Pre-pre-RFC: `NoDrop` marker trait

[felix.s]

@CAD97

In a best case world, we make every for<T> bound by Drop by default (the way they are Sized , and in this world T : Drop means droppable, not implements Drop explicitly), and every generic needs to consider if it can opt in to ?Drop types.

This was discussed before:

Fixing the `Drop` trait bound

What would I intuitively expect the Drop trait to behave like: When copying values, there are three categories of types: Cannot be copied (no traits) Can be copied using used-defined code (implements Clone) Can be trivially copied with mem-copy (implements Copy which derives from Clone) For dropping the situation should be similar, with the equivalent categories: Cannot be dropped (linear types). Those currently do not exist in Rust. Can be dropped using user-defined code (implements Drop)…