add preliminary support for incremental compilation to rustbuild.py

This implements the integration described in #37929. It requires the use of a local nightly as your bootstrap compiler. The setup is described in `src/bootstrap/README.md`.

This does NOT implement the "copy stage0 libs to stage1" optimization described in #37929, just because that seems orthogonal to me.

In local testing, I do not yet see any incremental re-use when building rustc. I'm not sure why that is, more investigation needed.

(For these reasons, this is not marked as fixing the relevant issue.)

r? @alexcrichton -- I included one random cleanup (`Step::noop()`) that turned out to not be especially relevant. Feel free to tell me you liked it better the old way.

For example:

- we now support `-vv` to get very verbose output.
- RUSTFLAGS is respected by `x.py`
- better error messages for some cases

rustbuild: Fix `copy` helper with existing files

This erroneously truncated files when the destination already existed and was an
existing hard link to the source. This in turn caused weird bugs!

Closes #37745

Rollup of 9 pull requests

- Successful merges: #38334, #38397, #38413, #38421, #38422, #38433, #38438, #38445, #38459
- Failed merges:

Use exec for the wrapper on UNIXes

This not only avoids the small – and unnecessary – constant overhead for each compiler invocation,
but also helps somewhat by only having “correct” rustc processes to look for in `/proc/`.

This also makes the wrapper behave effectively as a regular exec wrapper its intended to be.

I also took liberty to change the fallback error code from `1` to `0xfe` (now only relevant on windows) so that when people complain about “compiler exited with code 254”, its obvious where the issue lies (wrapper losing the exit code somehow).

r? @alexcrichton

stage0.txt: typo fix

Update beta bootstrap compiler

The current beta that rustc is bootstrapping from contains a bug in Cargo that
erroneously links to OpenSSL in /usr/local, but this is fixed in the most recent
1.14 beta, so let's use that.

Thread docs

r? @frewscvx

tidy features: use 2-parameter form of internal try macro for open err

This tiny patch merely applies @bluss's suggestion for how to get a more
informative error message when the feature check can't open a file, a
matter that had briefly annoyed the present author, leading to the
filing of #38417.

Resolves #38417.

Replace invalid use of `&mut` with `UnsafeCell` in `std::sync::mpsc`

Closes #36934

r? @alexcrichton

rustc: Disable NEON on armv7 android.

We thought Google's ABI for arvm7 required neon, but it is
currently optional, perhaps because there is a significant
population of Tegra 2 devices still in use.

This turns off neon code generation outside #[target-feature]
blocks just like we do on armv7-unknown-linux-gnu, but unlike
most other armv7 targets. LLVM defaults to +neon for this target,
so an explicit disable is necessary.

See https://developer.android.com/ndk/guides/abis.html#v7a
for instruction set extension requirements.

Closes #38402.

Document platform-specific differences for `std::process::exit`.

Fixes https://github.com/rust-lang/rust/issues/35046.

Rewrite, improve documentation for `core::hash::BuildHasherDefault`.

Fixes https://github.com/rust-lang/rust/issues/31242.

llvm: backport r280651

fixes #38406

r? @alexcrichton

(I'm secretly hoping this will error in the same way as #38314. That would sort of confirm that the problem is OOM and not enabling the SPARC backend)

Don't perform span mangling when building field/tup access nodes

There are no guarantees that the two spans used to create the new one
come from the same place or are even valid.

Fixes #36081.

Require `#[proc_macro_derive]` functions to be `pub`

r? @nrc

 Warn unused type aliases, reimplemented

Reimplementation of #37631. Fix #37455.

This not only avoids the small – and unnecessary – constant overhead for each compiler invocation,
but also helps somewhat by only having “correct” rustc processes to look for in `/proc/`.

This also makes the wrapper behave effectively as a regular exec wrapper its intended to be.

Fixes https://github.com/rust-lang/rust/issues/35046.

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.

The current beta that rustc is bootstrapping from contains a bug in Cargo that
erroneously links to OpenSSL in /usr/local, but this is fixed in the most recent
1.14 beta, so let's use that.

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

This tiny patch merely applies @bluss's suggestion for how to get a more
informative error message when the feature check can't open a file, a
matter that had briefly annoyed the present author, leading to the
filing of #38417.

Closes #36934