Correct error message

src/librustc_typeck/collect.rs

@@ -2397,7 +2397,7 @@ fn codegen_fn_attrs<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, id: DefId) -> Codegen
                     tcx.sess,
                     attr.span,
                     E0723,
-                    "`#[ffi_returns_twice]` may only be used on `extern fn`s"
+                    "`#[ffi_returns_twice]` may only be used on foreign functions"
                 ).emit();
             }
         } else if attr.check_name("rustc_allocator_nounwind") {

src/librustc_typeck/diagnostics.rs

@@ -4738,5 +4738,5 @@ fn make_recursive_type() -> impl Sized {
     E0698, // type inside generator must be known in this context
     E0719, // duplicate values for associated type binding
     E0722, // Malformed #[optimize] attribute
-    E0723, // `#[ffi_returns_twice]` is only allowed in `extern fn`
+    E0723, // `#[ffi_returns_twice]` is only allowed in foreign functions
 }

src/libsyntax/feature_gate.rs

@@ -290,7 +290,7 @@ pub fn walk_feature_fields<F>(&self, mut f: F)
     // The `repr(i128)` annotation for enums.
     (active, repr128, "1.16.0", Some(35118), None),
 
-    // Allows the use of `#[ffi_returns_twice]` on extern functions.
+    // Allows the use of `#[ffi_returns_twice]` on foreign functions.
     (active, ffi_returns_twice, "1.34.0", Some(58314), None),
 
     // The `unadjusted` ABI; perma-unstable.

src/test/ui/ffi_returns_twice.rs

@@ -2,5 +2,5 @@
 #![feature(ffi_returns_twice)]
 #![crate_type = "lib"]
 
-#[ffi_returns_twice] //~ ERROR `#[ffi_returns_twice]` may only be used on `extern fn`s
+#[ffi_returns_twice] //~ ERROR `#[ffi_returns_twice]` may only be used on foreign functions
 pub fn foo() {}

src/test/ui/ffi_returns_twice.stderr

-error[E0723]: `#[ffi_returns_twice]` may only be used on `extern fn`s
+error[E0723]: `#[ffi_returns_twice]` may only be used on foreign functions
   --> $DIR/ffi_returns_twice.rs:5:1
    |
-LL | #[ffi_returns_twice] //~ ERROR `#[ffi_returns_twice]` may only be used on `extern fn`s
+LL | #[ffi_returns_twice] //~ ERROR `#[ffi_returns_twice]` may only be used on foreign functions
    | ^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to previous error