Emits loc, and node count - before and after expansion.

E.g.,

```
rustc: x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore
Lines of code:             32060
Pre-expansion node count:  120205
Post-expansion node count: 482749
```

Currently the compiler supports the ability to emit multiple output types as
part of one compilation (e.g. asm, LLVM IR, bytecode, link, dep-info, etc). It
does not, however, support the ability to customize the output filename for each
of these output types. The `-o` flag is ignored if multiple emit types are
specified (and the compiler emits a warning about this).

Normally this doesn't matter too much, but in the case of `dep-info` it can lead
to a number of problems (e.g. see #28716). By allowing customization of the
output filename for each emit type we're able to solve the problems in that
issue.

This commit adds support for the `--emit` option to the compiler to look like:

    rustc foo.rs --emit dep-info=.deps/foo.d,link

This indicates that the `dep-info` output type will be placed at `.deps/foo.d`
and the `link` output type will otherwise be determined via the `--out-dir` and
`-o` flags.

Closes #28716

This adds a new target property, `target_vendor` which can be used as a
matcher for conditional compilation. The vendor is part of the autoconf
target triple: <arch><sub>-<vendor>-<os>-<env>

The default value for `target_vendor` is "unknown".

Matching against the `target_vendor` with `#[cfg]` is currently feature
gated as `cfg_target_vendor`.

This is so that the resolution results can be used after analysis, potentially for tool support

This could be a [breaking-change] if your lint or syntax extension (is that even possible?) uses HIR attributes or literals.

Currently, `early_error` and `early_warn` in `librustc::session` always
use `ColorConfig::Auto`. Modify them to follow the color configuration
set by the `--color` option.

As colored output is also printed during the early stage, parsing the
`--color` option should be done as early as possible. However, there are
still some cases when the output needs to be colored before knowing the
exact color settings. In these cases, it will be defaulted to
`ColorConfig::Auto`, which is the same as before.

Fixes #27879.

This is a temporary workaround for the bugs that have been found in
the implementation of PR #26173.

 * pnkfelix is unavailable in the short-term (i.e. for the next week) to fix them.

 * When the bugs are fixed, we will turn this back on by default.

(If you want to play with the known-to-be-buggy optimization in the
meantime, you can opt-back in via the debugging option that this
commit is toggling.)

This is a temporary workaround for the bugs that have been found in
the implementation of PR #26173.

 * pnkfelix is unavailable in the short-term (i.e. for the next week) to fix them.

 * When the bugs are fixed, we will turn this back on by default.

(If you want to play with the known-to-be-buggy optimization in the
meantime, you can opt-back in via the debugging option that this
commit is toggling.)

(Inspired by discussion with Gankro.)

This commit is an implementation of [RFC 1193][rfc] which adds the ability to
the compiler to cap the lint level for the entire compilation session. This flag
will ensure that no lints will go above this level, and Cargo will primarily use
this flag passing `--cap-lints allow` to all upstream dependencies.

[rfc]: https://github.com/rust-lang/rfcs/pull/1193

Closes #27259

(already thumbs-upped pre-rebase by nikomatsakis)

It now says '#[feature] may not be used on the stable release channel'.

I had to convert this error from a lint to a normal compiler error.

I left the lint previously-used for this in place since removing it is
a breaking change. It will just go unused until the end of time.

Fixes #24125

Conflicts:
	mk/platform.mk
	src/librustc/session/config.rs
	src/librustc_back/target/aarch64_apple_ios.rs
	src/librustc_back/target/aarch64_linux_android.rs
	src/librustc_back/target/arm_linux_androideabi.rs
	src/librustc_back/target/arm_unknown_linux_gnueabi.rs
	src/librustc_back/target/arm_unknown_linux_gnueabihf.rs
	src/librustc_back/target/armv7_apple_ios.rs
	src/librustc_back/target/armv7s_apple_ios.rs
	src/librustc_back/target/i386_apple_ios.rs
	src/librustc_back/target/i686_apple_darwin.rs
	src/librustc_back/target/i686_pc_windows_gnu.rs
	src/librustc_back/target/i686_unknown_dragonfly.rs
	src/librustc_back/target/i686_unknown_linux_gnu.rs
	src/librustc_back/target/mips_unknown_linux_gnu.rs
	src/librustc_back/target/mipsel_unknown_linux_gnu.rs
	src/librustc_back/target/mod.rs
	src/librustc_back/target/powerpc_unknown_linux_gnu.rs
	src/librustc_back/target/x86_64_apple_darwin.rs
	src/librustc_back/target/x86_64_apple_ios.rs
	src/librustc_back/target/x86_64_pc_windows_gnu.rs
	src/librustc_back/target/x86_64_unknown_dragonfly.rs
	src/librustc_back/target/x86_64_unknown_freebsd.rs
	src/librustc_back/target/x86_64_unknown_linux_gnu.rs
	src/librustc_back/target/x86_64_unknown_openbsd.rs
	src/librustc_llvm/lib.rs
	src/librustc_trans/back/link.rs
	src/librustc_trans/trans/base.rs
	src/libstd/os.rs
	src/rustllvm/RustWrapper.cpp

This adds a new `#[cfg]` matcher against the `target_env` property of the
destination target triple. For example all windows triples today end with `-gnu`
but we will also hopefully support non-`gnu` targets for Windows, at which point
we'll need to differentiate between the two. This new `target_env` matches is
provided and filled in with the target's environment name.

Currently the only non-empty value of this name is `gnu`, but `musl` will be
shortly added for the linux triples.

Changes the style guidelines regarding unit tests to recommend using a
sub-module named "tests" instead of "test" for unit tests as "test"
might clash with imports of libtest.

For now, words() is left in (but deprecated), and Words is a type alias for
struct SplitWhitespace.

Also cleaned up references to s.words() throughout codebase.

Closes #15628

`s/(?<!\{ self)(?<=\.)len\(\) == 0/is_empty()/g`

Fixes #22619

This commit is an implementation of [RFC 979][rfc] which changes the meaning of
the count parameter to the `splitn` function on strings and slices. The
parameter now means the number of items that are returned from the iterator, not
the number of splits that are made.

[rfc]: https://github.com/rust-lang/rfcs/pull/979

Closes #23911
[breaking-change]

This syntax has been renamed to `-l static=foo` some time ago.

Now that support has been removed, all lingering use cases are renamed.

(That is, added config and debugflag a la check-overflow but for drop
flag sanity-check.)

Remove now-unused import of NoDebugInfo from trans::glue.

This commit:

* Introduces `std::convert`, providing an implementation of
RFC 529.

* Deprecates the `AsPath`, `AsOsStr`, and `IntoBytes` traits, all
in favor of the corresponding generic conversion traits.

  Consequently, various IO APIs now take `AsRef<Path>` rather than
`AsPath`, and so on. Since the types provided by `std` implement both
traits, this should cause relatively little breakage.

* Deprecates many `from_foo` constructors in favor of `from`.

* Changes `PathBuf::new` to take no argument (creating an empty buffer,
  as per convention). The previous behavior is now available as
  `PathBuf::from`.

* De-stabilizes `IntoCow`. It's not clear whether we need this separate trait.

Closes #22751
Closes #14433

[breaking-change]

This commit is an implementation of [RFC 563][rfc] which adds a new
`cfg(debug_assertions)` directive which is specially recognized and calculated
by the compiler. The flag is turned off at any optimization level greater than 1
and may also be explicitly controlled through the `-C debug-assertions`
flag.

[rfc]: https://github.com/rust-lang/rfcs/pull/563

The `debug_assert!` and `debug_assert_eq!` macros now respect this instead of
the `ndebug` variable and `ndebug` no longer holds any meaning to the standard
library.

Code which was previously relying on `not(ndebug)` to gate expensive code should
be updated to rely on `debug_assertions` instead.

Closes #22492
[breaking-change]

This commit deprecates the majority of std::old_io::fs in favor of std::fs and
its new functionality. Some functions remain non-deprecated but are now behind a
feature gate called `old_fs`. These functions will be deprecated once
suitable replacements have been implemented.

The compiler has been migrated to new `std::fs` and `std::path` APIs where
appropriate as part of this change.

During my clean-up of rebase errors, I took the opportunity to implement
parse_opt_bool so that it isn't identical to parse_bool wrapped in
`Some`.

parse_opt_bool considers no value to be true, a value of 'y', 'yes' or
'on' to be true and 'n', 'no' or 'off' to be false. All other values are
an error.

Adds overflow checking to integer addition, multiplication, and subtraction
when `-Z force-overflow-checks` is true, or if `--cfg ndebug` is not passed to
the compiler. On overflow, it panics with `arithmetic operation overflowed`.
Also adds `overflowing_add`, `overflowing_sub`, and `overflowing_mul`
intrinsics for doing unchecked arithmetic.

[breaking-change]