Fixed panic in missing_copyright_notice (#7029)

crates/ruff_source_file/src/locator.rs

@@ -388,6 +388,59 @@ impl<'a> Locator<'a> {
         &self.contents[usize::from(offset)..]
     }
 
+    /// Finds the closest [`TextSize`] not exceeding the offset for which `is_char_boundary` is
+    /// `true`.
+    ///
+    /// Can be replaced with `str#floor_char_boundary` once it's stable.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// # use ruff_text_size::{Ranged, TextRange, TextSize};
+    /// # use ruff_source_file::Locator;
+    ///
+    /// let locator = Locator::new("Hello");
+    ///
+    /// assert_eq!(
+    ///     locator.floor_char_boundary(TextSize::from(0)),
+    ///     TextSize::from(0)
+    /// );
+    ///
+    /// assert_eq!(
+    ///     locator.floor_char_boundary(TextSize::from(5)),
+    ///     TextSize::from(5)
+    /// );
+    ///
+    /// let locator = Locator::new("α");
+    ///
+    /// assert_eq!(
+    ///     locator.floor_char_boundary(TextSize::from(0)),
+    ///     TextSize::from(0)
+    /// );
+    ///
+    /// assert_eq!(
+    ///     locator.floor_char_boundary(TextSize::from(1)),
+    ///     TextSize::from(0)
+    /// );
+    ///
+    /// assert_eq!(
+    ///     locator.floor_char_boundary(TextSize::from(2)),
+    ///     TextSize::from(2)
+    /// );
+    /// ```
+    pub fn floor_char_boundary(&self, offset: TextSize) -> TextSize {
+        if offset >= self.text_len() {
+            self.text_len()
+        } else {
+            // We know that the character boundary is within four bytes.
+            (0u32..=3u32)
+                .map(TextSize::from)
+                .filter_map(|index| offset.checked_sub(index))
+                .find(|offset| self.contents.is_char_boundary(offset.to_usize()))
+                .unwrap_or_default()
+        }
+    }
+
     /// Take the source code between the given [`TextRange`].
     #[inline]
     pub fn slice<T: Ranged>(&self, ranged: T) -> &'a str {