Most of the Rust community agrees that the vec! macro is clearer when
called using square brackets [] instead of regular brackets (). Most of
these ocurrences are from before macros allowed using different types of
brackets.

There is one left unchanged in a pretty-print test, as the pretty
printer still wants it to have regular brackets.

It's now passing again.

Fixes #20673

macro_rules! is like an item that defines a macro.  Other items don't have a
trailing semicolon, or use a paren-delimited body.

If there's an argument for matching the invocation syntax, e.g. parentheses for
an expr macro, then I think that applies more strongly to the *inner*
delimiters on the LHS, wrapping the individual argument patterns.

followed by a semicolon.

This allows code like `vec![1i, 2, 3].len();` to work.

This breaks code that uses macros as statements without putting
semicolons after them, such as:

    fn main() {
        ...
        assert!(a == b)
        assert!(c == d)
        println(...);
    }

It also breaks code that uses macros as items without semicolons:

    local_data_key!(foo)

    fn main() {
        println("hello world")
    }

Add semicolons to fix this code. Those two examples can be fixed as
follows:

    fn main() {
        ...
        assert!(a == b);
        assert!(c == d);
        println(...);
    }

    local_data_key!(foo);

    fn main() {
        println("hello world")
    }

RFC #378.

Closes #18635.

[breaking-change]

This breaks code that referred to variant names in the same namespace as
their enum. Reexport the variants in the old location or alter code to
refer to the new locations:

```
pub enum Foo {
    A,
    B
}

fn main() {
    let a = A;
}
```
=>
```
pub use self::Foo::{A, B};

pub enum Foo {
    A,
    B
}

fn main() {
    let a = A;
}
```
or
```
pub enum Foo {
    A,
    B
}

fn main() {
    let a = Foo::A;
}
```

[breaking-change]

[breaking-change]

[breaking-change]

(And fix some tests.)

`~[T]` in test, libgetopts, compiletest, librustdoc, and libnum.