checker: disallow `mut` keyword in right-hand side of assignment (#12318)

vlib/v/checker/checker.v

@@ -3717,6 +3717,11 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 					right.pos)
 			}
 		}
+		if right is ast.Ident {
+			if right.is_mut {
+				c.error('unexpected `mut` on right-hand side of assignment', right.mut_pos)
+			}
+		}
 	}
 	if node.left.len != right_len {
 		if right_first is ast.CallExpr {

vlib/v/checker/tests/right_hand_side_mut.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/right_hand_side_mut.vv:4:19: error: unexpected `mut` on right-hand side of assignment
+    2 |     mut flubbel := 123
+    3 |     mut wubbel := 234
+    4 |     _, _ := flubbel, mut wubbel
+      |                      ~~~
+    5 |     print(wubbel)
+    6 | }

vlib/v/checker/tests/right_hand_side_mut.vv

@@ -0,0 +1,6 @@
+fn main() {
+	mut flubbel := 123
+	mut wubbel := 234
+	_, _ := flubbel, mut wubbel
+	print(wubbel)
+}