[flake8-future-annotations] Add autofix (FA100) (#18903)

Summary
--

This PR resolves the easiest part of
https://github.com/astral-sh/ruff/issues/18502 by adding an autofix that
just adds
`from __future__ import annotations` at the top of the file, in the same
way
as FA102, which already has an identical unsafe fix.

Test Plan
--

Existing snapshots, updated to add the fixes.

crates/ruff_linter/src/rules/flake8_future_annotations/rules/future_rewritable_type_annotation.rs

@@ -1,9 +1,11 @@
+use ruff_diagnostics::Fix;
 use ruff_python_ast::Expr;
 
 use ruff_macros::{ViolationMetadata, derive_message_formats};
+use ruff_python_semantic::{MemberNameImport, NameImport};
 use ruff_text_size::Ranged;
 
-use crate::Violation;
+use crate::AlwaysFixableViolation;
 use crate::checkers::ast::Checker;
 
 /// ## What it does
@@ -61,6 +63,10 @@ use crate::checkers::ast::Checker;
 /// def func(obj: dict[str, int | None]) -> None: ...
 /// ```
 ///
+/// ## Fix safety
+/// This rule's fix is marked as unsafe, as adding `from __future__ import annotations`
+/// may change the semantics of the program.
+///
 /// ## Options
 /// - `target-version`
 #[derive(ViolationMetadata)]
@@ -68,12 +74,16 @@ pub(crate) struct FutureRewritableTypeAnnotation {
     name: String,
 }
 
-impl Violation for FutureRewritableTypeAnnotation {
+impl AlwaysFixableViolation for FutureRewritableTypeAnnotation {
     #[derive_message_formats]
     fn message(&self) -> String {
         let FutureRewritableTypeAnnotation { name } = self;
         format!("Add `from __future__ import annotations` to simplify `{name}`")
     }
+
+    fn fix_title(&self) -> String {
+        "Add `from __future__ import annotations`".to_string()
+    }
 }
 
 /// FA100
@@ -83,7 +93,17 @@ pub(crate) fn future_rewritable_type_annotation(checker: &Checker, expr: &Expr)
         .resolve_qualified_name(expr)
         .map(|binding| binding.to_string());
 
-    if let Some(name) = name {
-        checker.report_diagnostic(FutureRewritableTypeAnnotation { name }, expr.range());
-    }
+    let Some(name) = name else { return };
+
+    let import = &NameImport::ImportFrom(MemberNameImport::member(
+        "__future__".to_string(),
+        "annotations".to_string(),
+    ));
+    checker
+        .report_diagnostic(FutureRewritableTypeAnnotation { name }, expr.range())
+        .set_fix(Fix::unsafe_edit(
+            checker
+                .importer()
+                .add_import(import, ruff_text_size::TextSize::default()),
+        ));
 }

crates/ruff_linter/src/rules/flake8_future_annotations/snapshots/ruff_linter__rules__flake8_future_annotations__tests__edge_case.py.snap

@@ -1,19 +1,32 @@
 ---
 source: crates/ruff_linter/src/rules/flake8_future_annotations/mod.rs
-snapshot_kind: text
 ---
-edge_case.py:5:13: FA100 Add `from __future__ import annotations` to simplify `typing.List`
+edge_case.py:5:13: FA100 [*] Add `from __future__ import annotations` to simplify `typing.List`
   |
 5 | def main(_: List[int]) -> None:
   |             ^^^^ FA100
 6 |     a_list: t.List[str] = []
 7 |     a_list.append("hello")
   |
+  = help: Add `from __future__ import annotations`
 
-edge_case.py:6:13: FA100 Add `from __future__ import annotations` to simplify `typing.List`
+ℹ Unsafe fix
+  1 |+from __future__ import annotations
+1 2 | from typing import List
+2 3 | import typing as t
+3 4 | 
+
+edge_case.py:6:13: FA100 [*] Add `from __future__ import annotations` to simplify `typing.List`
   |
 5 | def main(_: List[int]) -> None:
 6 |     a_list: t.List[str] = []
   |             ^^^^^^ FA100
 7 |     a_list.append("hello")
   |
+  = help: Add `from __future__ import annotations`
+
+ℹ Unsafe fix
+  1 |+from __future__ import annotations
+1 2 | from typing import List
+2 3 | import typing as t
+3 4 |

crates/ruff_linter/src/rules/flake8_future_annotations/snapshots/ruff_linter__rules__flake8_future_annotations__tests__from_typing_import.py.snap

@@ -1,11 +1,17 @@
 ---
 source: crates/ruff_linter/src/rules/flake8_future_annotations/mod.rs
-snapshot_kind: text
 ---
-from_typing_import.py:5:13: FA100 Add `from __future__ import annotations` to simplify `typing.List`
+from_typing_import.py:5:13: FA100 [*] Add `from __future__ import annotations` to simplify `typing.List`
   |
 4 | def main() -> None:
 5 |     a_list: List[str] = []
   |             ^^^^ FA100
 6 |     a_list.append("hello")
   |
+  = help: Add `from __future__ import annotations`
+
+ℹ Unsafe fix
+  1 |+from __future__ import annotations
+1 2 | from typing import List
+2 3 | 
+3 4 |

crates/ruff_linter/src/rules/flake8_future_annotations/snapshots/ruff_linter__rules__flake8_future_annotations__tests__from_typing_import_many.py.snap

@@ -1,8 +1,7 @@
 ---
 source: crates/ruff_linter/src/rules/flake8_future_annotations/mod.rs
-snapshot_kind: text
 ---
-from_typing_import_many.py:5:13: FA100 Add `from __future__ import annotations` to simplify `typing.List`
+from_typing_import_many.py:5:13: FA100 [*] Add `from __future__ import annotations` to simplify `typing.List`
   |
 4 | def main() -> None:
 5 |     a_list: List[Optional[str]] = []
@@ -10,8 +9,15 @@ from_typing_import_many.py:5:13: FA100 Add `from __future__ import annotations`
 6 |     a_list.append("hello")
 7 |     a_dict = cast(Dict[int | None, Union[int, Set[bool]]], {})
   |
+  = help: Add `from __future__ import annotations`
 
-from_typing_import_many.py:5:18: FA100 Add `from __future__ import annotations` to simplify `typing.Optional`
+ℹ Unsafe fix
+  1 |+from __future__ import annotations
+1 2 | from typing import Dict, List, Optional, Set, Union, cast
+2 3 | 
+3 4 | 
+
+from_typing_import_many.py:5:18: FA100 [*] Add `from __future__ import annotations` to simplify `typing.Optional`
   |
 4 | def main() -> None:
 5 |     a_list: List[Optional[str]] = []
@@ -19,3 +25,10 @@ from_typing_import_many.py:5:18: FA100 Add `from __future__ import annotations`
 6 |     a_list.append("hello")
 7 |     a_dict = cast(Dict[int | None, Union[int, Set[bool]]], {})
   |
+  = help: Add `from __future__ import annotations`
+
+ℹ Unsafe fix
+  1 |+from __future__ import annotations
+1 2 | from typing import Dict, List, Optional, Set, Union, cast
+2 3 | 
+3 4 |

crates/ruff_linter/src/rules/flake8_future_annotations/snapshots/ruff_linter__rules__flake8_future_annotations__tests__import_typing.py.snap

@@ -1,11 +1,17 @@
 ---
 source: crates/ruff_linter/src/rules/flake8_future_annotations/mod.rs
-snapshot_kind: text
 ---
-import_typing.py:5:13: FA100 Add `from __future__ import annotations` to simplify `typing.List`
+import_typing.py:5:13: FA100 [*] Add `from __future__ import annotations` to simplify `typing.List`
   |
 4 | def main() -> None:
 5 |     a_list: typing.List[str] = []
   |             ^^^^^^^^^^^ FA100
 6 |     a_list.append("hello")
   |
+  = help: Add `from __future__ import annotations`
+
+ℹ Unsafe fix
+  1 |+from __future__ import annotations
+1 2 | import typing
+2 3 | 
+3 4 |

crates/ruff_linter/src/rules/flake8_future_annotations/snapshots/ruff_linter__rules__flake8_future_annotations__tests__import_typing_as.py.snap

@@ -1,11 +1,17 @@
 ---
 source: crates/ruff_linter/src/rules/flake8_future_annotations/mod.rs
-snapshot_kind: text
 ---
-import_typing_as.py:5:13: FA100 Add `from __future__ import annotations` to simplify `typing.List`
+import_typing_as.py:5:13: FA100 [*] Add `from __future__ import annotations` to simplify `typing.List`
   |
 4 | def main() -> None:
 5 |     a_list: t.List[str] = []
   |             ^^^^^^ FA100
 6 |     a_list.append("hello")
   |
+  = help: Add `from __future__ import annotations`
+
+ℹ Unsafe fix
+  1 |+from __future__ import annotations
+1 2 | import typing as t
+2 3 | 
+3 4 |