Emacs mode indentation is incorrect in several common cases

[nikomatsakis]

There are several common cases where the emacs mode gets the indentation incorrect. I'll list the examples as I believe they should be formatted.

1. One indent after open paren with no argument:

call_to_func(
    a,
    b,
    c)

1. Align -> with arguments:

fn a_func_decl(a: int,
               b: int)
               -> int {
}

1. Align field names:

foo(SomeStructName { a: int,
                     b: int })

foo(SomeStructName {a: int,
                    b: int})

[nikomatsakis]

Oh, and the motivating example for filing the bug: comments don't work quite right.

/*!
 * Foo
 * Bar
 */ 

and also you should be able to hit M-q and have it reflow correctly.

[huonw]

I've found trailing line comments breaks the indentation of the next line:

fn main() {
    println("foo"); // bar
        println("baz");
    println("qux");
}

(Not sure if this is worth a separate bug.)

[nikomatsakis]

Another example is a pattern relative to an indented match:

let x =
    match expr {
        Pattern => ... 
    };

Also, I believe patterns in general are indented incorrectly, though admittedly I'm not sure what the standard is. My preference is to keep all patterns at the same level of indentation, as shown here:

match foo {
    Pattern1 |
    Pattern2 => {
    }
}

[pnkfelix]

If we're going to fix /*! ... */, we should try to also take the time to fix the other doc comment variations, namely //!, if possible.

[pnkfelix]

part of #8793