Treat form feed as valid whitespace before a semicolon (#19343)

fixes #19310

crates/ruff_linter/resources/test/fixtures/isort/required_imports/whitespace.py

@@ -0,0 +1,5 @@
+# This is a regression test for https://github.com/astral-sh/ruff/issues/19310
+# there is a (potentially invisible) unicode formfeed character (000C) between "docstring" and the semicolon
+"docstring"; print(
+    f"{__doc__=}",
+)

crates/ruff_linter/src/importer/insertion.rs

@@ -288,7 +288,7 @@ fn match_docstring_end(body: &[Stmt]) -> Option<TextSize> {
 fn match_semicolon(s: &str) -> Option<TextSize> {
     for (offset, c) in s.char_indices() {
         match c {
-            ' ' | '\t' => continue,
+            _ if is_python_whitespace(c) => continue,
             ';' => return Some(TextSize::try_from(offset).unwrap()),
             _ => break,
         }

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

@@ -801,6 +801,7 @@ mod tests {
     #[test_case(Path::new("existing_import.py"))]
     #[test_case(Path::new("multiline_docstring.py"))]
     #[test_case(Path::new("off.py"))]
+    #[test_case(Path::new("whitespace.py"))]
     fn required_import(path: &Path) -> Result<()> {
         let snapshot = format!("required_import_{}", path.to_string_lossy());
         let diagnostics = test_path(

crates/ruff_linter/src/rules/isort/snapshots/ruff_linter__rules__isort__tests__required_import_whitespace.py.snap

@@ -0,0 +1,11 @@
+---
+source: crates/ruff_linter/src/rules/isort/mod.rs
+---
+whitespace.py:1:1: I002 [*] Missing required import: `from __future__ import annotations`
+ℹ Safe fix
+1 1 | # This is a regression test for https://github.com/astral-sh/ruff/issues/19310
+2 2 | # there is a (potentially invisible) unicode formfeed character (000C) between "docstring" and the semicolon
+3   |-"docstring"; print(
+  3 |+"docstring"; from __future__ import annotations; print(
+4 4 |     f"{__doc__=}",
+5 5 | )