Declarative macro_rules! attribute macros

[joshtriplett]

Many crates provide attribute macros. Today, this requires defining proc
macros, in a separate crate, typically with several additional dependencies
adding substantial compilation time, and typically guarded by a feature that
users need to remember to enable.

However, many common cases of attribute macros don't require any more power
than an ordinary macro_rules! macro. Supporting these common cases would
allow many crates to avoid defining proc macros, reduce dependencies and
compilation time, and provide these macros unconditionally without requiring a
the user to enable a feature.

I've reviewed several existing proc-macro-based attributes in the ecosystem,
and it appears that many would be able to use this feature to avoid needing
proc macros at all.

Rendered

[joshtriplett]

Nominated as a follow-up to recent lang discussions about this.

[kennytm]

how will this work in macro form? or is rust-lang/rust#39412 decl_macro still a thing

// ok
macro main {
    attr() ($func:item) => { make_async_main!($func) },
    attr(threads = $threads:literal) ($func:item) => { make_async_main!($func, $threads) },
}

// ?
macro stub attr() ($func:item) {
    make_stub_func!($func)
}

[joshtriplett]

@kennytm

how will this work in macro form?

macro is unstable and incomplete; if someone is working on it they can determine that. Your syntax proposal seems like a fine existence proof that it's possible to provide syntax for this.