diff --git a/src/lines.rs b/src/lines.rs
index 830faed59..b595b2657 100644
--- a/src/lines.rs
+++ b/src/lines.rs
@@ -151,20 +151,6 @@ pub fn codepoint_len(s: &str) -> usize {
     s.chars().count()
 }
 
-/// Slice `s` from `start` to `end` by codepoint. This is safer than
-/// slicing by bytes, which panics if the byte isn't on a codepoint
-/// boundary.
-pub fn substring_by_codepoint(s: &str, start: usize, end: usize) -> &str {
-    assert!(end > start);
-
-    let mut char_idx_iter = s.char_indices();
-    let byte_start = char_idx_iter.nth(start).expect("Expected a codepoint index inside `s`.").0;
-    match char_idx_iter.nth(end - 1 - start) {
-        Some(byte_end) => &s[byte_start..byte_end.0],
-        None => &s[byte_start..],
-    }
-}
-
 pub trait MaxLine {
     fn max_line(&self) -> LineNumber;
 }
diff --git a/src/style.rs b/src/style.rs
index 2c9c9c5ef..1d87688df 100644
--- a/src/style.rs
+++ b/src/style.rs
@@ -1,7 +1,7 @@
 //! Apply colours and styling to strings.
 
 use crate::{
-    lines::{codepoint_len, substring_by_codepoint, LineNumber},
+    lines::{codepoint_len, LineNumber},
     positions::SingleLineSpan,
     syntax::{AtomKind, MatchKind, MatchedPos, TokenKind},
 };
@@ -18,6 +18,20 @@ pub enum BackgroundColor {
     Light,
 }
 
+/// Slice `s` from `start` to `end` by codepoint. This is safer than
+/// slicing by bytes, which panics if the byte isn't on a codepoint
+/// boundary.
+fn substring_by_codepoint(s: &str, start: usize, end: usize) -> &str {
+    assert!(end > start);
+
+    let mut char_idx_iter = s.char_indices();
+    let byte_start = char_idx_iter.nth(start).expect("Expected a codepoint index inside `s`.").0;
+    match char_idx_iter.nth(end - 1 - start) {
+        Some(byte_end) => &s[byte_start..byte_end.0],
+        None => &s[byte_start..],
+    }
+}
+
 /// Split a string into equal length parts, padding the last part if
 /// necessary.
 ///