error: incorrect function inside `extern` block
  --> $DIR/issue-91370-foreign-fn-block-impl.rs:5:8
   |
LL |   extern {
   |   ------ `extern` blocks define existing foreign functions and functions inside of them cannot have a body
LL |
LL |       fn f() {
   |  ________^___-
   | |        |
   | |        cannot have a body
LL | |
LL | |
LL | |         impl Copy for u8 {}
LL | |     }
   | |_____- help: remove the invalid body: `;`
   |
   = help: you might have meant to write a function accessible through FFI, which can be done by writing `extern fn` outside of the `extern` block
   = note: for more information, visit https://doc.rust-lang.org/std/keyword.extern.html

error: aborting due to previous error