Add test for eval order for a+=b

src/test/ui/expr/compound-assignment/eval-order.rs

@@ -0,0 +1,76 @@
+// Test evaluation order of operands of the compound assignment operators
+
+// run-pass
+
+use std::ops::AddAssign;
+
+enum Side {
+    Lhs,
+    Rhs,
+}
+
+// In the following tests, we place our value into a wrapper type so that we
+// can do an element access as the outer place expression. If we just had the
+// block expression, it'd be a value expression and not compile.
+struct Wrapper<T>(T);
+
+// Evaluation order for `a op= b` where typeof(a) and typeof(b) are primitives
+// is first `b` then `a`.
+fn primitive_compound() {
+    let mut side_order = vec![];
+    let mut int = Wrapper(0);
+
+    {
+        side_order.push(Side::Lhs);
+        int
+    }.0 += {
+        side_order.push(Side::Rhs);
+        0
+    };
+
+    assert!(matches!(side_order[..], [Side::Rhs, Side::Lhs]));
+}
+
+// Evaluation order for `a op=b` otherwise is first `a` then `b`.
+fn generic_compound<T: AddAssign<T> + Default>() {
+    let mut side_order = vec![];
+    let mut add_assignable: Wrapper<T> = Wrapper(Default::default());
+
+    {
+        side_order.push(Side::Lhs);
+        add_assignable
+    }.0 += {
+        side_order.push(Side::Rhs);
+        Default::default()
+    };
+
+    assert!(matches!(side_order[..], [Side::Lhs, Side::Rhs]));
+}
+
+fn custom_compound() {
+    struct Custom;
+
+    impl AddAssign<()> for Custom {
+        fn add_assign(&mut self, _: ()) {
+            // this block purposely left blank
+        }
+    }
+
+    let mut side_order = vec![];
+    let mut custom = Wrapper(Custom);
+
+    {
+        side_order.push(Side::Lhs);
+        custom
+    }.0 += {
+        side_order.push(Side::Rhs);
+    };
+
+    assert!(matches!(side_order[..], [Side::Lhs, Side::Rhs]));
+}
+
+fn main() {
+    primitive_compound();
+    generic_compound::<i32>();
+    custom_compound();
+}