Function check 

pub fn check(
    base: &str,
    target: &str,
    proposed: &str,
    result: &str,
) -> SafetyVerdict

Checks a resolved merge against the safety invariant.

base is what the author wrote against, proposed is what they wrote, target is the state being merged onto (the speculative train state, or a ref head), and result is the strategy’s resolved output. Returns SafetyVerdict::Upholds when target -> result removes and adds only lines that base -> proposed removes and adds, counted as bags.

An author who explicitly proposes reverting target-side content passes this check: the lines are attributable to their proposal, so nothing is silent. Refusing deliberate reverts is policy (review, D24), not safety.