The parser needs to be extended to support the new syntax.
Assignments can now happen inside of expressions, which was previously impossible. Several syntax tree transformations assume that they can stop at an ExprNode, which they cannot any more in the future.
The control flow analysis needs updating to detect assigned names in expressions and correctly track their usage.
An assignment expression would probably always move the assigned expression into a temp variable and continue from there. There may be expressions from that rule (for simple C types) since C also supports assignments in expressions, but that sounds more like an optimisation than a requirement.