Prefer lifetime suggestion over generic error
syntax: add a some docs/clarification to the fields of ExpnInfo.
syntax: add a missing span rewrite in fold.

This was leaving Decls without the new spans; this is a minor change,
since literally nothing reads in the code base reads the span of a Decl
itself, always just its contents.
rustc: Relax restriction on privacy for fields

This is a necessary change in preparation for switching the defaults as part
of #8122.

RFC: 0004-private-fields
syntax: Permit visibility on tuple fields

This change is in preparation for #8122. Nothing is currently done with these
visibility qualifiers, they are just parsed and accepted by the compiler.

RFC: 0004-private-fields
auto merge of #13071 : ktt3ja/rust/deterministic-lifetime-suggestion, r=cmr

Close #13057
auto merge of #13152 : huonw/rust/wtf-are-things-in-spans, r=alexcrichton

Add some docs to ExpnInfo. Add a single overlooked `new_span` call to the folder (I'm pretty sure nothing reads this span, though, so it's probably pointless).
auto merge of #13145 : alexcrichton/rust/flip-some-defaults, r=brson

This change prepares `rustc` to accept private fields by default. These changes will have to go through a snapshot before the rest of the changes can happen.