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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