gccrs: offset_of: Compile the offset properly

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::visit): Add proper handling
	of the node.
	* rust-backend.h (lookup_field): Declare it.
	* rust-gcc.cc (lookup_field): Add forked implementation from gcc/c/.

gcc/testsuite/ChangeLog:

	* rust/execute/torture/offset_of1.rs: New test.

1 files changed, 16 insertions, 0 deletions

diff --git a/gcc/testsuite/rust/execute/torture/offset_of1.rs b/gcc/testsuite/rust/execute/torture/offset_of1.rs
new file mode 100644
index 0000000..7d39483
--- /dev/null
+++ b/gcc/testsuite/rust/execute/torture/offset_of1.rs
@@ -0,0 +1,16 @@
+// { dg-do run { target x86_64*-*-* } }
+// { dg-additional-options "-frust-assume-builtin-offset-of" }
+
+pub struct Foo {
+    pub a: i32,
+    pub b: i32,
+}
+
+fn main() -> i32 {
+    let a = offset_of!(Foo, a); // valid
+    let b = offset_of!(Foo, b); // valid
+
+    let res = a + b - 4;
+
+    res as i32
+}