Subpar error message on pub (ident) fn foo

[pnkfelix]

Someone who had code with the old pub_restricted feature is going to get pretty bad error message feedback:

#![feature(pub_restricted)]
mod a { pub (a) fn foo() -> i32 { 13 } }

fn main() {}

Here's the message you get (ever since pub(in path) was implemented, sometime after March 4th:

error: unmatched visibility `pub`
 --> pub_restricted.rs:2:9
  |
2 | mod a { pub (a) fn foo() -> i32 { 13 } }
  |         ^^^

(At the very least, it would be good if it highlighted pub (a) in its entirety, not just pub...)

Also, someone who doesn't know about pub restricted and just has a syntactic typo is also going to have problems here, since it is very confusing that the compiler seems to be saying that it doesn't know about the pub visibility, when that is one of the most fundamental visibilities.

[estebank]

#40627 proposes the following output for a given statement pub (a) fn afn() {}:

error: incorrect restriction in `pub`
  --> file.rs:15:1
   |
15 | pub (a) fn afn() {}
   |     ^^^
   |
   = help: valid visibility restrictions are:
           `pub`: visible on every module that imports this module
           `pub(crate)`: visible only on the current crate
           `pub(super)`: visible only in the current module's parent
           `pub(in path::to::module)`: visible only on the specified path
           `pub(self)` or ``: visible only in the current module
help: to make this visible only to module `a`, add `in` before the path:
   | pub (in a) fn afn() {}