Fixes rust-lang#77105

This keeps only the `std` artifacts compiled by the given stage, not the
compiler. This is useful when working on the latter stages of the
compiler in tandem with the standard library, since you don't have to
rebuild the *entire* compiler when the standard library changes.

0.4.2 -> 0.4.3

- Suggest `x.py setup` if config.toml doesn't exist yet (twice, once
before and once after the build)
- Prompt for a profile if not given on the command line
- Print the configuration file that will be used
- Print helpful starting commands after setup
- Link to the dev-guide after finishing
- Note that distro maintainers will see the changelog warning

The syscalls returning a new file descriptors generally use
lowest-numbered file descriptor not currently opened, without any
exceptions for those corresponding to the standard streams.

Previously when any of standard streams has been closed before starting
the application, operations on std::io::{stderr,stdin,stdout} objects
were likely to operate on other logically unrelated file resources
opened afterwards.

Avoid the issue by reopening the standard streams when they are closed.

Reopen standard file descriptors when they are missing on Unix

The syscalls returning a new file descriptors generally return lowest-numbered
file descriptor not currently opened, without any exceptions for those
corresponding to stdin, sdout, or stderr.

Previously when any of standard file descriptors has been closed before starting
the application, operations on std::io::{stderr,stdin,stdout} were likely to
either succeed while being performed on unrelated file descriptor, or fail with
EBADF which is silently ignored.

Avoid the issue by using /dev/null as a replacement when the standard file
descriptors are missing.

The implementation is based on the one found in musl. It was selected among a
few others on the basis of the lowest overhead in the case when all descriptors
are already present (measured on GNU/Linux).

Closes rust-lang#57728.
Closes rust-lang#46981.
Closes rust-lang#60447.

Benefits:
* Makes applications robust in the absence of standard file descriptors.
* Upholds IntoRawFd / FromRawFd safety contract (which was broken previously).

Drawbacks:
* Additional syscall during startup.
* The standard descriptors might have been closed intentionally.
* Requires /dev/null.

Alternatives:
* Check if stdin, stdout, stderr are opened and provide no-op substitutes in std::io::{stdin,stdout,stderr} without reopening them directly.
* Leave the status quo, expect robust applications to reopen them manually.

Add `x.py setup`

Closes rust-lang#76503.

- Suggest `x.py setup` if config.toml doesn't exist yet
- Prompt for a profile if not given on the command line
- Print the configuration that will be used
- Print helpful starting commands after setup
- Link to the dev-guide after finishing

…-obk

Unstably allow assume intrinsic in const contexts

Not sure much about this usage because there are concerns
about [blocking  optimization][1] and [slowing down LLVM][2] when using `assme` intrinsic
in inline functions.
But since Oli suggested in rust-lang#76960 (comment),
here we are.

[1]: rust-lang#54995 (comment)
[2]: rust-lang#49572 (comment)

Include libunwind in the rust-src component.

Some targets, such as musl, need the libunwind source to build the unwind crate (referenced [here](https://github.com/rust-lang/rust/blob/0da58007451a154da2480160429e1604a1f5f0ec/library/unwind/build.rs#L142)).

Fixes rust-lang/wg-cargo-std-aware#59

…-Simulacrum

clarify that `changelog-seen = 1` goes to the beginning of config.toml

Fixes rust-lang#77105

…rk-Simulacrum

Add `--keep-stage-std` to `x.py` for keeping only standard library artifacts

Unlike `--keep-stage 0`, `--keep-stage-std 0` will allow the stage 0 compiler artifacts (i.e., stage1/bin/rustc) to be rebuilt if it has changed. This allows contributors to iterate on later stages of the compiler in tandem with the standard library without needing to to rebuild the entire compiler. I often run into this when working on const-checking, since I may need to add a feature gate or make a small tweak to the standard library.

…alexcrichton

Invalidate local LLVM cache less often

This avoids a download of LLVM after every rebase. The downside to this is that if we land some patch affecting LLVM built in CI that breaks this option, but that PR does not update the LLVM submodule, we'll likely not notice until the next update -- but this seems unlikely to happen in practice and I am not personally worried about it.

r? @alexcrichton

Update mdBook

0.4.2 -> 0.4.3

Also updated version requirement in `Cargo.toml` from 0.4.0 to 0.4.3.

remove enum name from ImplSource variants

This is quite a lot cleaner in my opinion.

update Miri

Fixes rust-lang#77130

…tion-error, r=jyn514

Removing erroneous semicolon in transmute documentation

There is a semicolon in the example code that causes the expected value to not be returned.