Add test for using is_lvalue from AttributeContext

Author: brianschubert

test-data/unit/check-custom-plugin.test

@@ -232,6 +232,8 @@ reveal_type(magic.non_magic_method())  # N: Revealed type is "builtins.int"
 reveal_type(magic.non_magic_field)  # N: Revealed type is "builtins.int"
 magic.nonexistent_field  # E: Field does not exist
 reveal_type(magic.fallback_example)  # N: Revealed type is "Any"
+reveal_type(magic.no_assignment_field)  # N: Revealed type is "builtins.float"
+magic.no_assignment_field = "bad"  # E: Cannot assign to field
 
 derived = DerivedMagic()
 reveal_type(derived.magic_field)  # N: Revealed type is "builtins.str"
@@ -250,6 +252,7 @@ class Magic:
     def __getattr__(self, x: Any) -> Any: ...
     def non_magic_method(self) -> int: ...
     non_magic_field: int
+    no_assignment_field: float
 
 class DerivedMagic(Magic): ...
 [file mypy.ini]

test-data/unit/plugins/attrhook2.py

@@ -12,6 +12,8 @@ def get_attribute_hook(self, fullname: str) -> Callable[[AttributeContext], Type
             return magic_field_callback
         if fullname == "m.Magic.nonexistent_field":
             return nonexistent_field_callback
+        if fullname == "m.Magic.no_assignment_field":
+            return no_assignment_field
         return None
 
 
@@ -24,5 +26,12 @@ def nonexistent_field_callback(ctx: AttributeContext) -> Type:
     return AnyType(TypeOfAny.from_error)
 
 
+def no_assignment_field(ctx: AttributeContext) -> Type:
+    if ctx.is_lvalue:
+        ctx.api.fail(f"Cannot assign to field", ctx.context)
+        return AnyType(TypeOfAny.from_error)
+    return ctx.default_attr_type
+
+
 def plugin(version: str) -> type[AttrPlugin]:
     return AttrPlugin