Make too many imports an instantiation error (#1478)

* Make too many imports an instantiation error

Previously we'd accidentally only take the head of the list when
instantiating, but instead this changes the API to require exactly the
right number of imports.

Author: alexcrichton

crates/api/src/instance.rs

@@ -124,6 +124,14 @@ impl Instance {
             }
         }
 
+        if imports.len() != module.imports().len() {
+            bail!(
+                "wrong number of imports provided, {} != {}",
+                imports.len(),
+                module.imports().len()
+            );
+        }
+
         module.register_frame_info();
         let config = store.engine().config();
         let instance_handle = instantiate(

crates/api/tests/instance.rs

@@ -0,0 +1,13 @@
+use anyhow::Result;
+use wasmtime::*;
+
+#[test]
+fn wrong_import_numbers() -> Result<()> {
+    let store = Store::default();
+    let module = Module::new(&store, r#"(module (import "" "" (func)))"#)?;
+
+    assert!(Instance::new(&module, &[]).is_err());
+    let func = Func::wrap(&store, || {});
+    assert!(Instance::new(&module, &[func.clone().into(), func.into()]).is_err());
+    Ok(())
+}