wasm2es6js: Fix handling of start function

This is split out from #1002 and is intended to fix the tool's handling
of the `start` function. For the most accurate emulation of the wasm ESM
spec I believe we need to defer execution of the start function until
all our exports are wired up which should allow valid cyclical
references during instantiation.

The fix here is to remove the start function, if one is present, and
inject an invocation of it at the end of initialization (after our
exports are wired up). This fixes tests on #1002, but doesn't have any
direct analogue for tests here just yet.

Along the way because multiple files now come out of `wasm2es6js` by
default I've added an `--out-dir` argument as well as `-o` to ensure
that a folder for all outputs can be specified.

crates/cli-support/src/wasm2es6js.rs

@@ -121,7 +121,7 @@ impl Output {
         return ts
     }
 
-    pub fn js(self) -> Result<String, Error> {
+    pub fn js_and_wasm(mut self) -> Result<(String, Option<Vec<u8>>), Error> {
         let mut js_imports = String::new();
         let mut exports = String::new();
         let mut set_exports = String::new();
@@ -136,7 +136,7 @@ impl Output {
 
                 let name = (b'a' + (set.len() as u8)) as char;
                 js_imports.push_str(&format!(
-                    "import * as import_{} from '{}';",
+                    "import * as import_{} from '{}';\n",
                     name,
                     entry.module()
                 ));
@@ -155,6 +155,29 @@ impl Output {
                 set_exports.push_str(";\n");
             }
         }
+
+        // This is sort of tricky, but the gist of it is that if there's a start
+        // function we want to defer execution of the start function until after
+        // all our module's exports are bound. That way we'll execute it as soon
+        // as we're ready, but the module's imports and such will be able to
+        // work as everything is wired up.
+        //
+        // This ends up helping out in situations such as:
+        //
+        // * The start function calls an imported function
+        // * That imported function in turn tries to access the wasm module
+        //
+        // If we don't do this then the second step won't work because the start
+        // function is automatically executed before the promise of
+        // instantiation resolves, meaning that we won't actually have anything
+        // bound for it to access.
+        //
+        // If we remove the start function here (via `unstart`) then we'll
+        // reexport it as `__wasm2es6js_start` so be manually executed here.
+        if self.unstart() {
+            set_exports.push_str("wasm.exports.__wasm2es6js_start();\n");
+        }
+
         let inst = format!(
             "
             WebAssembly.instantiate(bytes,{{ {imports} }})
@@ -166,8 +189,8 @@ impl Output {
             imports = imports,
             set_exports = set_exports,
         );
+        let wasm = serialize(self.module).expect("failed to serialize");
         let (bytes, booted) = if self.base64 {
-            let wasm = serialize(self.module).expect("failed to serialize");
             (
                 format!(
                     "
@@ -199,8 +222,8 @@ impl Output {
         } else {
             bail!("the option --base64 or --fetch is required");
         };
-        Ok(format!(
-            "
+        let js = format!(
+            "\
             {js_imports}
             {bytes}
             export const booted = {booted};
@@ -210,6 +233,32 @@ impl Output {
             booted = booted,
             js_imports = js_imports,
             exports = exports,
-        ))
+        );
+        let wasm = if self.base64 { None } else { Some(wasm) };
+        Ok((js, wasm))
+    }
+
+    /// See comments above for what this is doing, but in a nutshell this
+    /// removes the start section, if any, and moves it to an exported function.
+    /// Returns whether a start function was found and removed.
+    fn unstart(&mut self) -> bool {
+        let mut start = None;
+        for (i, section) in self.module.sections().iter().enumerate() {
+            if let Section::Start(idx) = section  {
+                start = Some((i, *idx));
+                break;
+            }
+        }
+        let (i, idx) = match start {
+            Some(p) => p,
+            None => return false,
+        };
+        self.module.sections_mut().remove(i);
+        let entry = ExportEntry::new(
+            "__wasm2es6js_start".to_string(),
+            Internal::Function(idx),
+        );
+        self.module.export_section_mut().unwrap().entries_mut().push(entry);
+        true
     }
 }