Skip BOM when inserting start-of-file imports (#7622)

See:
https://github.com/astral-sh/ruff/issues/7455#issuecomment-1732387485.

crates/ruff_linter/resources/test/fixtures/flake8_simplify/SIM105_4.py

@@ -0,0 +1,4 @@
+#!/usr/bin/env python
+try:
+   from __builtin__ import bytes, str, open, super, range, zip, round, int, pow, object, input
+except ImportError:    pass

crates/ruff_linter/src/importer/insertion.rs

@@ -64,7 +64,7 @@ impl<'a> Insertion<'a> {
             // Otherwise, advance to the next row.
             locator.full_line_end(location)
         } else {
-            TextSize::default()
+            locator.contents_start()
         };
 
         // Skip over commented lines, with whitespace separation.

crates/ruff_linter/src/rules/flake8_simplify/mod.rs

@@ -19,6 +19,7 @@ mod tests {
     #[test_case(Rule::SuppressibleException, Path::new("SIM105_1.py"))]
     #[test_case(Rule::SuppressibleException, Path::new("SIM105_2.py"))]
     #[test_case(Rule::SuppressibleException, Path::new("SIM105_3.py"))]
+    #[test_case(Rule::SuppressibleException, Path::new("SIM105_4.py"))]
     #[test_case(Rule::ReturnInTryExceptFinally, Path::new("SIM107.py"))]
     #[test_case(Rule::IfElseBlockInsteadOfIfExp, Path::new("SIM108.py"))]
     #[test_case(Rule::CompareWithTuple, Path::new("SIM109.py"))]

crates/ruff_linter/src/rules/flake8_simplify/snapshots/ruff_linter__rules__flake8_simplify__tests__SIM105_SIM105_4.py.snap

@@ -0,0 +1,22 @@
+---
+source: crates/ruff_linter/src/rules/flake8_simplify/mod.rs
+---
+SIM105_4.py:2:1: SIM105 [*] Use `contextlib.suppress(ImportError)` instead of `try`-`except`-`pass`
+  |
+1 |   #!/usr/bin/env python
+2 | / try:
+3 | |    from __builtin__ import bytes, str, open, super, range, zip, round, int, pow, object, input
+4 | | except ImportError:    pass
+  | |___________________________^ SIM105
+  |
+  = help: Replace with `contextlib.suppress(ImportError)`
+
+ℹ Suggested fix
+1 1 | #!/usr/bin/env python
+2   |-try:
+  2 |+import contextlib
+  3 |+with contextlib.suppress(ImportError):
+3 4 |    from __builtin__ import bytes, str, open, super, range, zip, round, int, pow, object, input
+4   |-except ImportError:    pass
+
+

crates/ruff_source_file/src/locator.rs

@@ -76,7 +76,15 @@ impl<'a> Locator<'a> {
         if let Some(index) = memrchr2(b'\n', b'\r', bytes) {
             // SAFETY: Safe because `index < offset`
             TextSize::try_from(index).unwrap().add(TextSize::from(1))
-        } else if self.contents.starts_with('\u{feff}') {
+        } else {
+            self.contents_start()
+        }
+    }
+
+    /// Computes the start position of the file contents: either the first byte, or the byte after
+    /// the BOM.
+    pub fn contents_start(&self) -> TextSize {
+        if self.contents.starts_with('\u{feff}') {
             // Skip the BOM.
             '\u{feff}'.text_len()
         } else {
@@ -85,9 +93,9 @@ impl<'a> Locator<'a> {
         }
     }
 
+    /// Returns `true` if `offset` is at the start of a line.
     pub fn is_at_start_of_line(&self, offset: TextSize) -> bool {
-        offset == TextSize::from(0)
+        self.line_start(offset) == offset
-            || self.contents[TextRange::up_to(offset)].ends_with(['\n', '\r'])
     }
 
     /// Computes the offset that is right after the newline character that ends `offset`'s line.