Add unstable frontmatter support by epage · Pull Request #137193 · rust-lang/rust (original) (raw)

This is an implementation for #136889

This strips the frontmatter, like shebang, because nothing within rustc will be using this. rustfmt folks have said this should be good enough for now as rustfmt should successfully work with this (ie not remove it) even though proper formatting would require AST support. I had considered making this part of lexing of tokens but that led to problems with ambiguous tokens. See also zulip.

This does not do any fancy error reporting for invalid syntax. Besides keeping this first PR simple, we can punt on this because Cargo will be the front line of parsing in the majority of cases and it will need to do have as good or better error reporting than rustc, for this syntax, to avoid masking the quality of rustc's error reporting.

This includes supporting the syntax in

• the parser
• pretty printing
• rustfmt (nop, just needed a test)

This leaves to the future

• r-a support (called out in the tracking issue)