Poor error message for type definitions in `extern` blocks

[jimblandy]

Although work on RFC 1861 for extern types has begun in issue #43467, it's not complete, and in the mean time, Rust gives very strange messages if one accidentally includes a type in an extern block. For example:

use std::ffi::CString;
use std::os::raw::c_char;

extern {
    fn strlen(s: *const c_char) -> usize;
    pub struct S(c_char); 
}

fn main() {
    println!("{}", unsafe { strlen(CString::new("foo").unwrap().as_ptr()) });
}

On nightly, this elicits the following message:

   Compiling playground v0.0.1 (file:///playground)
error: unmatched visibility `pub`
 --> src/main.rs:7:5
  |
7 |     pub struct S(c_char); 
  |     ^^^

A reader of our book came across this error message by accident and asked me about it. We were both mystified.

[jimblandy]

Possibly related to issue #40599.

Perhaps this is deliberate, as the parse-fail/duplicate-visibility.rs test expects it. But I still think it's unclear.

[steveklabnik]

Triage: this error message got weirder:

error: expected one of `(`, `fn`, `static`, or `type`, found keyword `struct`
 --> src/main.rs:6:9
  |
6 |     pub struct S(c_char); 
  |         ^^^^^^ expected one of `(`, `fn`, `static`, or `type` here

[Centril]

We now recognize this as a struct and emit a good error:

error: struct is not supported in `extern` blocks
 --> src/main.rs:6:5
  |
6 |     pub struct S(c_char); 
  |     ^^^^^^^^^^^^^^^^^^^^^