This allows to parallel-install several versions of rust system-wide
Fixes #48263
Signed-off-by: NMarc-Antoine Perennou <Marc-Antoine@Perennou.com>

The *.md at the root directory in src/doc are no longer tested, but this
should be fine since all files there are deprecated.

This re-introduces a `Config.libdir_relative` field, now derived from
`libdir` and made relative to `prefix` if necessary.

This fixes a regression from #46592 when `--libdir` is given an absolute
path.  `Builder::sysroot_libdir` should always use a relative path so
its callers don't clobber system locations, and `librustc` also asserts
that `CFG_LIBDIR_RELATIVE` is really relative.

We'd previously assumed that these paths would be relative to the src
dir, and that for example our various CI scripts would, when calling
x.py, use `../x.py build ../src/tools/...` but this isn't the case --
they use `../x.py` without using the relevant source-relative path.

We eventually may want to make this (actually somewhat logical) change,
but this is not that time.

Primarily for CI purposes; this is intended to avoid cases where we
update rustbuild and unintentionally make CI stop running some builds to
the arguments being passed no longer applying for some reason.

This reverts commit 64a8730e.

Some Steps are by-default run but don't have any paths associated with
them. We need to have at least one PathSet per each Step, though, so we
add an empty one on calls to `never()`.

Previously, a Step would be able to tell on its own when it was invoked
"by-default" (that is, `./x.py test` was called instead of `./x.py test
some/path`). This commit replaces that functionality, invoking each Step
with each of the paths it has specified as "should be invoked by."

For example, if a step calls `path("src/tools/cargo")` and
`path("src/doc/cargo")` then it's make_run will be called twice, with
"src/tools/cargo" and "src/doc/cargo." This makes it so that default
handling logic is in builder, instead of spread across various Steps.

However, this meant that some Step specifications needed to be updated,
since for example `rustdoc` can be built by `./x.py build
src/librustdoc` or `./x.py build src/tools/rustdoc`. A `PathSet`
abstraction is added that handles this: now, each Step can not only list
`path(...)` but also `paths(&[a, b, ...])` which will make it so that we
don't invoke it with each of the individual paths, instead invoking it
with the first path in the list (though this shouldn't be depended on).

Future work likely consists of implementing a better/easier way for a
given Step to work with "any" crate in-tree, especially those that want
to run tests, build, or check crates in the std, test, or rustc crate
trees. Currently this is rather painful to do as most of the logic is
duplicated across should_run and make_run. It seems likely this can be
abstracted away into builder somehow.

Right now the ccache setting is only used for LLVM, but this tweaks it to also
be used for build scripts so C++ builds like `librustc_llvm` can be a bit
speedier.

Dist builds should always be as fast as we can make them, and since
those run on CI we don't care quite as much for the build being somewhat
slower. As such, we don't automatically enable ThinLTO on builds for the
dist builders.

Right now this directory is located under:

  $sysroot/lib/rustlib/$target/lib/codegen-backends

but after seeing what we do in a few other places it seems that a more
appropriate location would be:

  $sysroot/lib/rustlib/$target/codegen-backends

so this commit moves it!

Fixes #47658.

This currently only supports a limited subset of the full compilation,
but is likely 90% of what people will want and is possible without
building a full compiler (i.e., running LLVM). In theory, this means
that contributors who don't want to build LLVM now have an easy way to
compile locally, though running tests won't work.

This'll probably have a bunch of build errors, so let's try and head those off
and find them sooner rather than later!

These no longer work now that Cargo changes the cwd of rustc while it's running.
Instead use an absolute path that's set by rustbuild.

* Bump the release version to 1.25
* Bump the bootstrap compiler to the recent beta
* Allow using unstable rustdoc features on beta - this fix has been applied to
  the beta branch but needed to go to the master branch as well.

This commit updates LLVM to fix #45511 (https://reviews.llvm.org/D39981) and
also reenables ThinLTO for libtest now that we shouldn't hit #45768. This also
opportunistically enables ThinLTO for libstd which was previously blocked
(#45661) on test failures related to debuginfo with a presumed cause of #45511.

Closes #45511

Signed-off-by: NMarc-Antoine Perennou <Marc-Antoine@Perennou.com>

Right now ThinLTO is generating bad dwarf which is tracked by #45511, but this
is causing issues on OSX (#45768) where `dsymutil` is segfaulting and failing to
produce output.

Closes #45768

Now that the upstream LLVM bug is now fixed this commit cherry-picks the commit
onto our LLVM and then re-enables the ThinLTO paths for MIPS.

Closes #45654

Discovered in #45529 it looks like cross-module TLS imports aren't quite working
today, especially with `hidden` visibility which mostly comes up with multiple
codegen units. As a result this completely disables compiling with ThinLTO and
multiple codegen units on MIPS when bootstrapping.

cc #45654, the tracking issue for this

This commit enables ThinLTO for the compiler as well as multiple codegen units.
This is intended to get the benefits of parallel codegen while also avoiding
any major loss of perf. Finally this commit is also intended as further testing
for #45320 and shaking out bugs.

We copy built tool binaries into a dedicated directory to avoid deleting
them, stageN-tools-bin. These aren't ever cleared out by code, since
there should be no reason to do so, and we'll simply overwrite them as
necessary.

When clearing out the stageN-{std,rustc,tools} directories, make sure to
delete both Cargo directories -- per-target and build scripts. This
ensures that changing libstd doesn't cause problems due to build scripts
not being rebuilt, even though they should be.