Adding Drop impl to a type used in a public API type is a SemVer hazard

[obi1kenobi]

If a type in the public API has no components that impl Drop, adding the first impl Drop on that type or any of its components appears to be a major breaking change: playground

struct Foo;

// Adding this block is a breaking change downstream on `Bar`.
impl Drop for Foo {
    fn drop(&mut self) {}
}

pub struct Bar(Foo);

// Witness of the breakage.
// We don't need to use the value, we're just dropping it.
const fn example(_value: Bar) -> i64 {
    0
}

I don't believe we have a reliable way of detecting this via rustdoc JSON right now. We can check for impl Drop on the public type itself, but I don't see a way to reliably check that for its components.

Perhaps either rustdoc JSON could be extended with a new field with this information, or a new marker auto-trait could be added to the language that we could track?