Remove $:meta matcher in decl macros

[dtolnay]

$:meta is intended for matching the content of attributes, as in the following:

#![feature(decl_macro)]

macro m(
    $(#[$m:meta])*
    struct $n:ident;
) {
    $(#[$m])*
    struct $n(usize);
}

m! {
    #[derive(Debug)]
    struct S;
}

fn main() {
    println!("{:?}", S(1));
}

According to #34981 (comment) we intend to support arbitrary token streams inside of attributes rather than the limited grammar of $:meta. Let's have decl macros drop support for $:meta and encourage #[$($meta:tt)*] instead.

[eddyb]

The less fragment specifiers the better! We might even want to change the macro_rules meta to ident followed by any number of tt's, just to keep things simple.

[dtolnay]

Changing macro_rules's meta is tricky because it is currently allowed to be followed by tokens.

macro_rules! eddyb {
    ($m:meta $n:tt) => {}
}

eddyb!(a ~);

[eddyb]

So, the only generalization we can make is to have meta allow ident = tt and ident(tt*)?

[petrochenkov]

we intend to support arbitrary token streams inside of attributes

I thought the idea was to support attribute invocations identical to bang macro invocations

a::b::c!(tt*)
a::b::c![tt*]
a::b::c!{tt*}

=>

#[a::b::c(tt*)]
#[a::b::c[tt*]]
#[a::b::c{tt*}]

+ two "legacy" forms

#[a::b::c] // Maybe not so "legacy" with https://internals.rust-lang.org/t/idea-elide-parens-brackets-on-unparametrized-macros/6527
#[a::b::c = not_sure_what_exactly_but_probably_tt]

, but not arbitrary "token streams" (aka tt* without delimiters)

#[a::b::c e f + c ,,, ;_:] // Nope

In this case, I think, we can fit it into existing meta with all its following token rules.

[sgrif]

This seems like something that should get done for Rust 2018 -- Should this be nominated for more discussion?

[petrochenkov]

#50120 restricted the attribute grammar to delimited token trees, so it's at least technically possible to keep meta as a matcher now.

[petrochenkov]

#63674 supports the modern attribute syntax in meta.