Be sure to GC our imports as well as the module

After a module goes through its primary GC pass we need to look over the
set of remaining imports and use that to prune the set of imports that
we're binding.

Closes rustwasm#1613

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

@@ -204,8 +204,18 @@ impl<'a> Context<'a> {
         // After all we've done, especially
         // `unexport_unused_internal_exports()`, we probably have a bunch of
         // garbage in the module that's no longer necessary, so delete
-        // everything that we don't actually need.
+        // everything that we don't actually need. Afterwards make sure we don't
+        // try to emit bindings for now-nonexistent imports by pruning our
+        // `wasm_import_definitions` set.
         walrus::passes::gc::run(self.module);
+        let remaining_imports = self
+            .module
+            .imports
+            .iter()
+            .map(|i| i.id())
+            .collect::<HashSet<_>>();
+        self.wasm_import_definitions
+            .retain(|id, _| remaining_imports.contains(id));
 
         // Cause any future calls to `should_write_global` to panic, making sure
         // we don't ask for items which we can no longer emit.

crates/cli/tests/wasm-bindgen/main.rs

@@ -136,3 +136,18 @@ fn works_on_empty_project() {
 }
 
 mod npm;
+
+#[test]
+fn one_export_works() {
+    let (mut cmd, _out_dir) = Project::new("one_export_works")
+        .file(
+            "src/lib.rs",
+            r#"
+                use wasm_bindgen::prelude::*;
+                #[wasm_bindgen]
+                pub fn foo() {}
+            "#,
+        )
+        .wasm_bindgen("");
+    cmd.assert().success();
+}