[pyflakes] Avoid false positives in @no_type_check contexts (F821, F722)

crates/ruff_linter/resources/test/fixtures/pyflakes/F722_1.py

@@ -0,0 +1,18 @@
+"""Regression test for #13824.
+
+Don't report an error when the function being annotated has the
+`@no_type_check` decorator.
+"""
+
+from typing import no_type_check
+
+
+@no_type_check
+def f(arg: "this isn't python") -> "this isn't python either":
+    x: "this also isn't python" = 0
+
+
+@no_type_check
+class C:
+    def f(arg: "this isn't python") -> "this isn't python either":
+        x: "this also isn't python" = 1

crates/ruff_linter/resources/test/fixtures/pyflakes/F821_30.py

@@ -0,0 +1,18 @@
+"""Regression test for #13824.
+
+Don't report an error when the function being annotated has the
+`@no_type_check` decorator.
+"""
+
+from typing import no_type_check
+
+
+@no_type_check
+def f(arg: "A") -> "R":
+    x: "A" = 1
+
+
+@no_type_check
+class C:
+    def f(self, arg: "B") -> "S":
+        x: "B" = 1

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -1060,6 +1060,17 @@ impl<'a> Visitor<'a> for Checker<'a> {
         self.semantic.flags = flags_snapshot;
     }
 
+    fn visit_decorator(&mut self, decorator: &'a ruff_python_ast::Decorator) {
+        if decorator
+            .expression
+            .as_name_expr()
+            .is_some_and(|name| name.id.as_str() == "no_type_check")
+        {
+            self.semantic.flags |= SemanticModelFlags::NO_TYPE_CHECK;
+        }
+        visitor::walk_decorator(self, decorator);
+    }
+
     fn visit_expr(&mut self, expr: &'a Expr) {
         // Step 0: Pre-processing
         if self.source_type.is_stub()
@@ -1851,6 +1862,9 @@ impl<'a> Checker<'a> {
 
     /// Visit an [`Expr`], and treat it as a type definition.
     fn visit_type_definition(&mut self, expr: &'a Expr) {
+        if self.semantic.in_no_type_check() {
+            return;
+        }
         let snapshot = self.semantic.flags;
         self.semantic.flags |= SemanticModelFlags::TYPE_DEFINITION;
         self.visit_expr(expr);

crates/ruff_linter/src/rules/pyflakes/mod.rs

@@ -94,7 +94,8 @@ mod tests {
     #[test_case(Rule::YieldOutsideFunction, Path::new("F704.py"))]
     #[test_case(Rule::ReturnOutsideFunction, Path::new("F706.py"))]
     #[test_case(Rule::DefaultExceptNotLast, Path::new("F707.py"))]
-    #[test_case(Rule::ForwardAnnotationSyntaxError, Path::new("F722.py"))]
+    #[test_case(Rule::ForwardAnnotationSyntaxError, Path::new("F722_0.py"))]
+    #[test_case(Rule::ForwardAnnotationSyntaxError, Path::new("F722_1.py"))]
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_0.py"))]
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_1.py"))]
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_2.py"))]
@@ -159,6 +160,7 @@ mod tests {
     #[test_case(Rule::UndefinedName, Path::new("F821_26.pyi"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_27.py"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_28.py"))]
+    #[test_case(Rule::UndefinedName, Path::new("F821_30.py"))]
     #[test_case(Rule::UndefinedExport, Path::new("F822_0.py"))]
     #[test_case(Rule::UndefinedExport, Path::new("F822_0.pyi"))]
     #[test_case(Rule::UndefinedExport, Path::new("F822_1.py"))]

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F722_F722.py.snap → crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F722_F722_0.py.snap

@@ -2,14 +2,14 @@
 source: crates/ruff_linter/src/rules/pyflakes/mod.rs
 snapshot_kind: text
 ---
-F722.py:9:12: F722 Syntax error in forward annotation: `///`
+F722_0.py:9:12: F722 Syntax error in forward annotation: `///`
    |
  9 | def g() -> "///":
    |            ^^^^^ F722
 10 |     pass
    |
 
-F722.py:13:4: F722 Syntax error in forward annotation: `List[int]☃`
+F722_0.py:13:4: F722 Syntax error in forward annotation: `List[int]☃`
    |
 13 | X: """List[int]"""'☃' = []
    |    ^^^^^^^^^^^^^^^^^^ F722

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F722_F722_1.py.snap

@@ -0,0 +1,5 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+snapshot_kind: text
+---
+

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F821_F821_30.py.snap

@@ -0,0 +1,5 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+snapshot_kind: text
+---
+

crates/ruff_python_semantic/src/model.rs

@@ -1584,6 +1584,11 @@ impl<'a> SemanticModel<'a> {
         self.flags.intersects(SemanticModelFlags::ANNOTATION)
     }
 
+    /// Return `true` if the model is in a `@no_type_check` context.
+    pub const fn in_no_type_check(&self) -> bool {
+        self.flags.intersects(SemanticModelFlags::NO_TYPE_CHECK)
+    }
+
     /// Return `true` if the model is in a typing-only type annotation.
     pub const fn in_typing_only_annotation(&self) -> bool {
         self.flags
@@ -2222,6 +2227,23 @@ bitflags! {
         /// [PEP 257]: https://peps.python.org/pep-0257/#what-is-a-docstring
         const ATTRIBUTE_DOCSTRING = 1 << 25;
 
+        /// The model is in a [no_type_check] context.
+        ///
+        /// This is used to skip type checking when the `@no_type_check` decorator is found.
+        ///
+        /// For example (adapted from [#13824]):
+        /// ```python
+        /// from typing import no_type_check
+        ///
+        /// @no_type_check
+        /// def fn(arg: "A") -> "R":
+        ///     pass
+        /// ```
+        ///
+        /// [no_type_check]: https://docs.python.org/3/library/typing.html#typing.no_type_check
+        /// [#13824]: https://github.com/astral-sh/ruff/issues/13824
+        const NO_TYPE_CHECK = 1 << 26;
+
         /// The context is in any type annotation.
         const ANNOTATION = Self::TYPING_ONLY_ANNOTATION.bits() | Self::RUNTIME_EVALUATED_ANNOTATION.bits() | Self::RUNTIME_REQUIRED_ANNOTATION.bits();