Avoid marking `InitVar` as a typing-only annotation (#9688)

## Summary

Given:

```python
from dataclasses import InitVar, dataclass

@dataclass
class C:
    i: int
    j: int = None
    database: InitVar[DatabaseType] = None

    def __post_init__(self, database):
        if self.j is None and database is not None:
            self.j = database.lookup('j')

c = C(10, database=my_database)
```

We should avoid marking `InitVar` as typing-only, since it _is_ required
by the dataclass at runtime.

Note that by default, we _already_ don't flag this, since the
`@dataclass` member is needed at runtime too -- so it's only a problem
with `strict` mode.

Closes https://github.com/astral-sh/ruff/issues/9666.

crates/ruff_linter/resources/test/fixtures/flake8_type_checking/init_var.py

@@ -0,0 +1,18 @@
+"""Test: avoid marking an `InitVar` as typing-only."""
+
+from __future__ import annotations
+
+from dataclasses import FrozenInstanceError, InitVar, dataclass
+from pathlib import Path
+
+
+@dataclass
+class C:
+    i: int
+    j: int = None
+    database: InitVar[Path] = None
+
+    err: FrozenInstanceError = None
+
+    def __post_init__(self, database):
+        ...

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

@@ -721,6 +721,21 @@ where
                     AnnotationContext::RuntimeEvaluated => {
                         self.visit_runtime_evaluated_annotation(annotation);
                     }
+                    AnnotationContext::TypingOnly
+                        if flake8_type_checking::helpers::is_dataclass_meta_annotation(
+                            annotation,
+                            self.semantic(),
+                        ) =>
+                    {
+                        if let Expr::Subscript(subscript) = &**annotation {
+                            // Ex) `InitVar[str]`
+                            self.visit_runtime_required_annotation(&subscript.value);
+                            self.visit_annotation(&subscript.slice);
+                        } else {
+                            // Ex) `InitVar`
+                            self.visit_runtime_required_annotation(annotation);
+                        }
+                    }
                     AnnotationContext::TypingOnly => self.visit_annotation(annotation),
                 }
 

crates/ruff_linter/src/rules/flake8_type_checking/helpers.rs

@@ -2,11 +2,11 @@ use anyhow::Result;
 
 use ruff_diagnostics::Edit;
 use ruff_python_ast::call_path::from_qualified_name;
-use ruff_python_ast::helpers::map_callable;
+use ruff_python_ast::helpers::{map_callable, map_subscript};
 use ruff_python_ast::{self as ast, Decorator, Expr};
 use ruff_python_codegen::{Generator, Stylist};
 use ruff_python_semantic::{
-    analyze, Binding, BindingKind, NodeId, ResolvedReference, SemanticModel,
+    analyze, Binding, BindingKind, NodeId, ResolvedReference, ScopeKind, SemanticModel,
 };
 use ruff_source_file::Locator;
 use ruff_text_size::Ranged;
@@ -104,6 +104,35 @@ fn runtime_required_decorators(
     })
 }
 
+/// Returns `true` if an annotation will be inspected at runtime by the `dataclasses` module.
+///
+/// Specifically, detects whether an annotation is to either `dataclasses.InitVar` or
+/// `typing.ClassVar` within a `@dataclass` class definition.
+///
+/// See: <https://docs.python.org/3/library/dataclasses.html#init-only-variables>
+pub(crate) fn is_dataclass_meta_annotation(annotation: &Expr, semantic: &SemanticModel) -> bool {
+    // Determine whether the assignment is in a `@dataclass` class definition.
+    if let ScopeKind::Class(class_def) = semantic.current_scope().kind {
+        if class_def.decorator_list.iter().any(|decorator| {
+            semantic
+                .resolve_call_path(map_callable(&decorator.expression))
+                .is_some_and(|call_path| {
+                    matches!(call_path.as_slice(), ["dataclasses", "dataclass"])
+                })
+        }) {
+            // Determine whether the annotation is `typing.ClassVar` or `dataclasses.InitVar`.
+            return semantic
+                .resolve_call_path(map_subscript(annotation))
+                .is_some_and(|call_path| {
+                    matches!(call_path.as_slice(), ["dataclasses", "InitVar"])
+                        || semantic.match_typing_call_path(&call_path, "ClassVar")
+                });
+        }
+    }
+
+    false
+}
+
 /// Returns `true` if a function is registered as a `singledispatch` interface.
 ///
 /// For example, `fun` below is a `singledispatch` interface:

crates/ruff_linter/src/rules/flake8_type_checking/mod.rs

@@ -39,6 +39,7 @@ mod tests {
     #[test_case(Rule::RuntimeStringUnion, Path::new("TCH006_2.py"))]
     #[test_case(Rule::TypingOnlyFirstPartyImport, Path::new("TCH001.py"))]
     #[test_case(Rule::TypingOnlyStandardLibraryImport, Path::new("TCH003.py"))]
+    #[test_case(Rule::TypingOnlyStandardLibraryImport, Path::new("init_var.py"))]
     #[test_case(Rule::TypingOnlyStandardLibraryImport, Path::new("snapshot.py"))]
     #[test_case(Rule::TypingOnlyThirdPartyImport, Path::new("TCH002.py"))]
     #[test_case(Rule::TypingOnlyThirdPartyImport, Path::new("quote.py"))]
@@ -75,7 +76,9 @@ mod tests {
     }
 
     #[test_case(Rule::TypingOnlyThirdPartyImport, Path::new("strict.py"))]
+    #[test_case(Rule::TypingOnlyStandardLibraryImport, Path::new("init_var.py"))]
     fn strict(rule_code: Rule, path: &Path) -> Result<()> {
+        let snapshot = format!("strict_{}_{}", rule_code.as_ref(), path.to_string_lossy());
         let diagnostics = test_path(
             Path::new("flake8_type_checking").join(path).as_path(),
             &settings::LinterSettings {
@@ -86,7 +89,7 @@ mod tests {
                 ..settings::LinterSettings::for_rule(rule_code)
             },
         )?;
-        assert_messages!(diagnostics);
+        assert_messages!(snapshot, diagnostics);
         Ok(())
     }
 

crates/ruff_linter/src/rules/flake8_type_checking/snapshots/ruff_linter__rules__flake8_type_checking__tests__strict_typing-only-standard-library-import_init_var.py.snap

@@ -0,0 +1,50 @@
+---
+source: crates/ruff_linter/src/rules/flake8_type_checking/mod.rs
+---
+init_var.py:5:25: TCH003 [*] Move standard library import `dataclasses.FrozenInstanceError` into a type-checking block
+  |
+3 | from __future__ import annotations
+4 | 
+5 | from dataclasses import FrozenInstanceError, InitVar, dataclass
+  |                         ^^^^^^^^^^^^^^^^^^^ TCH003
+6 | from pathlib import Path
+  |
+  = help: Move into type-checking block
+
+ℹ Unsafe fix
+2  2  | 
+3  3  | from __future__ import annotations
+4  4  | 
+5     |-from dataclasses import FrozenInstanceError, InitVar, dataclass
+   5  |+from dataclasses import InitVar, dataclass
+6  6  | from pathlib import Path
+   7  |+from typing import TYPE_CHECKING
+   8  |+
+   9  |+if TYPE_CHECKING:
+   10 |+    from dataclasses import FrozenInstanceError
+7  11 | 
+8  12 | 
+9  13 | @dataclass
+
+init_var.py:6:21: TCH003 [*] Move standard library import `pathlib.Path` into a type-checking block
+  |
+5 | from dataclasses import FrozenInstanceError, InitVar, dataclass
+6 | from pathlib import Path
+  |                     ^^^^ TCH003
+  |
+  = help: Move into type-checking block
+
+ℹ Unsafe fix
+3  3  | from __future__ import annotations
+4  4  | 
+5  5  | from dataclasses import FrozenInstanceError, InitVar, dataclass
+6     |-from pathlib import Path
+   6  |+from typing import TYPE_CHECKING
+   7  |+
+   8  |+if TYPE_CHECKING:
+   9  |+    from pathlib import Path
+7  10 | 
+8  11 | 
+9  12 | @dataclass
+
+

crates/ruff_linter/src/rules/flake8_type_checking/snapshots/ruff_linter__rules__flake8_type_checking__tests__typing-only-standard-library-import_init_var.py.snap

@@ -0,0 +1,25 @@
+---
+source: crates/ruff_linter/src/rules/flake8_type_checking/mod.rs
+---
+init_var.py:6:21: TCH003 [*] Move standard library import `pathlib.Path` into a type-checking block
+  |
+5 | from dataclasses import FrozenInstanceError, InitVar, dataclass
+6 | from pathlib import Path
+  |                     ^^^^ TCH003
+  |
+  = help: Move into type-checking block
+
+ℹ Unsafe fix
+3  3  | from __future__ import annotations
+4  4  | 
+5  5  | from dataclasses import FrozenInstanceError, InitVar, dataclass
+6     |-from pathlib import Path
+   6  |+from typing import TYPE_CHECKING
+   7  |+
+   8  |+if TYPE_CHECKING:
+   9  |+    from pathlib import Path
+7  10 | 
+8  11 | 
+9  12 | @dataclass
+
+