Auto merge of #48056 - ExpHP:macro-commas, r=dtolnay

Comprehensively support trailing commas in std/core macros

I carefully organized the changes into four commits:

* Test cases
* Fixes for `macro_rules!` macros
* Fixes for builtin macros
* Docs for builtins

**I can easily scale this back to just the first two commits for now if such is desired.**

### Breaking (?) changes

* This fixes #48042, which is a breaking change that I hope people can agree is just a bugfix for an extremely dark corner case.

* To fix five of the builtins, this changes `syntax::ext::base::get_single_str_from_tts` to accept a trailing comma, and revises the documentation so that this aspect is not surprising. **I made this change under the (hopefully correct) understanding that `libsyntax` is private rustc implementation detail.** After reviewing all call sites (which were, you guessed it, *precisely those five macros*), I believe the revised semantics are closer to the intended spirit of the function.

### Changes which may require concensus

Up until now, it could be argued that some or all the following macros did not conceptually take a comma-separated list, because they only took one argument:

  * **`cfg(unix,)`** (most notable since cfg! is unique in taking a meta tag)
  * **`include{,_bytes,_str}("file.rs",)`**  (in item form this might be written as "`include!{"file.rs",}`" which is even slightly more odd)
  * **`compile_error("message",);`**
  * **`option_env!("PATH",)`**
  * **`try!(Ok(()),)`**

So I think these particular changes may require some sort of consensus.  **All of the fixes for builtins are included this list, so if we want to defer these decisions to later then I can scale this PR back to just the first two commits.**

### Other notes/general requests for comment

* Do we have a big checklist somewhere of "things to do when adding macros?" My hope is for `run-pass/macro-comma-support.rs` to remain comprehensive.
* Originally I wanted the tests to also comprehensively forbid double trailing commas.  However, this didn't work out too well: [see this gist and the giant FIXME in it](https://gist.github.com/ExpHP/6fc40e82f3d73267c4e590a9a94966f1#file-compile-fail_macro-comma-support-rs-L33-L50)
* I did not touch `select!`. It appears to me to be a complete mess, and its trailing comma mishaps are only the tip of the iceberg.
* There are [some compile-fail test cases](https://github.com/ExpHP/rust/blob/5fa97c35da2f0ee/src/test/compile-fail/macro-comma-behavior.rs#L49-L52) that didn't seem to work (rustc emits errors, but compile-fail doesn't acknowledge them), so they are disabled. Any clues? (Possibly related: These happen to be precisely the set of errors which are tagged by rustc as "this error originates in a macro outside of the current crate".)

---

Fixes #48042
Closes #46241

1 files changed, 2 insertions, 0 deletions

diff --git a/src/libsyntax_ext/cfg.rs b/src/libsyntax_ext/cfg.rs
index 1d8dc406468..1eeba9b30b8 100644
--- a/src/libsyntax_ext/cfg.rs
+++ b/src/libsyntax_ext/cfg.rs
@@ -28,6 +28,8 @@ pub fn expand_cfg<'cx>(cx: &mut ExtCtxt,
     let mut p = cx.new_parser_from_tts(tts);
     let cfg = panictry!(p.parse_meta_item());
 
+    let _ = p.eat(&token::Comma);
+
     if !p.eat(&token::Eof) {
         cx.span_err(sp, "expected 1 cfg-pattern");
         return DummyResult::expr(sp);