partial spec is an enum

crates/ty_python_semantic/src/types/generics.rs

@@ -568,7 +568,7 @@ impl<'db> GenericContext<'db> {
         loop {
             let mut any_changed = false;
             for i in 0..len {
-                let partial = PartialSpecialization {
+                let partial = PartialSpecialization::FromGenericContext {
                     generic_context: self,
                     types: &types,
                     // Don't recursively substitute type[i] in itself. Ideally, we could instead
@@ -646,7 +646,7 @@ impl<'db> GenericContext<'db> {
             // Typevars are only allowed to refer to _earlier_ typevars in their defaults. (This is
             // statically enforced for PEP-695 contexts, and is explicitly called out as a
             // requirement for legacy contexts.)
-            let partial = PartialSpecialization {
+            let partial = PartialSpecialization::FromGenericContext {
                 generic_context: self,
                 types: &expanded[0..idx],
                 skip: None,
@@ -1452,12 +1452,14 @@ impl<'db> Specialization<'db> {
 /// You will usually use [`Specialization`] instead of this type. This type is used when we need to
 /// substitute types for type variables before we have fully constructed a [`Specialization`].
 #[derive(Clone, Debug, Eq, Hash, PartialEq, get_size2::GetSize)]
-pub struct PartialSpecialization<'a, 'db> {
-    generic_context: GenericContext<'db>,
-    types: &'a [Type<'db>],
-    /// An optional typevar to _not_ substitute when applying the specialization. We use this to
-    /// avoid recursively substituting a type inside of itself.
-    skip: Option<usize>,
+pub enum PartialSpecialization<'a, 'db> {
+    FromGenericContext {
+        generic_context: GenericContext<'db>,
+        types: &'a [Type<'db>],
+        /// An optional typevar to _not_ substitute when applying the specialization. We use this to
+        /// avoid recursively substituting a type inside of itself.
+        skip: Option<usize>,
+    },
 }
 
 impl<'db> PartialSpecialization<'_, 'db> {
@@ -1468,14 +1470,21 @@ impl<'db> PartialSpecialization<'_, 'db> {
         db: &'db dyn Db,
         bound_typevar: BoundTypeVarInstance<'db>,
     ) -> Option<Type<'db>> {
-        let index = self
-            .generic_context
-            .variables_inner(db)
-            .get_index_of(&bound_typevar.identity(db))?;
-        if self.skip.is_some_and(|skip| skip == index) {
-            return Some(Type::Never);
+        match self {
+            PartialSpecialization::FromGenericContext {
+                generic_context,
+                types,
+                skip,
+            } => {
+                let index = generic_context
+                    .variables_inner(db)
+                    .get_index_of(&bound_typevar.identity(db))?;
+                if skip.is_some_and(|skip| skip == index) {
+                    return Some(Type::Never);
+                }
+                types.get(index).copied()
+            }
         }
-        self.types.get(index).copied()
     }
 }