Fix #2123 : check that the source and destination are different for manual memcpy

clippy_lints/src/loops.rs

@@ -749,7 +749,14 @@ fn get_indexed_assignments<'a, 'tcx>(
     ) -> Option<(FixedOffsetVar, FixedOffsetVar)> {
         if let Expr_::ExprAssign(ref lhs, ref rhs) = e.node {
             match (get_fixed_offset_var(cx, lhs, var), fetch_cloned_fixed_offset_var(cx, rhs, var)) {
-                (Some(offset_left), Some(offset_right)) => Some((offset_left, offset_right)),
+                (Some(offset_left), Some(offset_right)) => {
+                    // Source and destination must be different
+                    if offset_left.var_name != offset_right.var_name {
+                        Some((offset_left, offset_right))
+                    } else {
+                        None
+                    }
+                },
                 _ => None,
             }
         } else {

tests/ui/for_loop.rs

@@ -548,3 +548,11 @@ pub fn manual_clone(src: &[String], dst: &mut [String]) {
         dst[i] = src[i].clone();
     }
 }
+
+#[warn(needless_range_loop)]
+pub fn manual_copy_same_destination(dst: &mut [i32], d: usize, s: usize) {
+    // Same source and destination - don't trigger lint
+    for i in 0..dst.len() {
+        dst[d + i] = dst[s + i];
+    }
+}