Use E0724 instead of E0723 as an error code

src/librustc_typeck/collect.rs

@@ -2396,7 +2396,7 @@ fn codegen_fn_attrs<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, id: DefId) -> Codegen
                 struct_span_err!(
                     tcx.sess,
                     attr.span,
-                    E0723,
+                    E0724,
                     "`#[ffi_returns_twice]` may only be used on foreign functions"
                 ).emit();
             }

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 foreign functions
+    E0724, // `#[ffi_returns_twice]` is only allowed in foreign functions
 }

src/test/ui/ffi_returns_twice.stderr

-error[E0723]: `#[ffi_returns_twice]` may only be used on foreign functions
+error[E0724]: `#[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 foreign functions
@@ -6,4 +6,4 @@ LL | #[ffi_returns_twice] //~ ERROR `#[ffi_returns_twice]` may only be used on f
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0723`.
+For more information about this error, try `rustc --explain E0724`.