Detect fn with a body in an extern block

src/test/ui/extern/extern-ffi-fn-with-body.rs

@@ -0,0 +1,11 @@
+extern "C" {
+    fn foo() -> i32 { //~ ERROR incorrect `fn` inside `extern` block
+        return 0;
+    }
+}
+
+extern "C" fn bar() -> i32 {
+    return 0;
+}
+
+fn main() {}

src/test/ui/extern/extern-ffi-fn-with-body.stderr

@@ -0,0 +1,18 @@
+error: incorrect `fn` inside `extern` block
+  --> $DIR/extern-ffi-fn-with-body.rs:2:8
+   |
+LL |   extern "C" {
+   |   ------ `extern` blocks define existing foreign functions and `fn`s inside of them can't have a body
+LL |       fn foo() -> i32 {
+   |  ________^^^__________-
+   | |        |
+   | |        can't have a body
+LL | |         return 0;
+LL | |     }
+   | |_____- this body is invalid here
+   |
+   = 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
+