Document common manipulations and operations

[Michael-F-Bryan]

While playing around using rustc internals as a library I found it was pretty difficult to discover how to do a bunch of "common" operations. We might be able to improve discoverability by documenting these in an appendix.

Off the top of my head, it'd be nice to know how to:

• Get the fully qualified name of a type given its HIR representation and location (e.g. c_char -> libc::c_char or std::os::raw::c_char, depending on context)
• Get a Path for a given Item
• Find the DefId of a function or struct declaration
• How to pretty-print a function declaration

Feel free to add other "common" questions or shortcuts in the comments and I'll update this list accordingly.

CC: #74

[Michael-F-Bryan]

I'd be happy to write this up but I'll probably need a little guidance because I only know how to do the second thing (find DefId) at the moment.

[mark-i-m]

I don't even know enough to know what is a common operation, but something like that would definitely be helpful. Something like a cookbook appendix...

[Michael-F-Bryan]

To be honest, I was planning to use it as more of a "post questions here if Michael can't figure out how to do something seemingly simple" kind of thing.

For example, I know we do name resolution before rustc_driver fires the after_analysis callback, yet for the life of me I can't figure out how to get the fully qualified name for all a type. For context, I need to convert something like fn inc(a: c_int) -> c_int into fn inc(a: ::std::os::raw::c_int) -> ::std::os::raw::c_int.

[mark-i-m]

How to get a string from a Span (e.g. for printing errors)... Use codemap.span_to_snippet(span)

[mark-i-m]

How to get the parent of a defid (e.g. the module of an item): https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/How.20do.20I.20get.20the.20module.20that.20an.20Item.20belongs.20to.3F

[mark-i-m]

How to get an Item from a DefId: https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/How.20do.20I.20get.20an.20Item.20from.20a.20DefId.3F

Also, would be good to talk about def_kind