Function walk_expr 

pub fn walk_expr<'arena, 'src, V: Visitor<'arena, 'src> + ?Sized>(
    visitor: &mut V,
    expr: &Expr<'arena, 'src>,
) -> ControlFlow<()>

Dispatches expr to the appropriate child visitors based on its ExprKind.

Call this from Visitor::visit_expr to recurse into an expression’s children. Omit the call to skip the subtree entirely.