emit an empty module

src/emitter.rs

@@ -1,13 +1,23 @@
 use crate::wasm::*;
+use std::io;
 use std::io::Write;
 use std::result;
 use thiserror::Error;
 
 #[derive(Debug, Error)]
-pub enum Error {}
+pub enum Error {
+    #[error("io: {0}")]
+    IOError(#[from] io::Error),
+}
 
 pub type Result<T> = result::Result<T, Error>;
 
-pub fn emit<W: Write>(_w: W, _wasm: &[Wasm]) -> Result<()> {
+// https://webassembly.github.io/spec/core/binary/modules.html#binary-module
+const HEADER: &'static [u8] = &[0x00, 0x61, 0x73, 0x6d];
+const VERSION: &'static [u8] = &[0x01, 0x00, 0x00, 0x00];
+
+pub fn emit<W: Write>(mut w: W, _wasm: &[Wasm]) -> Result<()> {
+    w.write_all(HEADER)?;
+    w.write_all(VERSION)?;
     Ok(())
 }