4 thread parallelism by default

The Session default here is super unusual but seems to both compile and do what we expect as best as I can tell.

These depend on rustc being bug-free and it looks like that's not
currently entirely the case (e.g., we know of at least one bug that
introduces nondeterminism).

This also removes the unused NO_PARALLEL_COMPILER flag; if we want that
functionality we can readd it but this makes sure we really are parallel
everywhere.

This also patches a test that has differing output in the parallel case
(hopefully deterministically so!).

This avoids the problems of high thread counts (i.e., contention in the
kernel on the jobserver pipe due to thundering herd of readers) while
stil giving rustc some parallelism to work with.

Rollup of 8 pull requests

Successful merges:

 - #67249 (Improve code generated for `starts_with(<literal char>)`)
 - #67308 (Delete flaky test net::tcp::tests::fast_rebind)
 - #67318 (Improve typeck & lowering docs for slice patterns)
 - #67322 (use Self alias in place of macros)
 - #67323 (make transparent enums more ordinary)
 - #67336 (Fix JS error when loading page with search)
 - #67344 (.gitignore: Don't ignore a file that exists in the repository)
 - #67349 (Minor: update Unsize docs for dyn syntax)

Failed merges:

r? @ghost

Minor: update Unsize docs for dyn syntax

.gitignore: Don't ignore a file that exists in the repository

.gitignore should not ignore files that exist in the repository. The
ignore of .cargo applies to the committed .cargo directory used in an
example:

$ git ls-files --exclude-standard --ignored
src/test/run-make/thumb-none-qemu/example/.cargo/config

Explicitly un-ignore that file.

Fix JS error when loading page with search

For example when you load this page: https://doc.rust-lang.org/nightly/std/num/struct.NonZeroI8.html?search=foo, you get a nice JS error:

```
TypeError: ev is undefined
```

r? @kinnison

make transparent enums more ordinary

By recognizing that structs & unions have one variant, we can make the treatment of transparent enums less ad-hoc.

cc https://github.com/rust-lang/rust/issues/60405

r? @davidtwco

use Self alias in place of macros

Improve typeck & lowering docs for slice patterns

cc #62254

r? @matthewjasper

Delete flaky test net::tcp::tests::fast_rebind

This test is unreliable for at least 3 users on two platforms: see #57509 and #51006. It was added 5 years ago in #22015. Do we know whether this is testing something important that would indicate a bug in our implementation, or if it's fine to remove?

r? @sfackler @alexcrichton because this somewhat resembles #59018

Closes #57509. Closes #51006.

Improve code generated for `starts_with(<literal char>)`

This PR includes two minor improvements to the code generated when checking for string prefix/suffix.

The first commit simplifies the str/str operation, by taking advantage of the raw UTF-8 representation.

The second commit replaces the current str/char matching logic with a char->str encoding and then the previous method.

The resulting code should be equivalent in the generic case (one char is being encoded versus one char being decoded), but it becomes easy to optimize in the case of a literal char, which in most cases a developer might expect to be at least as simple as that of a literal string.

This PR should fix #41993

Rollup of 7 pull requests

Successful merges:

 - #65778 (Stabilize `std::{rc,sync}::Weak::{weak_count, strong_count}`)
 - #66570 (stabilize Result::map_or)
 - #66735 (Add str::strip_prefix and str::strip_suffix)
 - #66771 (Stabilize the `core::panic` module)
 - #67317 (fix type_name_of_val doc comment)
 - #67324 (Fix repetition in matches/mod.rs)
 - #67325 (cleanup with push_fake_read)

Failed merges:

r? @ghost

.gitignore should not ignore files that exist in the repository. The
ignore of .cargo applies to the committed .cargo directory used in an
example:

$ git ls-files --exclude-standard --ignored
src/test/run-make/thumb-none-qemu/example/.cargo/config

Explicitly un-ignore that file.

cleanup with push_fake_read

...and make things a bit more readable.

r? @matthewjasper

Fix repetition in matches/mod.rs

r? @matthewjasper

fix type_name_of_val doc comment

.

Stabilize the `core::panic` module

`std::panic` is already stable.

`core::panic::PanicInfo` and `core::panic::Location` are stable and can be used through that path because of a bug in stability checking: #15702

Add str::strip_prefix and str::strip_suffix

Introduces a counterpart for `Path::strip_prefix` on `str`.

This was also discussed in https://internals.rust-lang.org/t/pre-pr-path-strip-prefix-counterpart-in-str/11364/.

stabilize Result::map_or

r? @SimonSapin
Closes #66293

Stabilize `std::{rc,sync}::Weak::{weak_count, strong_count}`

* Original PR: #56696
* Tracking issue: #57977

Closes: #57977

Supporting comments:

> Although these were added for testing, it is occasionally useful to have a way to probe optimistically for whether a weak pointer has become dangling, without actually taking the overhead of manipulating atomics. Are there any plans to stabilize this?

_Originally posted by @bdonlan in https://github.com/rust-lang/rust/issues/57977#issuecomment-516970921_

> Having this stabilized would help. Currently, the only way to check if a weak pointer has become dangling is to call `upgrade`, which is by far expensive.

_Originally posted by @glebpom in https://github.com/rust-lang/rust/issues/57977#issuecomment-526934709_

Not sure if stabilizing these warrants a full RFC, so throwing this out here as a start for now.

Note: per CONTRIBUTING.md, I ran the tidy checks, but they seem to be failing on unchanged files (primarily in `src/stdsimd`).

Rollup of 6 pull requests

Successful merges:

 - #67255 (Remove i686-unknown-dragonfly target)
 - #67267 (Fix signature of `__wasilibc_find_relpath`)
 - #67282 (Fix example code of OpenOptions::open)
 - #67289 (Do not ICE on unnamed future)
 - #67300 (Restore original implementation of Vec::retain)
 - #67305 (Doc typo)

Failed merges:

r? @ghost

Doc typo

Restore original implementation of Vec::retain

This PR reverts #48065, which aimed to optimize `Vec::retain` by making use of `Vec::drain_filter`. Unfortunately at that time, `drain_filter` was unsound.

The soundness hole in `Vec::drain_filter` was fixed in #61224 by guaranteeing that cleanup logic runs via a nested `Drop`, even in the event of a panic. Implementing this nested drop affects codegen (apparently?) and results in slower code.

Fixes #65970

Do not ICE on unnamed future

Fix #67252.