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]

Unstable items used in a macro expansion will now always trigger
stability warnings, *unless* the unstable items are directly inside a
macro marked with `#[allow_internal_unstable]`. IOW, the compiler warns
unless the span of the unstable item is a subspan of the definition of a
macro marked with that attribute.

E.g.

    #[allow_internal_unstable]
    macro_rules! foo {
        ($e: expr) => {{
            $e;
            unstable(); // no warning
            only_called_by_foo!();
        }}
    }

    macro_rules! only_called_by_foo {
        () => { unstable() } // warning
    }

    foo!(unstable()) // warning

The unstable inside `foo` is fine, due to the attribute. But the
`unstable` inside `only_called_by_foo` is not, since that macro doesn't
have the attribute, and the `unstable` passed into `foo` is also not
fine since it isn't contained in the macro itself (that is, even though
it is only used directly in the macro).

In the process this makes the stability tracking much more precise,
e.g. previously `println!("{}", unstable())` got no warning, but now it
does. As such, this is a bug fix that may cause [breaking-change]s.

The attribute is definitely feature gated, since it explicitly allows
side-stepping the feature gating system.

The API this exposes is a little strange (being attached to `static`s),
so it makes sense to conservatively feature gate it. If it is highly
popular, it is possible to reverse this gating.

Closes #22698

Fixes #22424.

Update the reference page to the change in #20680.

Fixes #22152

The Rust Reference should include the tuple indexing (using a number
as a field) notation; currently it is only available on
http://doc.rust-lang.org/std/primitive.tuple.html and not easily
searchable.

Fixes #22386

book/syntax-extensions.html was renamed to book/plugins.html,
the link should be also updated.
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>

Fixes #19278

Fixes #14849

This isn't something we want to guarantee.

Added a second note about keeping the reference.md list up-to-date to
the bottom of the list, since not everyone (including me) reads the
big comment at the top of it.  :)

Ensured that the feature gate list in reference.md is kept in
alphabetical order.

Fixes #22136

Replaced outdated use of the `range(start, end)` function where
approriate with `start..end`, and tweaked the examples to compile and run with the latest rust. I also fixed two periphery compile issues in reference.md which were occluding whether there were any new errors created by these changes, so I fixed them.

Fixes #20172

    #[plugin] #[no_link] extern crate bleh;

becomes a crate attribute

    #![plugin(bleh)]

The feature gate is still required.

It's almost never correct to link a plugin into the resulting library /
executable, because it will bring all of libsyntax and librustc with it.
However if you really want this behavior, you can get it with a separate
`extern crate` item in addition to the `plugin` attribute.

Fixes #21043.
Fixes #20769.

[breaking-change]

Update reference.md:
 - derive() no longer supports Zero trait
 - derive() now supports Copy trait

This was correct in the EBNF, but not in the prose (which seems to have
been copied-and-pasted from regular string literals).

Fixes #21833.

[breaking-change]

This was particularly helpful in the time just after OIBIT's
implementation to make sure things that were supposed to be Copy
continued to be, but it's now creates a lot of noise for types that
intentionally don't want to be Copy.

As per rust-lang/rfcs#601, replace `be` with `become` as reserved
keyword for tail call optimization.

Unicode escapes were changed in [this
RFC](https://github.com/rust-lang/rfcs/blob/28aeb3c391c9afd344f124d3a69bdc2a420638b2/text/0446-es6-unicode-escapes.md)
to use the ES6 \u{00FFFF} syntax with a variable number of digits from
1-6, eliminating the need for two different syntaxes for unicode
literals.

Also explain integer fallback to `i32`.