Infer list[T] for starred target in unpacking (#18401)

## Summary

Closes: astral-sh/ty#191

## Test Plan

Update existing tests.

crates/ty_python_semantic/resources/mdtest/attributes.md

@@ -302,7 +302,7 @@ class C:
 
 c_instance = C()
 reveal_type(c_instance.a)  # revealed: Unknown | Literal[1]
-reveal_type(c_instance.b)  # revealed: Unknown
+reveal_type(c_instance.b)  # revealed: Unknown | list[Literal[2, 3]]
 ```
 
 #### Attributes defined in for-loop (unpacking)

crates/ty_python_semantic/resources/mdtest/unpacking.md

@@ -109,8 +109,7 @@ reveal_type(d)  # revealed: Literal[5]
 # error: [invalid-assignment] "Not enough values to unpack: Expected 3 or more"
 [a, *b, c, d] = (1, 2)
 reveal_type(a)  # revealed: Unknown
-# TODO: Should be list[Any] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: Unknown
+reveal_type(b)  # revealed: list[Unknown]
 reveal_type(c)  # revealed: Unknown
 reveal_type(d)  # revealed: Unknown
 ```
@@ -120,8 +119,7 @@ reveal_type(d)  # revealed: Unknown
 ```py
 [a, *b, c] = (1, 2)
 reveal_type(a)  # revealed: Literal[1]
-# TODO: Should be list[Any] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo(starred unpacking)
+reveal_type(b)  # revealed: list[Unknown]
 reveal_type(c)  # revealed: Literal[2]
 ```
 
@@ -130,8 +128,7 @@ reveal_type(c)  # revealed: Literal[2]
 ```py
 [a, *b, c] = (1, 2, 3)
 reveal_type(a)  # revealed: Literal[1]
-# TODO: Should be list[int] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo(starred unpacking)
+reveal_type(b)  # revealed: list[Literal[2]]
 reveal_type(c)  # revealed: Literal[3]
 ```
 
@@ -140,8 +137,7 @@ reveal_type(c)  # revealed: Literal[3]
 ```py
 [a, *b, c, d] = (1, 2, 3, 4, 5, 6)
 reveal_type(a)  # revealed: Literal[1]
-# TODO: Should be list[int] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo(starred unpacking)
+reveal_type(b)  # revealed: list[Literal[2, 3, 4]]
 reveal_type(c)  # revealed: Literal[5]
 reveal_type(d)  # revealed: Literal[6]
 ```
@@ -152,8 +148,7 @@ reveal_type(d)  # revealed: Literal[6]
 [a, b, *c] = (1, 2, 3, 4)
 reveal_type(a)  # revealed: Literal[1]
 reveal_type(b)  # revealed: Literal[2]
-# TODO: Should be list[int] once support for assigning to starred expression is added
-reveal_type(c)  # revealed: @Todo(starred unpacking)
+reveal_type(c)  # revealed: list[Literal[3, 4]]
 ```
 
 ### Starred expression (6)
@@ -164,7 +159,7 @@ reveal_type(c)  # revealed: @Todo(starred unpacking)
 reveal_type(a)  # revealed: Unknown
 reveal_type(b)  # revealed: Unknown
 reveal_type(c)  # revealed: Unknown
-reveal_type(d)  # revealed: Unknown
+reveal_type(d)  # revealed: list[Unknown]
 reveal_type(e)  # revealed: Unknown
 reveal_type(f)  # revealed: Unknown
 ```
@@ -247,8 +242,7 @@ reveal_type(b)  # revealed: Unknown
 # error: [invalid-assignment] "Not enough values to unpack: Expected 3 or more"
 (a, *b, c, d) = "ab"
 reveal_type(a)  # revealed: Unknown
-# TODO: Should be list[LiteralString] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: Unknown
+reveal_type(b)  # revealed: list[Unknown]
 reveal_type(c)  # revealed: Unknown
 reveal_type(d)  # revealed: Unknown
 ```
@@ -258,7 +252,7 @@ reveal_type(d)  # revealed: Unknown
 (a, b, *c, d) = "a"
 reveal_type(a)  # revealed: Unknown
 reveal_type(b)  # revealed: Unknown
-reveal_type(c)  # revealed: Unknown
+reveal_type(c)  # revealed: list[Unknown]
 reveal_type(d)  # revealed: Unknown
 ```
 
@@ -267,8 +261,7 @@ reveal_type(d)  # revealed: Unknown
 ```py
 (a, *b, c) = "ab"
 reveal_type(a)  # revealed: LiteralString
-# TODO: Should be list[Any] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo(starred unpacking)
+reveal_type(b)  # revealed: list[Unknown]
 reveal_type(c)  # revealed: LiteralString
 ```
 
@@ -277,8 +270,7 @@ reveal_type(c)  # revealed: LiteralString
 ```py
 (a, *b, c) = "abc"
 reveal_type(a)  # revealed: LiteralString
-# TODO: Should be list[LiteralString] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo(starred unpacking)
+reveal_type(b)  # revealed: list[LiteralString]
 reveal_type(c)  # revealed: LiteralString
 ```
 
@@ -287,8 +279,7 @@ reveal_type(c)  # revealed: LiteralString
 ```py
 (a, *b, c, d) = "abcdef"
 reveal_type(a)  # revealed: LiteralString
-# TODO: Should be list[LiteralString] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo(starred unpacking)
+reveal_type(b)  # revealed: list[LiteralString]
 reveal_type(c)  # revealed: LiteralString
 reveal_type(d)  # revealed: LiteralString
 ```
@@ -299,8 +290,7 @@ reveal_type(d)  # revealed: LiteralString
 (a, b, *c) = "abcd"
 reveal_type(a)  # revealed: LiteralString
 reveal_type(b)  # revealed: LiteralString
-# TODO: Should be list[int] once support for assigning to starred expression is added
-reveal_type(c)  # revealed: @Todo(starred unpacking)
+reveal_type(c)  # revealed: list[LiteralString]
 ```
 
 ### Unicode
@@ -411,8 +401,7 @@ def _(arg: tuple[int, tuple[str, bytes]] | tuple[tuple[int, bytes], Literal["ab"
 def _(arg: tuple[int, bytes, int] | tuple[int, int, str, int, bytes]):
     a, *b, c = arg
     reveal_type(a)  # revealed: int
-    # TODO: Should be `list[bytes | int | str]`
-    reveal_type(b)  # revealed: @Todo(starred unpacking)
+    reveal_type(b)  # revealed: list[bytes] | list[int | str]
     reveal_type(c)  # revealed: int | bytes
 ```
 
@@ -676,8 +665,7 @@ class ContextManager:
 
 with ContextManager() as (a, *b):
     reveal_type(a)  # revealed: int
-    # TODO: Should be list[int] once support for assigning to starred expression is added
-    reveal_type(b)  # revealed: @Todo(starred unpacking)
+    reveal_type(b)  # revealed: list[int]
 ```
 
 ### Unbound context manager expression

crates/ty_python_semantic/src/types/unpacker.rs

@@ -8,12 +8,12 @@ use ruff_python_ast::{self as ast, AnyNodeRef};
 use crate::Db;
 use crate::semantic_index::ast_ids::{HasScopedExpressionId, ScopedExpressionId};
 use crate::semantic_index::symbol::ScopeId;
-use crate::types::{Type, TypeCheckDiagnostics, infer_expression_types, todo_type};
+use crate::types::{Type, TypeCheckDiagnostics, infer_expression_types};
 use crate::unpack::{UnpackKind, UnpackValue};
 
 use super::context::InferContext;
 use super::diagnostic::INVALID_ASSIGNMENT;
-use super::{TupleType, UnionType};
+use super::{KnownClass, TupleType, UnionType};
 
 /// Unpacks the value expression type to their respective targets.
 pub(crate) struct Unpacker<'db> {
@@ -253,12 +253,17 @@ impl<'db> Unpacker<'db> {
             let starred_end_index = tuple_ty.len(self.db()) - remaining;
 
             // SAFETY: Safe because of the length check above.
-            let _starred_element_types =
+            let starred_element_types =
                 &tuple_ty.elements(self.db())[starred_index..starred_end_index];
-            // TODO: Combine the types into a list type. If the
-            // starred_element_types is empty, then it should be `List[Any]`.
-            // combine_types(starred_element_types);
-            element_types.push(todo_type!("starred unpacking"));
+
+            element_types.push(KnownClass::List.to_specialized_instance(
+                self.db(),
+                [if starred_element_types.is_empty() {
+                    Type::unknown()
+                } else {
+                    UnionType::from_elements(self.db(), starred_element_types)
+                }],
+            ));
 
             // Insert the types remaining that aren't consumed by the starred expression.
             element_types.extend_from_slice(
@@ -278,7 +283,18 @@ impl<'db> Unpacker<'db> {
                 );
             }
 
-            Cow::Owned(vec![Type::unknown(); targets.len()])
+            Cow::Owned(
+                targets
+                    .iter()
+                    .map(|target| {
+                        if target.is_starred_expr() {
+                            KnownClass::List.to_specialized_instance(self.db(), [Type::unknown()])
+                        } else {
+                            Type::unknown()
+                        }
+                    })
+                    .collect(),
+            )
         }
     }