add unannotated-attribute error kind

Summary: closes #1779

Reviewed By: stroxler

Differential Revision: D88562236

fbshipit-source-id: b6a89b1a615cf7e81ff5fc3daf5799c16be1f195

Author: yangdanny97

crates/pyrefly_config/src/error_kind.rs

@@ -232,6 +232,8 @@ pub enum ErrorKind {
     RedundantCondition,
     /// Raised by a call to reveal_type().
     RevealType,
+    /// An attribute is missing a type annotation and is initialized with the `None` literal.
+    UnannotatedAttribute,
     /// A function parameter is missing a type annotation.
     UnannotatedParameter,
     /// A function is missing a return type annotation.
@@ -304,6 +306,7 @@ impl ErrorKind {
             ErrorKind::ImplicitAny => Severity::Ignore,
             ErrorKind::UnannotatedParameter => Severity::Ignore,
             ErrorKind::UnannotatedReturn => Severity::Ignore,
+            ErrorKind::UnannotatedAttribute => Severity::Ignore,
             ErrorKind::MissingSource => Severity::Ignore,
             ErrorKind::OpenUnpacking => Severity::Ignore,
             _ => Severity::Error,

pyrefly/lib/alt/class/class_field.rs

@@ -1737,7 +1737,7 @@ impl<'a, Ans: LookupAnswer> AnswersSolver<'a, Ans> {
             }
             // We interpret `self.foo = None` to mean the type of foo is None or some unknown type.
             (None, Expr::NoneLiteral(_)) => {
-                self.error(errors, x.range(), ErrorInfo::Kind(ErrorKind::ImplicitAny), "This expression is implicitly inferred to be `Any | None`. Please provide an explicit type annotation.".to_owned());
+                self.error(errors, x.range(), ErrorInfo::Kind(ErrorKind::UnannotatedAttribute), "This expression is implicitly inferred to be `Any | None`. Please provide an explicit type annotation.".to_owned());
                 self.union(Type::None, Type::any_implicit())
             }
             (None, _) => self.expr_infer(x, errors),

pyrefly/lib/test/inference.rs

@@ -97,7 +97,7 @@ x2 = {}
 
 testcase!(
     test_warn_on_implicit_any_in_attribute,
-    TestEnv::new().enable_implicit_any_error(),
+    TestEnv::new().enable_unannotated_attribute_error(),
     r#"
 from typing import Any
 class A:

pyrefly/lib/test/util.rs

@@ -108,6 +108,7 @@ pub struct TestEnv {
     implicit_any_error: bool,
     unannotated_return_error: bool,
     unannotated_parameter_error: bool,
+    unannotated_attribute_error: bool,
     implicit_abstract_class_error: bool,
     open_unpacking_error: bool,
     default_require_level: Require,
@@ -127,6 +128,7 @@ impl TestEnv {
             implicit_any_error: false,
             unannotated_return_error: false,
             unannotated_parameter_error: false,
+            unannotated_attribute_error: false,
             implicit_abstract_class_error: false,
             open_unpacking_error: false,
             default_require_level: Require::Exports,
@@ -167,6 +169,11 @@ impl TestEnv {
         self
     }
 
+    pub fn enable_unannotated_attribute_error(mut self) -> Self {
+        self.unannotated_attribute_error = true;
+        self
+    }
+
     pub fn enable_unannotated_return_error(mut self) -> Self {
         self.unannotated_return_error = true;
         self
@@ -268,6 +275,9 @@ impl TestEnv {
         if self.implicit_any_error {
             errors.set_error_severity(ErrorKind::ImplicitAny, Severity::Error);
         }
+        if self.unannotated_attribute_error {
+            errors.set_error_severity(ErrorKind::UnannotatedAttribute, Severity::Error);
+        }
         if self.unannotated_return_error {
             errors.set_error_severity(ErrorKind::UnannotatedReturn, Severity::Error);
         }

website/docs/error-kinds.mdx

@@ -975,6 +975,22 @@ if "abc":
 
 Pyrefly uses this error to communicate the output of the [`reveal_type`](https://typing.python.org/en/latest/spec/directives.html#reveal-type) function.
 
+## unannotated-attribute
+
+This error is raised when a class attribute is missing a type annotation and is initialized with the `None` literal. Without an explicit annotation, Pyrefly infers the type as `Any | None`, which reduces type safety.
+
+This error is off by default. To fix it, add an explicit type annotation to the attribute.
+
+```python
+class MyClass:
+    # error: This expression is implicitly inferred to be `Any | None`. Please provide an explicit type annotation.
+    value = None
+
+# Fixed version:
+class MyClass:
+    value: int | None = None
+```
+
 ## unannotated-parameter
 
 This error is raised when a function parameter is missing a type annotation. This helps enforce fully-typed codebases by ensuring all parameters have explicit types.