Don't put anything before ES module imports

Because of some incorrect use of `js.push_str(..)`, we could sometimes emit code
before the ES modules imports, which is syntactically invalid:

    const __exports = {};
    import { Thing } from '...'; // Syntax error!

This has been fixed by making sure that the correct `imports` or `imports_post`
string is built up. We now also assert that the `js` string is empty at the
location where we add imports if we're using ES modules.

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

@@ -354,7 +354,8 @@ impl<'a> Context<'a> {
             | OutputMode::Node {
                 experimental_modules: true,
             } => {
-                js.push_str(&format!("import * as wasm from './{}_bg';\n", module_name));
+                self.imports
+                    .push_str(&format!("import * as wasm from './{}_bg';\n", module_name));
                 if needs_manual_start {
                     self.footer.push_str("wasm.__wbindgen_start();\n");
                 }
@@ -365,7 +366,7 @@ impl<'a> Context<'a> {
             // expose the same initialization function as `--target no-modules`
             // as the default export of the module.
             OutputMode::Web => {
-                js.push_str("const __exports = {};\n");
+                self.imports_post.push_str("const __exports = {};\n");
                 self.imports_post.push_str("let wasm;\n");
                 init = self.gen_init(&module_name, needs_manual_start);
                 self.footer.push_str("export default init;\n");
@@ -377,6 +378,10 @@ impl<'a> Context<'a> {
         ts.push_str(&init_ts);
 
         // Emit all the JS for importing all our functionality
+        assert!(
+            !self.config.mode.uses_es_modules() || js.is_empty(),
+            "ES modules require imports to be at the start of the file"
+        );
         js.push_str(&self.imports);
         js.push_str("\n");
         js.push_str(&self.imports_post);

crates/cli-support/src/lib.rs

@@ -42,6 +42,19 @@ enum OutputMode {
     Node { experimental_modules: bool },
 }
 
+impl OutputMode {
+    fn uses_es_modules(&self) -> bool {
+        match self {
+            OutputMode::Bundler { .. }
+            | OutputMode::Web
+            | OutputMode::Node {
+                experimental_modules: true,
+            } => true,
+            _ => false,
+        }
+    }
+}
+
 enum Input {
     Path(PathBuf),
     Module(Module, String),