Fixes #30521 and #30627 by adding new snapshots
edunham uploaded the snapshots.

This adds a constructor for a Weak that can never be upgraded. These are
mostly useless, but for example are required when deserializing.

- Successful merges: #30373, #30502, #30511, #30546, #30556, #30620
- Failed merges:

As discussed in issue #30568.

Use proper URLs as given by Github.

So far we relied on redirect by Github which is not guaranteed to work.

Resolves #30507

r? @steveklabnik

Rewrite of a paragraph in in the `match` section.

The colon `:` should be used only when the sentence preceeding it is a
complete sentence. If this is not the case, then a `;` should be used;
this denotes that the following fragment is a part of the previous
fragment.

I got a new bike; it has two wheels. (Similar to I got a new bike, it has two wheels)
The ice cream truck has great flavours; blueberry, blackberry, berryberry.

Writing a complete sentence:
- with a list under it
- You can join two sentences with it: Much like this.

r? @steveklabnik

The current explanation for scan() is not very clear as to how it works, especially when it compares itself to fold().

I believe these changes makes it all a bit more clear for the reader, and makes it easier to understand the example code.

r? @steveklabnik

As discussed [here](https://internals.rust-lang.org/t/more-structured-errors/3005)

r? @nikomatsakis or anyone else on the @rust-lang/compiler team

This PR siliences some warnings when compiling stdlib with --test. Mostly remove some unused imports and added a few `#[allow(..)]`.

I also marked some signal handling functions with `#[cfg(not(test))]`, because they are only called through `rt::lang_start`, which is also marked as  `#[cfg(not(test))]`

When using `cc` for linking rustc will, if gold is available (by looking for `/usr/bin/ld.gold`), pass `-fuse-ld=gold` to `cc`.

In some scenarios gold links much faster than ld. Servo uses it to considerably speed up linking. gold behaves nearly identically to ld (though I think there are rare corner cases that don't work still). I've run this through crater and everything there continues to link.

To disable, pass `-C disable-gold`.

- Successful merges: #30253, #30390, #30405, #30549, #30603, #30610
- Failed merges:

Also, make its panic description identical in wording to assert_eq.

Fixes #30600

@ubsan brought up this relatively poor error message. This adds a
help message hinting when the problem actually is, and how to fix
it.

(reported in #30477)

Fixes #30217.

None of my devices/browsers renders this correctly.

r? @steveklabnik

r? @steveklabnik

Also, make its panic description identical in wording to assert_eq.

Fixes #30600

@ubsan brought up this relatively poor error message. This adds a
help message hinting when the problem actually is, and how to fix
it.

There's no need for us to redeclare it in an extern block.

We should probably put the syscall number constants in libc too.

To disable, pass `-C disable-gold`

A dumb mistake on my part from previous PR fixing unit enums in MIR.

r? @steveklabnik

`auto_ref()` currently returns an Rvalue datum for the ref'd value,
which is fine for thin pointers, but for fat pointers this means that
once the pointer is moved out of that datum, its memory will be marked
as dead. And because there is not necessarily an intermediate temporary
involved we can end up marking memory as dead that is actually still
used.

As I don't want to break the micro-optimization for thin pointers by
always returning an Lvalue datum, I decided to only do so for fat
pointers.

Fix #30478

the old code was *so terrible*.

r? @EddyB

I forgot about it in the previous commit