Enable Final instance attributes for attrs (#14232)

A quick patch to enable the following scenario:

```python
@define
class C:
    a: Final[int]  # `a` is a final instance attribute
```

There are some edge cases I haven't covered here that would be complex
to handle and not add much value IMO, so I think this will be useful
like this.

Author: Tinche

mypy/plugins/attrs.py

@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 from typing import Iterable, List, cast
-from typing_extensions import Final
+from typing_extensions import Final, Literal
 
 import mypy.plugin  # To avoid circular imports.
 from mypy.exprtotype import TypeTranslationError, expr_to_unanalyzed_type
@@ -756,20 +756,28 @@ def _add_init(
     ctx: mypy.plugin.ClassDefContext,
     attributes: list[Attribute],
     adder: MethodAdder,
-    method_name: str,
+    method_name: Literal["__init__", "__attrs_init__"],
 ) -> None:
     """Generate an __init__ method for the attributes and add it to the class."""
-    # Convert attributes to arguments with kw_only arguments at the  end of
+    # Convert attributes to arguments with kw_only arguments at the end of
     # the argument list
     pos_args = []
     kw_only_args = []
+    sym_table = ctx.cls.info.names
     for attribute in attributes:
         if not attribute.init:
             continue
         if attribute.kw_only:
             kw_only_args.append(attribute.argument(ctx))
         else:
             pos_args.append(attribute.argument(ctx))
+
+        # If the attribute is Final, present in `__init__` and has
+        # no default, make sure it doesn't error later.
+        if not attribute.has_default and attribute.name in sym_table:
+            sym_node = sym_table[attribute.name].node
+            if isinstance(sym_node, Var) and sym_node.is_final:
+                sym_node.final_set_in_init = True
     args = pos_args + kw_only_args
     if all(
         # We use getattr rather than instance checks because the variable.type

test-data/unit/check-attr.test

@@ -1834,3 +1834,36 @@ class Sub(Base):
 # This matches runtime semantics
 reveal_type(Sub)  # N: Revealed type is "def (*, name: builtins.str, first_name: builtins.str, last_name: builtins.str) -> __main__.Sub"
 [builtins fixtures/property.pyi]
+
+[case testFinalInstanceAttribute]
+from attrs import define
+from typing import Final
+
+@define
+class C:
+    a: Final[int]
+
+reveal_type(C) # N: Revealed type is "def (a: builtins.int) -> __main__.C"
+
+C(1).a = 2 # E: Cannot assign to final attribute "a"
+
+[builtins fixtures/property.pyi]
+
+[case testFinalInstanceAttributeInheritance]
+from attrs import define
+from typing import Final
+
+@define
+class C:
+    a: Final[int]
+
+@define
+class D(C):
+    b: Final[str]
+
+reveal_type(D) # N: Revealed type is "def (a: builtins.int, b: builtins.str) -> __main__.D"
+
+D(1, "").a = 2 # E: Cannot assign to final attribute "a"
+D(1, "").b = "2" # E: Cannot assign to final attribute "b"
+
+[builtins fixtures/property.pyi]