Most of the changes are cleanup facilitated by straight-forward attribute handling.
This is a minor [breaking-change] for users of `quote_stmt!` (returns `Option<P<Stmt>>` now) and some of the public methods in `Parser` (a few `Vec<Attribute>` arguments/returns were removed).

r? @nikomatsakis

closes #23327

This commit deprecates the `count`, `range` and `range_step` functions
in `iter`, in favor of range notation. To recover all existing
functionality, a new `step_by` adapter is provided directly on `ops::Range`
and `ops::RangeFrom`.

[breaking-change]

r? @alexcrichton 

Remove the unused parts of the tuple macro in libcore (for `val0` and similar removed methods). Also adjust the module docs accordingly.

closes #23327

Finally making progress on a fully native toolchain. Specifically, this makes it possible to move forward by building librustllvm on the target platform.

Closes #7950

Closes #7950

r? @alexcrichton 

cc @alexcrichton 

r? @alexcrichton or @aturon 

This still needs to somehow figure out how to avoid unstable warnings arising from the use of unstable functions. I tried to use `#[allow_internal_unstable]` but it still spits out warnings as far as I can see. @huonw (I think you implemented it) does `#[allow_internal_unstable]` not work for some reason or am I using it incorrectly?

This commit starts to organize the `std::os::$platform` modules and in the
process stabilizes some of the functionality contained within. The organization
of these modules will reflect the organization of the standard library itself
with extension traits for primitives in the same corresponding module.

The OS-specific modules will grow more functionality over time including
concrete types that are not extending functionality of other structures, and
these will either go into the closest module in `std::os::$platform` or they
will grow a new module in the hierarchy.

The following items are now stable:

* `os::{unix, windows}`
* `unix::ffi`
* `unix::ffi::OsStrExt`
* `unix::ffi::OsStrExt::{from_bytes, as_bytes, to_cstring}`
* `unix::ffi::OsString`
* `unix::ffi::OsStringExt::{from_vec, into_vec}`
* `unix::process`
* `unix::process::CommandExt`
* `unix::process::CommandExt::{uid, gid}`
* `unix::process::ExitStatusExt`
* `unix::process::ExitStatusExt::signal`
* `unix::prelude`
* `windows::ffi`
* `windows::ffi::OsStringExt`
* `windows::ffi::OsStringExt::from_wide`
* `windows::ffi::OsStrExt`
* `windows::ffi::OsStrExt::encode_wide`
* `windows::prelude`

The following items remain unstable:

* `unix::io`
* `unix::io::{Fd, AsRawFd}`
* `unix::fs::{PermissionsExt, OpenOptionsExt}`
* `windows::io`
* `windows::io::{Handle, AsRawHandle}`
* `windows::io::{Socket, AsRawSocket}`
* `windows::fs`
* `windows::fs::OpenOptionsExt`

Due to the reorgnization of the platform extension modules, this commit is a
breaking change. Most imports can be fixed by adding the relevant libstd module
in the `use` path (such as `ffi` or `fs`).

[breaking-change]

This commit starts to organize the `std::os::$platform` modules and in the
process stabilizes some of the functionality contained within. The organization
of these modules will reflect the organization of the standard library itself
with extension traits for primitives in the same corresponding module.

The OS-specific modules will grow more functionality over time including
concrete types that are not extending functionality of other structures, and
these will either go into the closest module in `std::os::$platform` or they
will grow a new module in the hierarchy.

The following items are now stable:

* `os::{unix, windows}`
* `unix::ffi`
* `unix::ffi::OsStrExt`
* `unix::ffi::OsStrExt::{from_bytes, as_bytes, to_cstring}`
* `unix::ffi::OsString`
* `unix::ffi::OsStringExt::{from_vec, into_vec}`
* `unix::process`
* `unix::process::CommandExt`
* `unix::process::CommandExt::{uid, gid}`
* `unix::process::ExitStatusExt`
* `unix::process::ExitStatusExt::signal`
* `unix::prelude`
* `windows::ffi`
* `windows::ffi::OsStringExt`
* `windows::ffi::OsStringExt::from_wide`
* `windows::ffi::OsStrExt`
* `windows::ffi::OsStrExt::encode_wide`
* `windows::prelude`

The following items remain unstable:

* `unix::io`
* `unix::io::{Fd, AsRawFd}`
* `unix::fs::{PermissionsExt, OpenOptionsExt}`
* `windows::io`
* `windows::io::{Handle, AsRawHandle}`
* `windows::io::{Socket, AsRawSocket}`
* `windows::fs`
* `windows::fs::OpenOptionsExt`

Due to the reorgnization of the platform extension modules, this commit is a
breaking change. Most imports can be fixed by adding the relevant libstd module
in the `use` path (such as `ffi` or `fs`).

[breaking-change]

Remove mention of removed methods `.val0` etc.

- Successful merges: #23375, #23379, #23382, #23384
- Failed merges: 

r? @alexcrichton cc @vhbit

This changed `an String` to `a String`. Very minor change!

The usage of `an String` was introduced in https://github.com/rust-lang/rust/commit/a828e7948069f310dc5b33be8edb65e5e8e0cf9a#diff-b596503c7c33ce457b6d047e351ac12bR423, which changed `an OsString` to `an String`.

`std::io` does not currently expose the `stdin_raw`, `stdout_raw`, or
`stderr_raw` functions. According to the current plans for stdio (see
rust-lang/rfcs#517), raw access will likely be provided using the
platform-specific `std::os::{unix,windows}` modules. At the moment we
don't expose any way to do this. As such, delete all mention of the
`*_raw` functions from the `stdin`/`stdout`/`stderr` function
documentation.

While we're at it, remove a few `pub`s from items that aren't exposed.
This is done just to lessen the confusion experienced by anyone who
looks at the source in an attempt to find the `*_raw` functions.

Now that `old_io` is deprecated and `std::io` is stable, we should stop
recommending the use of `old_io` in the module documentation.

It is a frequent pattern among I/O functions to take `P: AsPath + ?Sized` or
`AsOsStr` instead of `AsPath`. Most of these functions do not need to take
ownership of their argument, but for libraries in general it's much more
ergonomic to not deal with `?Sized` at all and simply require an argument `P`
instead of `&P`.

This change is aimed at removing unsightly `?Sized` bounds while retaining the
same level of usability as before. All affected functions now take ownership of
their arguments instead of taking them by reference, but due to the forwarding
implementations of `AsOsStr` and `AsPath` all code should continue to work as it
did before.

This is strictly speaking a breaking change due to the signatures of these
functions changing, but normal idiomatic usage of these APIs should not break in
practice.

[breaking-change]

`sys/ttycom.h` in both:
`#define TIOCGWINSZ  _IOR('t', 104, struct winsize)  /* get window size */`

Super minor change!

The `an String` was introduced in https://github.com/rust-lang/rust/commit/a828e7948069f310dc5b33be8edb65e5e8e0cf9a#diff-b596503c7c33ce457b6d047e351ac12bR423, which changed `an OsString` to `an String`.

It is a frequent pattern among I/O functions to take `P: AsPath + ?Sized` or
`AsOsStr` instead of `AsPath`. Most of these functions do not need to take
ownership of their argument, but for libraries in general it's much more
ergonomic to not deal with `?Sized` at all and simply require an argument `P`
instead of `&P`.

This change is aimed at removing unsightly `?Sized` bounds while retaining the
same level of usability as before. All affected functions now take ownership of
their arguments instead of taking them by reference, but due to the forwarding
implementations of `AsOsStr` and `AsPath` all code should continue to work as it
did before.

This is strictly speaking a breaking change due to the signatures of these
functions changing, but normal idiomatic usage of these APIs should not break in
practice.

[breaking-change]

r? @Manishearth