Confusing error message when trying to implement a function in an extern block

[Gaelan]

A programmer new to Rust (yours truly, for example) may assume that these two syntaxes are equivalent:

extern "C" {
    fn foo() -> i32 {
        return 0;
    }
}

extern "C" fn foo() -> i32 {
    return 0;
}

This is not the case, of course—the first is a syntax error. However, the compiler points this out rather unhelpfully:

error: expected one of `!`, `(`, `+`, `::`, `;`, `<`, or `where`, found `{`
 --> src/lib.rs:2:21
  |
2 |     fn foo() -> i32 {
  |                     ^ expected one of 7 possible tokens here

A message such as "If you're trying to declare a function callable from other languages, use extern "C" fn foo() (wording is bikesheddable, obviously) would have saved me a lot of hair-tearing.