diff --git a/src/tools/compiletest/src/runtest.rs b/src/tools/compiletest/src/runtest.rs
index 6b5147cea6626..86ec8d52ac2e7 100644
--- a/src/tools/compiletest/src/runtest.rs
+++ b/src/tools/compiletest/src/runtest.rs
@@ -22,7 +22,7 @@ use crate::directives::{AuxCrate, TestProps};
 use crate::errors::{Error, ErrorKind, load_errors};
 use crate::output_capture::ConsoleOut;
 use crate::read2::{Truncated, read2_abbreviated};
-use crate::runtest::compute_diff::{DiffLine, make_diff, write_diff};
+use crate::runtest::compute_diff::{DiffLine, diff_by_lines, make_diff, write_diff};
 use crate::util::{Utf8PathBufExt, add_dylib_path, static_regex};
 use crate::{json, stamp_file_path};
 
@@ -2794,6 +2794,7 @@ impl<'test> TestCx<'test> {
                     expected,
                     actual,
                     actual_unnormalized,
+                    compare_output_by_lines || compare_output_by_lines_subset,
                 );
             }
         } else {
@@ -2831,6 +2832,7 @@ impl<'test> TestCx<'test> {
         expected: &str,
         actual: &str,
         actual_unnormalized: &str,
+        show_diff_by_lines: bool,
     ) {
         writeln!(self.stderr, "diff of {stream}:\n");
         if let Some(diff_command) = self.config.diff_command.as_deref() {
@@ -2897,6 +2899,10 @@ impl<'test> TestCx<'test> {
                 write_diff(&mismatches_unnormalized, &mismatches_normalized, 0)
             );
         }
+
+        if show_diff_by_lines {
+            write!(self.stderr, "{}", diff_by_lines(expected, actual));
+        }
     }
 
     fn check_and_prune_duplicate_outputs(
diff --git a/src/tools/compiletest/src/runtest/compute_diff.rs b/src/tools/compiletest/src/runtest/compute_diff.rs
index 97ccdb989d437..a724d6a691764 100644
--- a/src/tools/compiletest/src/runtest/compute_diff.rs
+++ b/src/tools/compiletest/src/runtest/compute_diff.rs
@@ -104,3 +104,50 @@ pub(crate) fn write_diff(expected: &str, actual: &str, context_size: usize) -> S
     }
     output
 }
+
+pub(crate) fn diff_by_lines(expected: &str, actual: &str) -> String {
+    use std::collections::HashMap;
+    use std::fmt::Write;
+    let mut output = String::new();
+    let mut expected_counts: HashMap<&str, usize> = HashMap::new();
+    let mut actual_counts: HashMap<&str, usize> = HashMap::new();
+
+    for line in expected.lines() {
+        *expected_counts.entry(line).or_insert(0) += 1;
+    }
+    for line in actual.lines() {
+        *actual_counts.entry(line).or_insert(0) += 1;
+    }
+
+    fn write_only_lines(
+        output: &mut String,
+        left_lines: &HashMap<&str, usize>,
+        right_lines: &HashMap<&str, usize>,
+    ) {
+        let mut left_only: Vec<(&str, usize)> = left_lines
+            .iter()
+            .filter_map(|(&line, &left_count)| {
+                let right_count = right_lines.get(line).copied().unwrap_or(0);
+                if left_count > right_count { Some((line, left_count - right_count)) } else { None }
+            })
+            .collect();
+        left_only.sort_by(|(a, _), (b, _)| a.cmp(b));
+
+        if left_only.is_empty() {
+            writeln!(output, "(no lines found)").unwrap();
+        } else {
+            for (line, diff) in left_only {
+                for _ in 0..diff {
+                    writeln!(output, "{line}").unwrap();
+                }
+            }
+        }
+    }
+
+    writeln!(output, "Compare output by lines enabled, diff by lines:").unwrap();
+    writeln!(output, "Expected contains these lines that are not in actual:").unwrap();
+    write_only_lines(&mut output, &expected_counts, &actual_counts);
+    writeln!(output, "Actual contains these lines that are not in expected:").unwrap();
+    write_only_lines(&mut output, &actual_counts, &expected_counts);
+    output
+}