[ty] guard against recursion in determining completion kind (#20354)

## Summary

Fixes https://github.com/astral-sh/ty/issues/1158

## Test Plan

Added test.

crates/ty_ide/src/completion.rs

@@ -551,6 +551,8 @@ mod tests {
 
     use super::{CompletionSettings, token_suffix_by_kinds};
 
+    use ty_python_semantic::CompletionKind;
+
     #[test]
     fn token_suffixes_match() {
         insta::assert_debug_snapshot!(
@@ -3093,6 +3095,27 @@ from os.<CURSOR>
         );
     }
 
+    #[test]
+    fn completion_kind_recursive_type_alias() {
+        let test = cursor_test(
+            r#"
+            type T = T | None
+            def f(rec: T):
+                re<CURSOR>
+            "#,
+        );
+
+        let completions = completion(
+            &test.db,
+            &CompletionSettings::default(),
+            test.cursor.file,
+            test.cursor.offset,
+        );
+        let completion = completions.iter().find(|c| c.name == "rec").unwrap();
+
+        assert_eq!(completion.kind(&test.db), Some(CompletionKind::Struct));
+    }
+
     // NOTE: The methods below are getting somewhat ridiculous.
     // We should refactor this by converting to using a builder
     // to set different modes. ---AG

crates/ty_python_semantic/src/semantic_model.rs

@@ -11,7 +11,7 @@ use crate::semantic_index::definition::Definition;
 use crate::semantic_index::scope::FileScopeId;
 use crate::semantic_index::semantic_index;
 use crate::types::ide_support::all_declarations_and_bindings;
-use crate::types::{Type, binding_type, infer_scope_types};
+use crate::types::{CycleDetector, Type, binding_type, infer_scope_types};
 
 pub struct SemanticModel<'db> {
     db: &'db dyn Db,
@@ -319,7 +319,11 @@ impl<'db> Completion<'db> {
     /// the client uses this information to help improve the UX (perhaps by
     /// assigning an icon of some kind to the completion).
     pub fn kind(&self, db: &'db dyn Db) -> Option<CompletionKind> {
-        fn imp<'db>(db: &'db dyn Db, ty: Type<'db>) -> Option<CompletionKind> {
+        fn imp<'db>(
+            db: &'db dyn Db,
+            ty: Type<'db>,
+            visitor: &CompletionKindVisitor<'db>,
+        ) -> Option<CompletionKind> {
             Some(match ty {
                 Type::FunctionLiteral(_)
                 | Type::DataclassDecorator(_)
@@ -346,23 +350,33 @@ impl<'db> Completion<'db> {
                 Type::EnumLiteral(_) => CompletionKind::Enum,
                 Type::ProtocolInstance(_) => CompletionKind::Interface,
                 Type::NonInferableTypeVar(_) | Type::TypeVar(_) => CompletionKind::TypeParameter,
-                Type::Union(union) => union.elements(db).iter().find_map(|&ty| imp(db, ty))?,
-                Type::Intersection(intersection) => {
-                    intersection.iter_positive(db).find_map(|ty| imp(db, ty))?
-                }
+                Type::Union(union) => union
+                    .elements(db)
+                    .iter()
+                    .find_map(|&ty| imp(db, ty, visitor))?,
+                Type::Intersection(intersection) => intersection
+                    .iter_positive(db)
+                    .find_map(|ty| imp(db, ty, visitor))?,
                 Type::Dynamic(_)
                 | Type::Never
                 | Type::SpecialForm(_)
                 | Type::KnownInstance(_)
                 | Type::AlwaysTruthy
                 | Type::AlwaysFalsy => return None,
-                Type::TypeAlias(alias) => imp(db, alias.value_type(db))?,
+                Type::TypeAlias(alias) => {
+                    visitor.visit(ty, || imp(db, alias.value_type(db), visitor))?
+                }
             })
         }
-        self.kind.or_else(|| self.ty.and_then(|ty| imp(db, ty)))
+        self.kind.or_else(|| {
+            self.ty
+                .and_then(|ty| imp(db, ty, &CompletionKindVisitor::default()))
+        })
     }
 }
 
+type CompletionKindVisitor<'db> = CycleDetector<CompletionKind, Type<'db>, Option<CompletionKind>>;
+
 /// The "kind" of a completion.
 ///
 /// This is taken directly from the LSP completion specification:
@@ -372,7 +386,7 @@ impl<'db> Completion<'db> {
 /// `Type` (and possibly other information), which might be interesting and
 /// contentious. Then the outer edges map this to the LSP types, which is
 /// expected to be mundane and boring.
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum CompletionKind {
     Text,
     Method,