Completing support for trailing commas in `std` macros

[ExpHP]

I really want to see complete support for trailing commas in std macros.

As I described on Internals, all (stable) "variadic" macros correctly support trailing commas for sufficiently long argument lists. It is only a few, select, specific argument counts that trip them up.

Here is a check list of all test cases that currently fail to compile:

Failing cases that obviously need fixing

These macros already support trailing commas when given longer argument lists, they just fail for these specific lengths:

• {debug_,}assert!(true, );
• {debug_,}assert!(true, "hello",);
• {debug_,}assert_{eq,ne}!(1, 1,);
  • Status: Fixed in Beta
• panic!("hello",);
• writeln!(&mut stdout,);
• unreachable!("hello",);

Worth discussion

These macros technically have not yet committed to the "function-call-like" syntax and we still currently have the option to extend these with an entirely different syntax. Like, I don't know, compile_error!("a" => b) or something.

• include{,_bytes,_str}!("dumdum.rs",);
• compile_error!("lel",);
• try!(Ok(()),);
• option_env!("PATH",);
  • note: env! handles the trailing comma so we might want this just for consistency's sake
• cfg!(unix,);

Oddball cases

• the unstable mpsc select! does not support trailing commas in general...
  • ...but it also contains numerous other papercuts and probably deserves an issue all of its own.

---

I offer to help with at least those macros which are implemented via macro_rules!.

[ExpHP]

Concerns

Of the top of my head I can think of at least the following concerns which should be addressed during implementation.

• Instastability. (I assume changes to macro_rules! macros are instastable...)
• The recursion limit. Macro invocations which were previously valid should go through the same number of recursive expansions (or fewer) or else this technically becomes a breaking change.
• Impact on compile times. Will adding cases to ubiquitous macros like assert! negatively impact compile times?
• Testing for double commas. There should probably be compile-fail tests that give double trailing commas. (note that rust is currently missing such tests in general, see Test coverage for double trailing commas is poor #46238)
• Appearance of documentation. Since macro patterns appear in docs.