Warn unused type aliases, reimplemented

Reimplementation of #37631. Fix #37455.

Library stabilizations/deprecations for 1.15 release

Stabilized:

- `std::iter::Iterator::{min_by, max_by}`
- `std::os::*::fs::FileExt`
- `std::sync::atomic::Atomic*::{get_mut, into_inner}`
- `std::vec::IntoIter::{as_slice, as_mut_slice}`
- `std::sync::mpsc::Receiver::try_iter`
- `std::os::unix::process::CommandExt::before_exec`
- `std::rc::Rc::{strong_count, weak_count}`
- `std::sync::Arc::{strong_count, weak_count}`
- `std::char::{encode_utf8, encode_utf16}`
- `std::cell::Ref::clone`
- `std::io::Take::into_inner`

Deprecated:

- `std::rc::Rc::{would_unwrap, is_unique}`
- `std::cell::RefCell::borrow_state`

Closes #23755
Closes #27733
Closes #27746
Closes #27784
Closes #28356
Closes #31398
Closes #34931
Closes #35601
Closes #35603
Closes #35918
Closes #36105

struct field reordering and optimization

This is work in progress.  The goal is to divorce the order of fields in source code from the order of fields in the LLVM IR, then optimize structs (and tuples/enum variants)by always ordering fields from least to most aligned.  It does not work yet.  I intend to check compiler memory usage as a benchmark, and a crater run will probably be required.

I don't know enough of the compiler to complete this work unaided.  If you see places that still need updating, please mention them.  The only one I know of currently is debuginfo, which I'm putting off intentionally until a bit later.

r? @EddyB

rustbuild: Fix LC_ID_DYLIB directives on OSX

Currently libraries installed by rustbuild on OSX have an incorrect
`LC_ID_DYLIB` directive located in the dynamic libraries that are
installed. The directive we expect looks like:

    @rpath/libstd.dylib

Which means that if you want to find that dynamic library you should
look at the dylib's other `@rpath` directives. Typically our `@rpath`
directives look like `@loader_path/../lib` for the compiler as that's
where the installed libraries will be located. Currently, though,
rustbuild produces dylibs with the directive that looks like:

    /Users/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-mac/build/build/x86_64-apple-darwin/stage1-std/x86_64-apple-darwin/release/deps/libstd-713ad88203512705.dylib

In other words, the build directory is encoded erroneously. The compiler
already [knows how] to change this directive, but it only passes that
argument when `-C rpath` is also passed. The rustbuild system, however,
explicitly [does not pass] this option explicitly and instead bakes its
own. This logic then also erroneously didn't pass `-Wl,-install_name`
like the compiler.

[knows how]: https://github.com/rust-lang/rust/blob/4a008cccaabc8b3fe65ccf5868b9d16319c9ac58/src/librustc_trans/back/linker.rs#L210-L214
[does not pass]: https://github.com/rust-lang/rust/blob/4a008cccaabc8b3fe65ccf5868b9d16319c9ac58/src/bootstrap/bin/rustc.rs#L133-L158

To fix this regression this patch introduces a new `-Z` flag, `-Z
osx-rpath-install-name` which basically just forces the compiler to take
the previous `-install_name` branch when creating a dynamic library.
Hopefully we can sort out a better rpath story in the future, but for
now this "hack" should suffice in getting our nightly builds back to the
same state as before.

Closes #38430

rustbuild: Create directories in mingw dist

Previously we accidentally relied on the mingw dist step running last, but the
step just needed to ensure the directories were created.

Currently libraries installed by rustbuild on OSX have an incorrect
`LC_ID_DYLIB` directive located in the dynamic libraries that are
installed. The directive we expect looks like:

    @rpath/libstd.dylib

Which means that if you want to find that dynamic library you should
look at the dylib's other `@rpath` directives. Typically our `@rpath`
directives look like `@loader_path/../lib` for the compiler as that's
where the installed libraries will be located. Currently, though,
rustbuild produces dylibs with the directive that looks like:

    /Users/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-mac/build/build/x86_64-apple-darwin/stage1-std/x86_64-apple-darwin/release/deps/libstd-713ad88203512705.dylib

In other words, the build directory is encoded erroneously. The compiler
already [knows how] to change this directive, but it only passes that
argument when `-C rpath` is also passed. The rustbuild system, however,
explicitly [does not pass] this option explicitly and instead bakes its
own. This logic then also erroneously didn't pass `-Wl,-install_name`
like the compiler.

[knows how]: https://github.com/rust-lang/rust/blob/4a008cccaabc8b3fe65ccf5868b9d16319c9ac58/src/librustc_trans/back/linker.rs#L210-L214
[does not pass]: https://github.com/rust-lang/rust/blob/4a008cccaabc8b3fe65ccf5868b9d16319c9ac58/src/bootstrap/bin/rustc.rs#L133-L158

To fix this regression this patch introduces a new `-Z` flag, `-Z
osx-rpath-install-name` which basically just forces the compiler to take
the previous `-install_name` branch when creating a dynamic library.
Hopefully we can sort out a better rpath story in the future, but for
now this "hack" should suffice in getting our nightly builds back to the
same state as before.

Closes #38430

Previously we accidentally relied on the mingw dist step running last, but the
step just needed to ensure the directories were created.

macros: allow a `path` fragment to be parsed as a type parameter bound

Allow a `path` fragment to be parsed as a type parameter bound.
Fixes #8521.

Update book for rustup

Supersedes https://github.com/rust-lang/rust/pull/37934

Don't land yet. Needs coordination with https://github.com/rust-lang/rust-www/pull/621

Fixes https://github.com/rust-lang/rust/issues/35653

r? @steveklabnik

travis: Fix testing 32-bit OSX target

We passed --target when we meant to pass --build, meaning we tested only the
standard library for 32-bit, not the whole compiler like we intended.

macros: fix the expected paths for a non-inline module matched by an `item` fragment

Fixes #38190.
r? @nrc

We passed --target when we meant to pass --build, meaning we tested only the
standard library for 32-bit, not the whole compiler like we intended.

Fixes #35653

cc https://github.com/rust-lang/rust-www/pull/621

rustbuild: Add sccache support

This commit adds support for sccache, a ccache-like compiler which works on MSVC
and stores results into an S3 bucket. This also switches over all Travis and
AppVeyor automation to using sccache to ensure a shared and unified cache over
time which can be shared across builders.

The support for sccache manifests as a new `--enable-sccache` option which
instructs us to configure LLVM differently to use a 'sccache' binary instead of
a 'ccache' binary. All docker images for Travis builds are updated to download
Mozilla's tooltool builds of sccache onto various containers and systems.
Additionally a new `rust-lang-ci-sccache` bucket is configured to hold all of
our ccache goodies.

---

Note that this does not currently change Windows [due to previously written up issues](https://github.com/rust-lang/rust/issues/38119#issuecomment-266631585). Despite that, however, I was curious to get timings for the builds on Travis to see what ranges we're working with. As a result, this is a WIP PR I'm using to gauge build times and such.

make deterministic_hash host-architecture-independent

`DefPath::deterministic_hash` used to call `std::hash::Hash`, which depends on the current architecture in several ways, which would prevent metadata written on one host architecture from being successfully read on another one.

Use a hasher we control instead.

Fixes #38177.

r? @michaelwoerister

Fix unaligned loads in librustc_metadata

Creating a `&[u32]` from an `&[u8]` is not necessarily valid, and crashes
on certain platforms if the data is not well aligned.

rustbuild: Package rust-mingw by default

This fixes the `make dist` step on MinGW to package the `rust-mingw` component
by default. This should hopefully be the last step in fixing nightlies.

- `std::rc::Rc::{strong_count, weak_count}`
- `std::sync::Arc::{strong_count, weak_count}`

Deprecate:

- `std::rc::Rc::{would_unwrap, is_unique}`

The standard implementations of Hasher have architecture-dependent
results when hashing integers. This causes problems when the hashes are
stored within metadata - metadata written by one host architecture can't
be read by another.

To fix that, implement an architecture-independent StableHasher and use
it in all places an architecture-independent hasher is needed.

Fixes #38177.

This fixes the `make dist` step on MinGW to package the `rust-mingw` component
by default. This should hopefully be the last step in fixing nightlies.

rustbuild: Add cli option --keep-stage

This option is intended to be used like:

./x.py build --stage 1 --keep-stage 0

Which skips all stage 0 steps, so that stage 1 can be recompiled
directly (even if for example libcore has changes).

This is useful when working on `cfg(not(stage0))` parts of the
libraries or when re-running stage 1 tests in libraries in general.

Fixes #38326

This code was introduced in #27565 to mark types in paths alive. It is now unnecessary since #37676.