Add regression test for manual_swap

Author: flip1995

tests/ui/swap.rs

@@ -46,6 +46,15 @@ fn slice() {
     foo.swap(0, 1);
 }
 
+fn unswappable_slice() {
+    let foo = &mut [vec![1, 2], vec![3, 4]];
+    let temp = foo[0][1];
+    foo[0][1] = foo[1][0];
+    foo[1][0] = temp;
+
+    // swap(foo[0][1], foo[1][0]) would fail
+}
+
 fn vec() {
     let mut foo = vec![1, 2];
     let temp = foo[0];

tests/ui/swap.stderr

@@ -17,15 +17,15 @@ LL | |     foo[1] = temp;
    | |_________________^ help: try: `foo.swap(0, 1)`
 
 error: this looks like you are swapping elements of `foo` manually
-  --> $DIR/swap.rs:51:5
+  --> $DIR/swap.rs:60:5
    |
 LL | /     let temp = foo[0];
 LL | |     foo[0] = foo[1];
 LL | |     foo[1] = temp;
    | |_________________^ help: try: `foo.swap(0, 1)`
 
 error: this looks like you are swapping `a` and `b` manually
-  --> $DIR/swap.rs:71:7
+  --> $DIR/swap.rs:80:7
    |
 LL |       ; let t = a;
    |  _______^
@@ -36,7 +36,7 @@ LL | |     b = t;
    = note: or maybe you should use `std::mem::replace`?
 
 error: this looks like you are swapping `c.0` and `a` manually
-  --> $DIR/swap.rs:80:7
+  --> $DIR/swap.rs:89:7
    |
 LL |       ; let t = c.0;
    |  _______^
@@ -47,7 +47,7 @@ LL | |     a = t;
    = note: or maybe you should use `std::mem::replace`?
 
 error: this looks like you are trying to swap `a` and `b`
-  --> $DIR/swap.rs:68:5
+  --> $DIR/swap.rs:77:5
    |
 LL | /     a = b;
 LL | |     b = a;
@@ -57,7 +57,7 @@ LL | |     b = a;
    = note: or maybe you should use `std::mem::replace`?
 
 error: this looks like you are trying to swap `c.0` and `a`
-  --> $DIR/swap.rs:77:5
+  --> $DIR/swap.rs:86:5
    |
 LL | /     c.0 = a;
 LL | |     a = c.0;