rust (original) (raw)

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
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 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