Fixes #18787

Fixes #22553

Fixes #22610

Fixes #22637

Fixes #23052

Fixes #22721.

 * "let met text" was previously of &String type.  Now it is of &str type.

 * Update the slicing syntax.  Both &text[] and text.slice_from() evaluate
   to a &str.

 * We were passing a u32 to expr_usize.  Call expr_u32 instead.

The previous code was passing "true" as the panic! error value.

We require the *deferred* loading, not just an opportunistic
asynchronous loading. I think `<script defer>` is safe to use,
according to <http://caniuse.com/#feat=script-defer>.

follow freebsd due to last deprecation of std::old_io::fs

This is a hack, but I don't think we can do much better as long as `derive` is
running at the syntax expansion phase.

If the custom_derive feature gate is enabled, this works with user-defined
traits and syntax extensions. Without the gate, you can't use e.g. #[derive_Clone]
directly, so this does not change the stable language.

This commit also cleans up the deriving code somewhat, and forbids some
previously-meaningless attribute syntax. For this reason it's technically a

    [breaking-change]

This is important because attributes can affect expansion.

These have been deprecated for quite some time, so we should be good to remove
them now.

This test is known to fail on windows.

Right now the rust upgrade in cargo is blocked on fixing this overflow. If a
this benchmark is run it will trigger an overflow error today:

    #[bench]
    fn foo(b: &mut test::Bencher) {}

This commit adds a check on each iteration of the loop that the maximum
multiplier (10) doesn't overflow, and if it does just return the results so far.

The `std::net` primitives should be ready for use now and as a result the old
ones are now deprecated and slated for removal. Most TCP/UDP functionality is
now available through `std::net` but the `std::old_io::net::pipe` module is
removed entirely from the standard library.

Unix socket funtionality can be found in sfackler's [`unix_socket`][unix] crate
and there is currently no replacement for named pipes on Windows.

[unix]: https://crates.io/crates/unix_socket

[breaking-change]

fixes #23080

Being a person who somehow has taken a liking to premature optimisation, my knee-jerk reaction to
locking in std handles was preamble resembling following snippet:

    let stdout = stdout();
    let lstdout = stdout.lock();
    let stdin = stdin();
    let lstdin = stdin.lock();

and then reading from the locked handle like this:

    let mut letter = [0; 1];
    lstdin.read(&mut letter).unwrap();

As it is now this code will deadlock because the `read` method attempts to lock stdout as well!

Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>

Fixes #23059

I was having trouble figuring out which functions had tests and which
didn't. This commit is just moving tests around and does not change
anything.

The `rsplitn` call was called with 2 instead of 1 so the iterator would yield 3
items in some cases, not the 2 that it should have.

Closes #23076

This commit performs a stabilization pass over the `std::fs` module now that
it's had some time to bake. The change was largely just adding `#[stable]` tags,
but there are a few APIs that remain `#[unstable]`.

The following apis are now marked `#[stable]`:

* `std::fs` (the name)
* `File`
* `Metadata`
* `ReadDir`
* `DirEntry`
* `OpenOptions`
* `Permissions`
* `File::{open, create}`
* `File::{sync_all, sync_data}`
* `File::set_len`
* `File::metadata`
* Trait implementations for `File` and `&File`
* `OpenOptions::new`
* `OpenOptions::{read, write, append, truncate, create}`
* `OpenOptions::open` - this function was modified, however, to not attempt to
  reject cross-platform openings of directories. This means that some platforms
  will succeed in opening a directory and others will fail.
* `Metadata::{is_dir, is_file, len, permissions}`
* `Permissions::{readonly, set_readonly}`
* `Iterator for ReadDir`
* `DirEntry::path`
* `remove_file` - like with `OpenOptions::open`, the extra windows code to
  remove a readonly file has been removed. This means that removing a readonly
  file will succeed on some platforms but fail on others.
* `metadata`
* `rename`
* `copy`
* `hard_link`
* `soft_link`
* `read_link`
* `create_dir`
* `create_dir_all`
* `remove_dir`
* `remove_dir_all`
* `read_dir`

The following apis remain `#[unstable]`.

* `WalkDir` and `walk` - there are many methods by which a directory walk can be
  constructed, and it's unclear whether the current semantics are the right
  ones. For example symlinks are not handled super well currently. This is now
  behind a new `fs_walk` feature.
* `File::path` - this is an extra abstraction which the standard library
  provides on top of what the system offers and it's unclear whether we should
  be doing so. This is now behind a new `file_path` feature.
* `Metadata::{accessed, modified}` - we do not currently have a good
  abstraction for a moment in time which is what these APIs should likely be
  returning, so these remain `#[unstable]` for now. These are now behind a new
  `fs_time` feature
* `set_file_times` - like with `Metadata::accessed`, we do not currently have
  the appropriate abstraction for the arguments here so this API remains
  unstable behind the `fs_time` feature gate.
* `PathExt` - the precise set of methods on this trait may change over time and
  some methods may be removed. This API remains unstable behind the `path_ext`
  feature gate.
* `set_permissions` - we may wish to expose a more granular ability to set the
  permissions on a file instead of just a blanket "set all permissions" method.
  This function remains behind the `fs` feature.

The following apis are now `#[deprecated]`

* The `TempDir` type is now entirely deprecated and is [located on
  crates.io][tempdir] as the `tempdir` crate with [its source][github] at
  rust-lang/tempdir.

[tempdir]: https://crates.io/crates/tempdir
[github]: https://github.com/rust-lang/tempdir

The stability of some of these APIs has been questioned over the past few weeks
in using these APIs, and it is intentional that the majority of APIs here are
marked `#[stable]`. The `std::fs` module has a lot of room to grow and the
material is [being tracked in a RFC issue][rfc-issue].

[rfc-issue]: https://github.com/rust-lang/rfcs/issues/939

[breaking-change]