wasmparser(CM+GC): Assert that SubtypeCxs are always using the same typing context (#2170)

A `SubtypeCx`, used for subtype-checking in the component model, is constructed
with two `TypesRef`s. These `TypesRef`s must be associated with the same
validator, so that we know that core types are canonicalized the same across the two of
them and we can compare `CoreTypeId`s for equality.

I had originally hoped to move the `SubtypeCx` to wrapping a single
`TypesRef`, but this cannot work because (unlike core types) new component model
types can be pushed into each different `TypesRef` for different components.

Author: fitzgen

crates/wasmparser/src/validator/component_types.rs

@@ -730,7 +730,13 @@ pub enum ComponentEntityType {
 
 impl ComponentEntityType {
     /// Determines if component entity type `a` is a subtype of `b`.
-    pub fn is_subtype_of(a: &Self, at: TypesRef, b: &Self, bt: TypesRef) -> bool {
+    ///
+    /// # Panics
+    ///
+    /// Panics if the two given `TypesRef`s are not associated with the same
+    /// `Validator`.
+    pub fn is_subtype_of(a: &Self, at: TypesRef<'_>, b: &Self, bt: TypesRef<'_>) -> bool {
+        assert_eq!(at.id(), bt.id());
         SubtypeCx::new(at.list, bt.list)
             .component_entity_type(a, b, 0)
             .is_ok()
@@ -2150,6 +2156,9 @@ where
 {
     /// Pushes a new anonymous type within this object, returning an identifier
     /// which can be used to refer to it.
+    ///
+    /// For internal use only!
+    #[doc(hidden)]
     fn push_ty<T>(&mut self, ty: T) -> T::Id
     where
         T: TypeData;
@@ -2496,7 +2505,13 @@ macro_rules! limits_match {
 
 impl<'a> SubtypeCx<'a> {
     /// Create a new instance with the specified type lists
+    ///
+    /// # Panics
+    ///
+    /// Panics if the two given `TypesRef`s are not associated with the same
+    /// `Validator`.
     pub fn new_with_refs(a: TypesRef<'a>, b: TypesRef<'a>) -> SubtypeCx<'a> {
+        assert_eq!(a.id(), b.id());
         Self::new(a.list, b.list)
     }
 

crates/wasmparser/src/validator/types.rs

@@ -783,7 +783,13 @@ impl<T> Index<usize> for SnapshotList<T> {
 
     #[inline]
     fn index(&self, index: usize) -> &T {
-        self.get(index).unwrap()
+        match self.get(index) {
+            Some(x) => x,
+            None => panic!(
+                "out-of-bounds indexing into `SnapshotList`: index is {index}, but length is {}",
+                self.len()
+            ),
+        }
     }
 }