From fe9408070ec51de32a609fbd1245b062a12509f4 Mon Sep 17 00:00:00 2001
From: Senthilnathan <senthilnathan_selvarajan@agnikul.in>
Date: Wed, 22 Jul 2026 19:04:22 +0530
Subject: [PATCH 1/2] tests: add UI test for `.swap()` suggestion

---
 tests/ui/suggestions/suggest-swap.rs     | 13 +++++++++++++
 tests/ui/suggestions/suggest-swap.stderr | 22 ++++++++++++++++++++++
 2 files changed, 35 insertions(+)
 create mode 100644 tests/ui/suggestions/suggest-swap.rs
 create mode 100644 tests/ui/suggestions/suggest-swap.stderr

diff --git a/tests/ui/suggestions/suggest-swap.rs b/tests/ui/suggestions/suggest-swap.rs
new file mode 100644
index 0000000000000..13d86a4b706f4
--- /dev/null
+++ b/tests/ui/suggestions/suggest-swap.rs
@@ -0,0 +1,13 @@
+use std::mem;
+
+fn main() {
+    let mut arr = [1, 2, 3];
+
+    let i = 0usize;
+    let j = 1usize;
+
+    mem::swap(
+        &mut arr[i],
+        &mut arr[j], //~ ERROR cannot borrow `arr[_]` as mutable more than once at a time
+    );
+}
diff --git a/tests/ui/suggestions/suggest-swap.stderr b/tests/ui/suggestions/suggest-swap.stderr
new file mode 100644
index 0000000000000..4c321ca8939f4
--- /dev/null
+++ b/tests/ui/suggestions/suggest-swap.stderr
@@ -0,0 +1,22 @@
+error[E0499]: cannot borrow `arr[_]` as mutable more than once at a time
+  --> $DIR/suggest-swap.rs:11:9
+   |
+LL |     mem::swap(
+   |     --------- first borrow later used by call
+LL |         &mut arr[i],
+   |         ----------- first mutable borrow occurs here
+LL |         &mut arr[j],
+   |         ^^^^^^^^^^^ second mutable borrow occurs here
+   |
+help: use `.swap()` to swap elements at the specified indices instead
+   |
+LL -     mem::swap(
+LL -         &mut arr[i],
+LL -         &mut arr[j],
+LL -     );
+LL +     arr.swap(j, i);
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0499`.

From d5482401250fef44346e03ab35df9e1fac005016 Mon Sep 17 00:00:00 2001
From: Senthilnathan <senthilnathan_selvarajan@agnikul.in>
Date: Thu, 23 Jul 2026 11:03:10 +0530
Subject: [PATCH 2/2] fix: add comment on top of test file to follow style spec

---
 tests/ui/suggestions/suggest-swap.rs     | 1 +
 tests/ui/suggestions/suggest-swap.stderr | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/tests/ui/suggestions/suggest-swap.rs b/tests/ui/suggestions/suggest-swap.rs
index 13d86a4b706f4..ef6604a42b2c0 100644
--- a/tests/ui/suggestions/suggest-swap.rs
+++ b/tests/ui/suggestions/suggest-swap.rs
@@ -1,3 +1,4 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/159664>
 use std::mem;
 
 fn main() {
diff --git a/tests/ui/suggestions/suggest-swap.stderr b/tests/ui/suggestions/suggest-swap.stderr
index 4c321ca8939f4..85fbbfb7e743c 100644
--- a/tests/ui/suggestions/suggest-swap.stderr
+++ b/tests/ui/suggestions/suggest-swap.stderr
@@ -1,5 +1,5 @@
 error[E0499]: cannot borrow `arr[_]` as mutable more than once at a time
-  --> $DIR/suggest-swap.rs:11:9
+  --> $DIR/suggest-swap.rs:12:9
    |
 LL |     mem::swap(
    |     --------- first borrow later used by call