feat: add codegen test

Ensure that code for generic `d_primal::<T>` is generated even if `primal::<T>`
is never used.

- incorporate feedback from @ZuseZ4

Author: haenoe

tests/codegen/autodiff/generic.rs

@@ -0,0 +1,42 @@
+//@ compile-flags: -Zautodiff=Enable -Zautodiff=NoPostopt -C opt-level=3 -Clto=fat
+//@ no-prefer-dynamic
+//@ needs-enzyme
+#![feature(autodiff)]
+
+use std::autodiff::autodiff;
+
+#[autodiff(d_square, Reverse, Duplicated, Active)]
+fn square<T: std::ops::Mul<Output = T> + Copy>(x: &T) -> T {
+    *x * *x
+}
+
+// Ensure that `d_square::<f64>` code is generated even if `square::<f64>` was never called
+//
+// CHECK: ; generic::square
+// CHECK-NEXT: ; Function Attrs:
+// CHECK-NEXT: define internal {{.*}} double
+// CHECK-NEXT: start:
+// CHECK-NOT: ret
+// CHECK: fmul double
+
+// Ensure that `d_square::<f32>` code is generated
+//
+// CHECK: ; generic::square
+// CHECK-NEXT: ; Function Attrs: {{.*}}
+// CHECK-NEXT: define internal {{.*}} float
+// CHECK-NEXT: start:
+// CHECK-NOT: ret
+// CHECK: fmul float
+
+fn main() {
+    let xf32: f32 = std::hint::black_box(3.0);
+    let xf64: f64 = std::hint::black_box(3.0);
+
+    let outputf32 = square::<f32>(&xf32);
+    assert_eq!(9.0, outputf32);
+
+    let mut df_dxf64: f64 = std::hint::black_box(0.0);
+
+    let output_f64 = d_square::<f64>(&xf64, &mut df_dxf64, 1.0);
+    assert_eq!(6.0, df_dxf64);
+}