[ty] Infer list[T] when unpacking non-literal list

Author: dhruvmanila

crates/ty_python_semantic/resources/mdtest/unpacking.md

@@ -1,6 +1,6 @@
 # Unpacking
 
-If there are not enough or too many values ​​when unpacking, an error will occur and the types of
+If there are not enough or too many values when unpacking, an error will occur and the types of
 all variables (if nested tuple unpacking fails, only the variables within the failed tuples) is
 inferred to be `Unknown`.
 
@@ -207,6 +207,57 @@ reveal_type(c)  # revealed: int
 reveal_type(d)  # revealed: Literal[2]
 ```
 
+## List
+
+### Literal unpacking
+
+```py
+a, b = [1, 2]
+# TODO: should be `int` for both `a` and `b`
+reveal_type(a)  # revealed: Unknown
+reveal_type(b)  # revealed: Unknown
+```
+
+### Simple unpacking
+
+```py
+def _(value: list[int]):
+    a, b = value
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: int
+```
+
+### Nested unpacking
+
+```py
+def _(value: list[list[int]]):
+    a, (b, c) = value
+    reveal_type(a)  # revealed: list[int]
+    reveal_type(b)  # revealed: int
+    reveal_type(c)  # revealed: int
+```
+
+### Invalid nested unpacking
+
+```py
+def _(value: list[int]):
+    # error: [not-iterable] "Object of type `int` is not iterable"
+    a, (b, c) = value
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: Unknown
+    reveal_type(c)  # revealed: Unknown
+```
+
+### Starred expression
+
+```py
+def _(value: list[int]):
+    a, *b, c = value
+    reveal_type(a)  # revealed: int
+    reveal_type(b)  # revealed: list[int]
+    reveal_type(c)  # revealed: int
+```
+
 ## String
 
 ### Simple unpacking
@@ -293,6 +344,18 @@ reveal_type(b)  # revealed: LiteralString
 reveal_type(c)  # revealed: list[LiteralString]
 ```
 
+### Starred expression (6)
+
+```py
+from typing_extensions import LiteralString
+
+def _(s: LiteralString):
+    a, b, *c = s
+    reveal_type(a)  # revealed: LiteralString
+    reveal_type(b)  # revealed: LiteralString
+    reveal_type(c)  # revealed: list[LiteralString]
+```
+
 ### Unicode
 
 ```py

crates/ty_python_semantic/src/types/unpacker.rs

@@ -192,8 +192,15 @@ impl<'db> Unpacker<'db> {
                                 err.fallback_element_type(self.db())
                             })
                         };
-                        for target_type in &mut target_types {
-                            target_type.push(ty);
+                        // Both `elts` and `target_types` are guaranteed to have the same length.
+                        for (element, target_type) in elts.iter().zip(&mut target_types) {
+                            if element.is_starred_expr() {
+                                target_type.push(
+                                    KnownClass::List.to_specialized_instance(self.db(), [ty]),
+                                );
+                            } else {
+                                target_type.push(ty);
+                            }
                         }
                     }
                 }