As a parent, you do NOT have the right to spy on your child NO MATTER WHAT. I don't care if they are suicidal or doing drugs, that doesn't give you the right to spy on them. You don't have the right to read their email or diary or texts.
Not only do you not have the right, literally no good will come from you violating their trust like this. All they will learn is hyper-vigilance against anyone who wants to get to know them, and they will learn to see you as the enemy and they will learn to hide things much better.
Instead of treating your child as a wild monster that you're trying to control, treat your child in ways that make them feel loved and trusted and able to trust you. Things like using drugs and having suicidal thoughts are a sign that they need more care, better care, more experienced care, not control. CONTROL ALWAYS MAKES IT WORSE.
edited to add:
sometimes I honestly feel grateful that my parents were so neglectful and disinterested in my life. Because they did not value consent at ALL and if they thought they could control me better by invading my intimate thoughts or alone time they absolutely would have.