Detect illegal hidden lifetimes in `impl Trait`
This branch fixes#46541 -- however, it presently doesn't build because it also *breaks* a number of existing usages of impl Trait. I'm opening it as a WIP for now, just because we want to move on impl Trait, but I'll try to fix the problem in a bit.
~~(The problem is due to the fact that we apparently infer stricter lifetimes in closures that we need to; for example, if you capture a variable of type `&'a &'b u32`, we will put *precisely* those lifetimes into the closure, even if the closure would be happy with `&'a &'a u32`. This causes the present chance to affect things that are not invariant.)~~ fixed
r? @cramertj
Download the GCC artifacts from the HTTP server instead of FTP server.
Try to bring back the `dist-i686-linux` and `dist-x86_64-linux alt` builders which has mysteriously lost their cache 14 hours ago and stuck forever unable to download `mpfr-2.4.2.tar.bz2` since it keeps getting
```
==> PASV ... couldn't connect to 209.132.180.131 port 10058: Connection timed out
```
Unfortunately we don't have sufficient time to rebuild the cache *and*
distribute everything in `dist-x86_64-linux alt`, the debug assertions are
really slow.
We will re-enable them after the PR has been successfully merged, thus
successfully updating the cache (freeing up 40 minutes), giving us enough
time to build these tools.
We used to make the upvar types in the closure `==` but that was
stronger than we needed. Subtyping suffices, since we are copying the
upvar value into the closure field. This in turn allows us to infer
smaller lifetimes in captured values in some cases (like the example
here), avoiding errors.
Fix ordering of auto-generated trait bounds in rustdoc output
While the order of the where clauses was deterministic, the
ordering of bounds and lifetimes was not. This made the order flip-
flop randomly when new traits and impls were added to libstd.
This PR makes the ordering of bounds and lifetimes deterministic,
and re-enables the test that was causing the issue.
Fixes#49123
