Remove $:meta matcher in decl macros

$: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.

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.

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 ~);

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

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.

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

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

#63674 supports the modern attribute syntax in meta.

Auto merge of #63674 - petrochenkov:meta2, r=<try>

Rollup merge of rust-lang#63674 - petrochenkov:meta2, r=Centril

Add documentation and make matchers more flexible

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Remove $:meta matcher in decl macros #49629

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Remove $:meta matcher in decl macros #49629

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eddyb commented on Apr 3, 2018

dtolnay commented on Apr 3, 2018

eddyb commented on Apr 3, 2018

petrochenkov commented on Apr 3, 2018

sgrif commented on Aug 2, 2018

petrochenkov commented on Aug 2, 2018

petrochenkov commented on Aug 17, 2019

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