Disallow non-type/instance aliases in component/instance types (#2462)

This fixes a bug in the validation of components where previously
invalid `alias` items were allowed to be present in `instance` and
`component` type definitions. The specification indicates that these
should be disallowed and `wasmparser` erroneously allowed them. The
impact of this change is expected to be nonexistent or negligible as
this only applies to handwritten components. Components produced by
`wit-component` do not use this feature so no previously produced
component should be rejected. Additionally the impact of allowing these
invalid `alias` annotations is also negligible as you couldn't actually
do anything with the item anyway so it was just sort of dead bloat in a
binary.

cc WebAssembly/component-model#621

Author: alexcrichton

crates/wasmparser/src/validator/component.rs

@@ -2446,29 +2446,42 @@ impl ComponentState {
         types: &mut TypeAlloc,
         offset: usize,
     ) -> Result<()> {
+        let component = components.last_mut().unwrap();
+
+        match component.kind {
+            // For a component itself all alias kind are allowed.
+            ComponentKind::Component => {}
+
+            // For instance/component types only aliases to `type` or `instance`
+            // items are allowed, and all other aliases are rejected.
+            ComponentKind::InstanceType | ComponentKind::ComponentType => match &alias {
+                crate::ComponentAlias::InstanceExport {
+                    kind: ComponentExternalKind::Type | ComponentExternalKind::Instance,
+                    ..
+                }
+                | crate::ComponentAlias::Outer {
+                    kind: ComponentOuterAliasKind::Type | ComponentOuterAliasKind::CoreType,
+                    ..
+                } => {}
+
+                _ => bail!(
+                    offset,
+                    "aliases in a component or instance type may only refer to types or instances"
+                ),
+            },
+        }
+
         match alias {
             crate::ComponentAlias::InstanceExport {
                 instance_index,
                 kind,
                 name,
-            } => components.last_mut().unwrap().alias_instance_export(
-                instance_index,
-                kind,
-                name,
-                types,
-                offset,
-            ),
+            } => component.alias_instance_export(instance_index, kind, name, types, offset),
             crate::ComponentAlias::CoreInstanceExport {
                 instance_index,
                 kind,
                 name,
-            } => components.last_mut().unwrap().alias_core_instance_export(
-                instance_index,
-                kind,
-                name,
-                types,
-                offset,
-            ),
+            } => component.alias_core_instance_export(instance_index, kind, name, types, offset),
             crate::ComponentAlias::Outer { kind, count, index } => match kind {
                 ComponentOuterAliasKind::CoreModule => {
                     Self::alias_module(components, count, index, offset)

tests/cli/component-model/alias.wast

@@ -299,3 +299,12 @@
     (instance (instantiate (component $i "x")))
   )
 )
+
+(assert_invalid
+  (component
+    (type (component
+      (import "x" (instance $I (export "f" (func))))
+      (alias export $I "f" (func))
+    ))
+  )
+  "aliases in a component or instance type may only refer to types or instances")

tests/snapshots/cli/component-model/alias.wast.json

@@ -183,6 +183,13 @@
       "line": 293,
       "filename": "alias.27.wasm",
       "module_type": "binary"
+    },
+    {
+      "type": "assert_invalid",
+      "line": 304,
+      "filename": "alias.28.wasm",
+      "module_type": "binary",
+      "text": "aliases in a component or instance type may only refer to types or instances"
     }
   ]
 }