thread_local does not support types that implement Drop

[carllerche]

I discussed this briefly with @aturon, but I thought I would submit an issue to track the discussion.

I have been trying to implement an equivalent to Java's thread park / unpark using #[thread_local] but it requires being able to run some cleanup code when the thread exits. This currently isn't possible but should be with a little help from rust's thread support.

[pythonesque]

Pretty sure this is a duplicate of issue #17572.

[thestinger]

@carllerche: I have an implementation of TLS with support for lazy initialization and destruction and planned on landing an initial version and working on it in-tree. The initial work was in #17583 but that doesn't include the work I planned on landing in subsequent pull requests including destructors.