diff --git a/crates/ruff/src/checkers/physical_lines.rs b/crates/ruff/src/checkers/physical_lines.rs
index 856b43b8d24da..adb2963e40494 100644
--- a/crates/ruff/src/checkers/physical_lines.rs
+++ b/crates/ruff/src/checkers/physical_lines.rs
@@ -4,7 +4,7 @@ use std::path::Path;
 
 use ruff_diagnostics::Diagnostic;
 use ruff_python_ast::newlines::StrExt;
-use ruff_python_ast::source_code::Stylist;
+use ruff_python_ast::source_code::{Locator, Stylist};
 
 use crate::registry::Rule;
 use crate::rules::flake8_executable::helpers::{extract_shebang, ShebangDirective};
@@ -22,8 +22,8 @@ use crate::settings::{flags, Settings};
 
 pub fn check_physical_lines(
     path: &Path,
+    locator: &Locator,
     stylist: &Stylist,
-    contents: &str,
     commented_lines: &[usize],
     doc_lines: &[usize],
     settings: &Settings,
@@ -57,7 +57,7 @@ pub fn check_physical_lines(
 
     let mut commented_lines_iter = commented_lines.iter().peekable();
     let mut doc_lines_iter = doc_lines.iter().peekable();
-    for (index, line) in contents.universal_newlines().enumerate() {
+    for (index, line) in locator.contents().universal_newlines().enumerate() {
         while commented_lines_iter
             .next_if(|lineno| &(index + 1) == *lineno)
             .is_some()
@@ -163,8 +163,8 @@ pub fn check_physical_lines(
 
     if enforce_no_newline_at_end_of_file {
         if let Some(diagnostic) = no_newline_at_end_of_file(
+            locator,
             stylist,
-            contents,
             autofix.into() && settings.rules.should_fix(&Rule::NoNewLineAtEndOfFile),
         ) {
             diagnostics.push(diagnostic);
@@ -200,8 +200,8 @@ mod tests {
         let check_with_max_line_length = |line_length: usize| {
             check_physical_lines(
                 Path::new("foo.py"),
+                &locator,
                 &stylist,
-                line,
                 &[],
                 &[],
                 &Settings {
diff --git a/crates/ruff/src/linter.rs b/crates/ruff/src/linter.rs
index 7c634b409012c..1ad51a0893602 100644
--- a/crates/ruff/src/linter.rs
+++ b/crates/ruff/src/linter.rs
@@ -191,8 +191,8 @@ pub fn check_path(
     {
         diagnostics.extend(check_physical_lines(
             path,
+            locator,
             stylist,
-            contents,
             indexer.commented_lines(),
             &doc_lines,
             settings,
diff --git a/crates/ruff/src/rules/pycodestyle/rules/no_newline_at_end_of_file.rs b/crates/ruff/src/rules/pycodestyle/rules/no_newline_at_end_of_file.rs
index 5a3b3de96ee94..9609c37895112 100644
--- a/crates/ruff/src/rules/pycodestyle/rules/no_newline_at_end_of_file.rs
+++ b/crates/ruff/src/rules/pycodestyle/rules/no_newline_at_end_of_file.rs
@@ -3,7 +3,7 @@ use rustpython_parser::ast::Location;
 use ruff_diagnostics::{AlwaysAutofixableViolation, Diagnostic, Fix};
 use ruff_macros::{derive_message_formats, violation};
 use ruff_python_ast::newlines::StrExt;
-use ruff_python_ast::source_code::Stylist;
+use ruff_python_ast::source_code::{Locator, Stylist};
 use ruff_python_ast::types::Range;
 
 /// ## What it does
@@ -38,16 +38,16 @@ impl AlwaysAutofixableViolation for NoNewLineAtEndOfFile {
 
 /// W292
 pub fn no_newline_at_end_of_file(
+    locator: &Locator,
     stylist: &Stylist,
-    contents: &str,
     autofix: bool,
 ) -> Option<Diagnostic> {
-    if !contents.ends_with(['\n', '\r']) {
+    if !locator.contents().ends_with(['\n', '\r']) {
         // Note: if `lines.last()` is `None`, then `contents` is empty (and so we don't
         // want to raise W292 anyway).
-        if let Some(line) = contents.universal_newlines().last() {
+        if let Some(line) = locator.contents().universal_newlines().last() {
             // Both locations are at the end of the file (and thus the same).
-            let location = Location::new(contents.universal_newlines().count(), line.len());
+            let location = Location::new(locator.count_lines(), line.len());
             let mut diagnostic =
                 Diagnostic::new(NoNewLineAtEndOfFile, Range::new(location, location));
             if autofix {
diff --git a/crates/ruff_python_ast/Cargo.toml b/crates/ruff_python_ast/Cargo.toml
index 635a315c80ef7..44df6c5fb9f26 100644
--- a/crates/ruff_python_ast/Cargo.toml
+++ b/crates/ruff_python_ast/Cargo.toml
@@ -24,5 +24,6 @@ regex = { workspace = true }
 rustc-hash = { workspace = true }
 rustpython-common = { workspace = true }
 rustpython-parser = { workspace = true }
+# TODO(charlie): See https://github.com/RustPython/RustPython/pull/4684.
 serde = { workspace = true }
 smallvec = { version = "1.10.0" }
diff --git a/crates/ruff_python_ast/src/source_code/locator.rs b/crates/ruff_python_ast/src/source_code/locator.rs
index 035110f78ed67..9acf3a95db894 100644
--- a/crates/ruff_python_ast/src/source_code/locator.rs
+++ b/crates/ruff_python_ast/src/source_code/locator.rs
@@ -56,10 +56,18 @@ impl<'a> Locator<'a> {
         self.contents
     }
 
+    /// Return the number of lines in the source code.
+    pub fn count_lines(&self) -> usize {
+        let index = self.get_or_init_index();
+        index.count_lines()
+    }
+
+    /// Return the number of bytes in the source code.
     pub const fn len(&self) -> usize {
         self.contents.len()
     }
 
+    /// Return `true` if the source code is empty.
     pub const fn is_empty(&self) -> bool {
         self.contents.is_empty()
     }
@@ -83,6 +91,14 @@ impl Index {
             Index::Utf8(utf8) => utf8.byte_offset(location, contents),
         }
     }
+
+    /// Return the number of lines in the source code.
+    fn count_lines(&self) -> usize {
+        match self {
+            Index::Ascii(ascii) => ascii.line_start_byte_offsets.len(),
+            Index::Utf8(utf8) => utf8.line_start_byte_offsets.len(),
+        }
+    }
 }
 
 impl From<&str> for Index {