diff --git a/compiler/rustc_lexer/src/unescape.rs b/compiler/rustc_lexer/src/unescape.rs
index d6ea4249247f3..d53bb456f4c0a 100644
--- a/compiler/rustc_lexer/src/unescape.rs
+++ b/compiler/rustc_lexer/src/unescape.rs
@@ -356,11 +356,10 @@ where
         let start = src.len() - chars.as_str().len() - c.len_utf8();
         let res = match c {
             '\\' => {
-                match chars.clone().next() {
-                    Some('\n') => {
-                        // Rust language specification requires us to skip whitespaces
-                        // if unescaped '\' character is followed by '\n'.
-                        // For details see [Rust language reference]
+                match chars.as_str().as_bytes().first() {
+                    Some(b'\n') => {
+                        let _ = chars.next();
+                        // skip whitespace for backslash newline, see [Rust language reference]
                         // (https://doc.rust-lang.org/reference/tokens.html#string-literals).
                         skip_ascii_whitespace(&mut chars, start, &mut |range, err| {
                             callback(range, Err(err))
@@ -379,30 +378,38 @@ where
     }
 }
 
+/// Skip ASCII whitespace, except for the formfeed character
+/// (see [this issue](https://github.com/rust-lang/rust/issues/136600)).
+/// Warns on unescaped newline and following non-ASCII whitespace.
 fn skip_ascii_whitespace<F>(chars: &mut Chars<'_>, start: usize, callback: &mut F)
 where
     F: FnMut(Range<usize>, EscapeError),
 {
-    let tail = chars.as_str();
-    let first_non_space = tail
-        .bytes()
-        .position(|b| b != b' ' && b != b'\t' && b != b'\n' && b != b'\r')
-        .unwrap_or(tail.len());
-    if tail[1..first_non_space].contains('\n') {
-        // The +1 accounts for the escaping slash.
-        let end = start + first_non_space + 1;
+    let mut spaces = 0;
+    let mut contains_nl = false;
+
+    for byte in chars.as_str().bytes() {
+        let is_space = b" \t\n\r".contains(&byte);
+        spaces += is_space as usize;
+        contains_nl |= byte == b'\n';
+        if !is_space {
+            break;
+        }
+    }
+    *chars = chars.as_str()[spaces..].chars();
+
+    // the escaping backslash and newline add 2 bytes
+    let end = start + 2 + spaces;
+
+    if contains_nl {
         callback(start..end, EscapeError::MultipleSkippedLinesWarning);
     }
-    let tail = &tail[first_non_space..];
-    if let Some(c) = tail.chars().next() {
+    if let Some(c) = chars.clone().next() {
         if c.is_whitespace() {
-            // For error reporting, we would like the span to contain the character that was not
-            // skipped. The +1 is necessary to account for the leading \ that started the escape.
-            let end = start + first_non_space + c.len_utf8() + 1;
-            callback(start..end, EscapeError::UnskippedWhitespaceWarning);
+            // for error reporting, include the character that was not skipped in the span
+            callback(start..end + c.len_utf8(), EscapeError::UnskippedWhitespaceWarning);
         }
     }
-    *chars = tail.chars();
 }
 
 /// Takes a contents of a string literal (without quotes) and produces a