Built-in derive macros `Send` and `Sync` exist

[petrochenkov]

Their single purpose is improving diagnostics.
When those macros expand they always produce an error saying that unsafe impl Send for Foo {} should be written explicitly rather than derived.

So, when #61877 is implemented you'll be able to write use Send or use std::Send without an error, but that's not an intent.

The macros should be removed, and derive(Send) should result in a resolution error, and the suggestion to write unsafe impl Send for Foo {} explicitly should be put on that resolution error.

[eddyb]

I would've half-expected e.g. use core::Clone; to never work, and instead the builtin macro be exposed from core::clone::Clone and core::prelude::v1::Clone (and both overlap with the Clone trait).
But it sounds like that's not going to be the case?

[petrochenkov]

@eddyb
I've answered in #62086 (comment).