Panic safety in drop

[bluss]

@Stebalien discovered panic safety issue bluss/arrayvec#3 and it applies to smallvec as well.

My understanding is: SmallVec::drop first attempts to drop every element, then it inhibits the drop of the inner array. The panic safety issue is that a panic during drop of an element means the inhibition is never reached, so the inner data can be dropped again.

Testcase adapted from @Stebalien

    #[test]
    #[should_panic]
    fn test_drop_panic_smallvec() {
        // This test should only panic once, and not double panic,
        // which would mean a double drop
        struct DropPanic;

        impl Drop for DropPanic {
            fn drop(&mut self) {
                panic!("drop");
            }
        }

        let mut v = SmallVec::<[_; 1]>::new();
        v.push(DropPanic);
    }

[bluss]

bluss/arrayvec/pull/4 shows an alternative way to solve it, but it is fragile due to bug rust-lang/rust/issues/27906

[bluss]

I guess any way to solve it is fragile 😦 due to panic-on-drop semantics not being completely clear.