Make UnsafeCell doc easier to follow

remove italic as per @GuillaumeGomez suggestion

Merge three rules into one following @cramertj

This is propagated from rust-lang/prev.rust-lang.org#1062.

based on @nikomatsakis texthg

…directories

These change from release to release and are impossible to get right,
since they are generated by Make magic.

(Meanwhile, a couple of parse-fail tests are moved to UI tests so that
the reader can see the new output, and an existing UI test is given a
more evocative name.)

The code for several of the core traits doesn't use hygenic macros.
This isn't a problem, except for the Debug trait, which is the only
one that uses a variable, named "builder".

Variables can't share names with unit structs, so attempting to
[derive(Debug)] on any type while a unit struct with the name
"builder" was in scope would result in an error.

This commit just changes the name of the variable to
"__debug_trait_builder", because I couldn't figure out how to get a
list of all unit structs in-scope from within the derive expansion
function. If someone wants to have a unit struct with
the exact name "__debug_trait_builder", they'll just have to do it
without a [derive(Debug)].

Pulls in a redesigned `std::simd` module as well as a replacement for the
`is_target_feature_detected!` macro

rephrase UnsafeCell doc

As shown by discussions on users.rust-lang.org [[1]], [[2]], UnsafeCell doc is not totally clear. I tried to made the doc univocal regarding what is allowed and what is not. The edits are based on my understanding following [[1]].

[1]: https://users.rust-lang.org/t/unsafecell-behavior-details/1560
[2]: https://users.rust-lang.org/t/is-there-a-better-way-to-overload-index-indexmut-for-a-rc-refcell/15591/12

Update Feature Request instructions

As noted in rust-lang#48393 the contribution instructions for submitting a Feature Request are a little hasty, suggesting that the user immediately create an issue in the RFC repository. For users that want to submit a feature request, let's instead point them directly to the README file for the RFC repository, which contains detailed instructions on how to submit a Feature Request.

Update L4Re target specification

Due to the dynamically generated linker arguments of the L4Re build system, it is not a good idea to hard-code them in Rust. This PR undoes this step. It also adds an empty implementation to retrieve the number of CPUs.

…labnik

Propagating upstream code of conduct changes

[This repository's code of conduct](https://github.com/rust-lang/rust/blob/master/CODE_OF_CONDUCT.md) is out of sync with the [rust-www code of conduct](https://github.com/rust-lang/rust-www/blob/master/en-US/conduct.md) due changes from rust-lang/prev.rust-lang.org#1062. This PR propagates those changes and brings the files back into sync.

r? @steveklabnik

…=QuietMisdreavus

Add missing urls

r? @QuietMisdreavus

tidy: Add a check for stray `.stderr` and `.stdout` files in UI test directories

Update stdsimd module

Pulls in a redesigned `std::simd` module as well as a replacement for the
`is_target_feature_detected!` macro

…y, r=estebank

in which some labels and notes are upgraded to structured suggestions

(Meanwhile, a couple of parse-fail tests are moved to UI tests so that
the reader can see the new output, and an existing UI test is given a
more evocative name.)

r? @estebank

…etrochenkov

Fix hygene issue when deriving Debug

The code for several of the core traits doesn't use hygenic macros.
This isn't a problem, except for the Debug trait, which is the only
one that uses a variable, named "builder".

Variables can't share names with unit structs, so attempting to
[derive(Debug)] on any type while a unit struct with the name
"builder" was in scope would result in an error.

This commit just changes the name of the variable to
"__debug_trait_builder", because I couldn't figure out how to get a
list of all unit structs in-scope from within the derive expansion
function. If someone wants to have a unit struct with
the exact name "__debug_trait_builder", they'll just have to do it
without a [derive(Debug)].

I also checked the implementations of the other built-in derives to
ensure they didn't declare any variables.

Clarify interfaction between File::set_len and file cursor

…g, r=QuietMisdreavus

Remember state of top-level collapse toggle widget

This change allows the big top-right expand/collapse toggle to remember its setting across navigation or page reloads. Prior to this change, there was this annoyance:

- browse to some docs
- Click the minus button to collapse them
- browse to other docs (or reload the page)
- Everything is expanded again

The solution is based on storing a simple boolean flag in localStorage. I think it's a good improvement, but it does introduce the following potentially surprising behavior:

- browse to some docs
- click the minus button to collapse them
- click to expand a particular item (not the main top-right big one)
- reload the page, everything is collapsed

Paired with @DebugSteven on this.

…, r=QuietMisdreavus

Remove auto trait implementation section when empty

Fixes rust-lang#48882.

…=QuietMisdreavus

Add missing links