metadata technical debt

[graydon]

Tracking bug for issues to fix in metadata that make it difficult to imagine larger scale fixes:

• Finish metadata::encoder::encode_info_for_mod #4573 : finish metadata::encode_info_for_mod
• Move metadata handling into its own crate #2213 : extract metadata into its own crate
• Don't use external node ids in crate metadata #2419 : don't use external node IDs in crate metadata
• The crate store does not deduplicate used crates #3474 : crate store doesn't deduplicate used crates
• absolute path names are not correct for crates not in root namespace #1920 : absolute paths are nor correct for crates not in root namespace

There are probably a lot of other bugs here. Please add them so we can get multiple hands cleaning this stuff up.

[huonw]

I think that metadata will need to move to libsyntax (or some change like that) to allow syntax extensions to be loaded in a structured way (#1762) since it needs to search for any libraries from which they are loaded, and then actually load the macros in a (hopefully) typesafe way.

Work on this bug seems like the perfect opportunity to get started on that move.

[bblum]

metametadatabug

[brson]

@huonw I prefer for metadata to move to its own crate.

[huonw]

@brson that'd be nice, but I don't think it's possible, unless it is independent of the AST, or macro expansion moves out of libsyntax; otherwise libsyntax will depend on libmetadata (to load macros from external crates), and vice versa.

[steveklabnik]

Triage: all but one of the original bugs are closed, so I'm just going to close this one. #2419 stands alone.