
+    // interact with the wasm world. Custom data can be stored in stores but for
+    // now we just use `()`.
+    let mut store = Store::new(&engine, ());
+
+    // With a compiled `Module` we can then instantiate it, creating
+    // an `Instance` which we can actually poke at functions on.
+    let instance = Instance::new(&mut store, &module, &[])?;
+
+    // The `Instance` gives us access to various exported functions and items,
+    // which we access here to pull out our `answer` exported function and
+    // run it.
+    let answer = instance.get_func(&mut store, "answer")
+        .expect("`answer` was not an exported function");
+
+    // There's a few ways we can call the `answer` `Func` value. The easiest
+    // is to statically assert its signature with `typed` (in this case
+    // asserting it takes no arguments and returns one i32) and then call it.
+    let answer = answer.typed::<(), i32>(&store)?;
+
+    // And finally we can call our function! Note that the error propagation
+    // with `?` is done to handle the case where the wasm function traps.
+    let result = answer.call(&mut store, ())?;
+    println!("Answer: {:?}", result);
+    Ok(())
+}