No longer does anything.

Closes #8142.

This is not the semantics we want long-term. You can continue to use
`#[unsafe_destructor]`, but you'll need to add
`#![feature(unsafe_destructor)]` to the crate attributes.

[breaking-change]

This removes all remnants of `@` pointers from rustc. Additionally, this removes
the `GC` structure from the prelude as it seems odd exporting an experimental
type in the prelude by default.

Closes #14193
[breaking-change]

Who doesn't like a massive renaming?

librustc: Forbid destructors from being attached to any structs that might contain non-Owned fields. r=nmatsakis

This results in a lot of warnings in rustc.  I left them in because
many are bugs and we should fix our code, but Graydon asked that
I not touch every file in the codebase.