add cast and trasmute example

integration/tests/examples/cast_and_transmute.rs

@@ -0,0 +1,37 @@
+#![feature(autodiff)]
+
+#[no_mangle]
+#[autodiff(b_dot_local, Reverse, Duplicated, Active)]
+fn dot_local(x: &usize) -> f64 {
+    // dereference x, and cast the usize back to the double ptr.
+    // Then load a double from the pointer and return sin(the value).
+    let x = unsafe { *x as *const f64 };
+    let x = unsafe { *x };
+    x.sin()
+}
+
+// now the same with std::mem::transmute
+#[no_mangle]
+#[autodiff(b_dot_local2, Reverse, Duplicated, Active)]
+fn dot_local2(x: &usize) -> f64 {
+    // dereference x, and std::mem::transmute the usize back to the double ptr.
+    // Then load a double from the pointer and return sin(the value).
+    let x = unsafe { std::mem::transmute::<usize, *const f64>(*x) };
+    let x = unsafe { *x };
+    x.sin()
+}
+
+fn main() {
+    let x = 1.34;
+    let mut dx = 0.0;
+    let mut dx2 = 0.0;
+    let x1: usize = &x as *const f64 as usize;
+    let mut dx1: usize = &mut dx as *mut f64 as usize;
+    let mut dx2: usize = &mut dx2 as *mut f64 as usize;
+    b_dot_local(&x1, &mut dx1, 1.0);
+    b_dot_local2(&x1, &mut dx2, 1.0);
+    let dx1f = unsafe { *(dx1 as *const f64) };
+    let dx2f = unsafe { *(dx2 as *const f64) };
+    assert_eq!(dx1f, f64::cos(x));
+    assert_eq!(dx2f, f64::cos(x));
+}