syntax: Unsupport foo! bar { ... } macros in the parser

[petrochenkov]

Their support in expansion was removed in #61606.

Also un-reserve macro_rules as a macro name, there's no ambiguity between macro_rules definitions and macro calls (it also wasn't reserved correctly).

cc rust-lang/wg-grammar#51

[rust-highfive]

r? @eddyb

(rust_highfive has picked a reviewer for you, use r? to override)

[Centril]

r=me with or without nits.

[Centril]

r? @Centril

[eddyb]

cc @rust-lang/lang This PR lets one define macros named macro_rules which would be used for an invocation like macro_rules!(...), while macro_rules! foo {...} would still refer to the builtin macro.

While from an implementation point of view this seems reasonable (treating macro_rules! foo {...} as a special syntax and not a macro invocation), I do want to make sure this is not a compatibility hazard (in case we want to, say, allow something like macro foo = macro_rules! {...}; in the future).

That said, I think we can land this PR as-is, and revert it later if we really don't want to do this.

[petrochenkov]

Two notes:

• macro_rules was already a special syntax, not a macro invocation, this PR doesn't change that
• the "reservation" could be worked around by use foo as macro_rules previously

  macro_rules! foo { () => (0) }
  
  use foo as macro_rules;
  
  fn main() {
      println!("{}", macro_rules!());
  }

[eddyb]

@petrochenkov Oh, and that use wouldn't affect macro_rules! ident {...} syntax anyway?
I can see how that makes this PR even more of a slam-dunk.

Like I said, I think we should just land it and if anyone has any concerns, they can be raised async.

[petrochenkov]

Oh, and that use wouldn't affect macro_rules! ident {...} syntax anyway?

Yes, macro_rules in macro_rules! foo { ... } doesn't go through name resolution, since it's a context-dependent keyword like union and processed during parsing.
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=aa35ddb773e577d4d2b9842d3f0059d1

[petrochenkov]

@bors r=Centril

[bors]

📌 Commit d0dc41a has been approved by Centril

[Centril]

Removing nomination per @eddyb's note re. async and since the whole team was pinged.

(It's clear to me that this is a slam dunk)