add a test to verify a good error for enabling autodiff without lto

Author: ZuseZ4

tests/ui/autodiff/no_lto_flag.no_lto.stderr

@@ -0,0 +1,4 @@
+error: using the autodiff feature requires setting `lto="fat"` in your Cargo.toml
+
+error: aborting due to 1 previous error
+

tests/ui/autodiff/no_lto_flag.rs

@@ -0,0 +1,31 @@
+//@ needs-enzyme
+//@ no-prefer-dynamic
+//@ revisions: with_lto no_lto
+//@[with_lto] compile-flags: -Zautodiff=Enable -C opt-level=3  -Clto=fat
+//@[no_lto] compile-flags: -Zautodiff=Enable -C opt-level=3 -Clto=thin
+
+#![feature(autodiff)]
+//@[no_lto] build-fail
+//@[with_lto] build-pass
+
+// Autodiff requires users to enable lto=fat (for now).
+// In the past, autodiff did not run if users forget to enable fat-lto, which caused functions to
+// returning zero-derivatives. That's obviously wrong and confusing to users. We now added a check
+// which will abort compilation instead.
+
+use std::autodiff::autodiff_reverse;
+//[no_lto]~? ERROR using the autodiff feature requires setting `lto="fat"` in your Cargo.toml
+
+#[autodiff_reverse(d_square, Duplicated, Active)]
+fn square(x: &f64) -> f64 {
+    *x * *x
+}
+
+fn main() {
+    let xf64: f64 = std::hint::black_box(3.0);
+
+    let mut df_dxf64: f64 = std::hint::black_box(0.0);
+
+    let _output_f64 = d_square(&xf64, &mut df_dxf64, 1.0);
+    assert_eq!(6.0, df_dxf64);
+}