This massively speeds up serialization. It also
seems to produce deterministic metadata hashes
(before I was seeing inconsistent results).

Fixes #35232.

apart the tests that tested many things at once.

now don't print duplicate errors within one context, so I sometimes
had to break functions into two functions.

No longer does anything.

Fixes #13041.

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]

Closes #2569

This is the backwards-incompatible part of per-binding-site "mut".

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