diff --git a/std/array_list.zig b/std/array_list.zig
index b71f5be6ab94..5deb3c5fb1e2 100644
--- a/std/array_list.zig
+++ b/std/array_list.zig
@@ -85,7 +85,7 @@ pub fn AlignedArrayList(comptime T: type, comptime A: u29) type {
             try self.ensureCapacity(self.len + 1);
             self.len += 1;
 
-            mem.copy(T, self.items[n + 1 .. self.len], self.items[n .. self.len - 1]);
+            mem.copyBackwards(T, self.items[n + 1 .. self.len], self.items[n .. self.len - 1]);
             self.items[n] = item;
         }
 
@@ -93,7 +93,7 @@ pub fn AlignedArrayList(comptime T: type, comptime A: u29) type {
             try self.ensureCapacity(self.len + items.len);
             self.len += items.len;
 
-            mem.copy(T, self.items[n + items.len .. self.len], self.items[n .. self.len - items.len]);
+            mem.copyBackwards(T, self.items[n + items.len .. self.len], self.items[n .. self.len - items.len]);
             mem.copy(T, self.items[n .. n + items.len], items);
         }
 
@@ -266,19 +266,36 @@ test "insert ArrayList test" {
     defer list.deinit();
 
     try list.append(1);
+    try list.append(2);
+    try list.append(3);
     try list.insert(0, 5);
     assert(list.items[0] == 5);
     assert(list.items[1] == 1);
+    assert(list.items[2] == 2);
+    assert(list.items[3] == 3);
+}
+
+test "insertSlice ArrayList test" {
+    var list = ArrayList(i32).init(debug.global_allocator);
+    defer list.deinit();
 
+    try list.append(1);
+    try list.append(2);
+    try list.append(3);
+    try list.append(4);
     try list.insertSlice(1, []const i32{
         9,
         8,
     });
-    assert(list.items[0] == 5);
+    assert(list.items[0] == 1);
     assert(list.items[1] == 9);
     assert(list.items[2] == 8);
+    assert(list.items[3] == 2);
+    assert(list.items[4] == 3);
+    assert(list.items[5] == 4);
 
     const items = []const i32{1};
     try list.insertSlice(0, items[0..0]);
-    assert(list.items[0] == 5);
+    assert(list.len == 6);
+    assert(list.items[0] == 1);
 }
diff --git a/std/mem.zig b/std/mem.zig
index 555e1e249db3..41c5d0c8a30a 100644
--- a/std/mem.zig
+++ b/std/mem.zig
@@ -125,6 +125,7 @@ pub const Allocator = struct {
 
 /// Copy all of source into dest at position 0.
 /// dest.len must be >= source.len.
+/// dest.ptr must be <= src.ptr.
 pub fn copy(comptime T: type, dest: []T, source: []const T) void {
     // TODO instead of manually doing this check for the whole array
     // and turning off runtime safety, the compiler should detect loops like
@@ -135,6 +136,23 @@ pub fn copy(comptime T: type, dest: []T, source: []const T) void {
         dest[i] = s;
 }
 
+/// Copy all of source into dest at position 0.
+/// dest.len must be >= source.len.
+/// dest.ptr must be >= src.ptr.
+pub fn copyBackwards(comptime T: type, dest: []T, source: []const T) void {
+    // TODO instead of manually doing this check for the whole array
+    // and turning off runtime safety, the compiler should detect loops like
+    // this and automatically omit safety checks for loops
+    @setRuntimeSafety(false);
+    assert(dest.len >= source.len);
+    var i = source.len;
+    while(i > 0){
+        i -= 1;
+        dest[i] = source[i];
+    }
+}
+
+
 pub fn set(comptime T: type, dest: []T, value: T) void {
     for (dest) |*d|
         d.* = value;