Add regression test

src/test/ui/foreign/issue-91370-foreign-fn-block-impl.rs

@@ -0,0 +1,12 @@
+// Regression test for issue #91370.
+
+extern {
+    //~^ `extern` blocks define existing foreign functions
+    fn f() {
+        //~^ incorrect function inside `extern` block
+        //~| cannot have a body
+        impl Copy for u8 {}
+    }
+}
+
+fn main() {}

src/test/ui/foreign/issue-91370-foreign-fn-block-impl.stderr

@@ -0,0 +1,21 @@
+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
+