verify that our rlib check works

Author: ZuseZ4

tests/ui/autodiff/autodiff_in_rlib.rs

@@ -0,0 +1,17 @@
+#![feature(autodiff)]
+#![crate_type = "rlib"]
+//@ needs-enzyme
+//@ compile-flags: -Zautodiff=Enable -C opt-level=3  -Clto=fat
+//@ build-fail
+
+// We test that we fail to compile if a user applied an autodiff_ macro in src/lib.rs,
+// since autodiff doesn't work in libraries yet. In the past we used to just return zeros in the
+// autodiffed functions, which is obviously confusing and wrong, so erroring is an improvement.
+
+use std::autodiff::autodiff_reverse;
+//~? ERROR: using the autodiff feature with library builds is not yet supported
+
+#[autodiff_reverse(d_square, Duplicated, Active)]
+pub fn square(x: &f64) -> f64 {
+    *x * *x
+}

tests/ui/autodiff/autodiff_in_rlib.stderr

@@ -0,0 +1,4 @@
+error: using the autodiff feature with library builds is not yet supported
+
+error: aborting due to 1 previous error
+