libfmt_macros should be renamed to have a rustc_ prefix or moved into librustc_builtin_macros.

[eddyb]

For historical reasons we have a crate that looks like it would be part of the standard library, but it's really just a parser for the implementation of format_args!.

cc @petrochenkov

[Mark-Simulacrum]

❤️ I would also deeply appreciate the removal/merge of this crate. It's what I always hit when trying to find the "real" formatting piece of the compiler, and it's essentially never what I want.

[petrochenkov]

or moved into librustc_builtin_macros

Unfortunately, fmt_macros is also used from rustc_trait_selection due to implementation of the #[on_unimplemented] attribute, and it's undesirable to introduce the dependency rustc_builtin_macros -> rustc_trait_selection.

Possible solutions:

• Keep it as a separate crate, rename to rustc_parse_format.
• Merge it into rustc_parse if it doesn't add extra dependencies (need to check).
• Turn #[rustc_on_unimplemented] into an built-in attribute macro so it does formatting during expansion and trait selection works with some lowered form of it, #[rustc_on_unimplemented_impl] or whatever. Then the crate can be merged into rustc_builtin_macros.

[eddyb]

• Keep it as a separate crate, rename to rustc_parse_format.
• Merge it into rustc_parse if it doesn't add extra dependencies (need to check).

Either of these seem fine with me, anything else would be overkill, IMO.