Implement `Component::define_unknown_imports_as_traps` (#8672)

* Implement Component::define_unknown_imports_as_traps

This will search through a components imports, find any imports
that have not yet been defined in the linker and add a definition
which will trap upon being called.

* Address PR feedback

Signed-off-by: Ryan Levick <ryan.levick@fermyon.com>

* Small refactoring

Signed-off-by: Ryan Levick <ryan.levick@fermyon.com>

---------

Signed-off-by: Ryan Levick <ryan.levick@fermyon.com>

crates/wasmtime/src/runtime/component/linker.rs

@@ -336,6 +336,73 @@ impl<T> Linker<T> {
             .instantiate_async(store)
             .await
     }
+
+    /// Implement any imports of the given [`Component`] with a function which traps.
+    ///
+    /// By default a [`Linker`] will error when unknown imports are encountered when instantiating a [`Component`].
+    /// This changes this behavior from an instant error to a trap that will happen if the import is called.
+    pub fn define_unknown_imports_as_traps(&mut self, component: &Component) -> Result<()> {
+        use wasmtime_environ::component::ComponentTypes;
+        use wasmtime_environ::component::TypeDef;
+        // Recursively stub out all imports of the component with a function that traps.
+        fn stub_item<T>(
+            linker: &mut LinkerInstance<T>,
+            item_name: &str,
+            item_def: &TypeDef,
+            parent_instance: Option<&str>,
+            types: &ComponentTypes,
+        ) -> Result<()> {
+            // Skip if the item isn't an instance and has already been defined in the linker.
+            if !matches!(item_def, TypeDef::ComponentInstance(_)) && linker.get(item_name).is_some()
+            {
+                return Ok(());
+            }
+
+            match item_def {
+                TypeDef::ComponentFunc(_) => {
+                    let fully_qualified_name = parent_instance
+                        .map(|parent| format!("{}#{}", parent, item_name))
+                        .unwrap_or_else(|| item_name.to_owned());
+                    linker.func_new(&item_name, move |_, _, _| {
+                        bail!("unknown import: `{fully_qualified_name}` has not been defined")
+                    })?;
+                }
+                TypeDef::ComponentInstance(i) => {
+                    let instance = &types[*i];
+                    let mut linker_instance = linker.instance(item_name)?;
+                    for (export_name, export) in instance.exports.iter() {
+                        stub_item(
+                            &mut linker_instance,
+                            export_name,
+                            export,
+                            Some(item_name),
+                            types,
+                        )?;
+                    }
+                }
+                TypeDef::Resource(_) => {
+                    let ty = crate::component::ResourceType::host::<()>();
+                    linker.resource(item_name, ty, |_, _| Ok(()))?;
+                }
+                TypeDef::Component(_) | TypeDef::Module(_) => {
+                    bail!("unable to define {} imports as traps", item_def.desc())
+                }
+                _ => {}
+            }
+            Ok(())
+        }
+
+        for (_, (import_name, import_type)) in &component.env_component().import_types {
+            stub_item(
+                &mut self.root(),
+                import_name,
+                import_type,
+                None,
+                component.types(),
+            )?;
+        }
+        Ok(())
+    }
 }
 
 impl<T> LinkerInstance<'_, T> {
@@ -616,6 +683,10 @@ impl<T> LinkerInstance<'_, T> {
         self.map
             .insert(name, &mut self.strings, self.allow_shadowing, item)
     }
+
+    fn get(&self, name: &str) -> Option<&Definition> {
+        self.map.get(name, &self.strings)
+    }
 }
 
 impl NameMap {

crates/wasmtime/src/runtime/linker.rs

@@ -256,8 +256,7 @@ impl<T> Linker<T> {
     /// Implement any imports of the given [`Module`] with a function which traps.
     ///
     /// By default a [`Linker`] will error when unknown imports are encountered
-    /// in a command module while using [`Linker::module`]. Use this function
-    /// when
+    /// in a command module while using [`Linker::module`].
     ///
     /// This method can be used to allow unknown imports from command modules.
     ///

tests/all/component_model/linker.rs

@@ -145,3 +145,39 @@ fn linker_substituting_types_issue_8003() -> Result<()> {
     }
     Ok(())
 }
+
+#[test]
+fn linker_defines_unknown_imports_as_traps() -> Result<()> {
+    let engine = Engine::default();
+    let mut linker = Linker::<()>::new(&engine);
+
+    let component = Component::new(
+        &engine,
+        r#"(component
+            (import "foo" (func))
+            (import "bar" (instance (export "baz" (func))))
+            (import "qux" (type (sub resource)))
+        )"#,
+    )?;
+    linker.define_unknown_imports_as_traps(&component)?;
+    let mut store = Store::new(&engine, ());
+    let _ = linker.instantiate(&mut store, &component)?;
+
+    Ok(())
+}
+
+#[test]
+fn linker_fails_to_define_unknown_core_module_imports_as_traps() -> Result<()> {
+    let engine = Engine::default();
+    let mut linker = Linker::<()>::new(&engine);
+
+    let component = Component::new(
+        &engine,
+        r#"(component
+            (import "foo" (core module))
+        )"#,
+    )?;
+    assert!(linker.define_unknown_imports_as_traps(&component).is_err());
+
+    Ok(())
+}