Rollup merge of #111665 - est31:offset_of_tests, r=WaffleLapkin

Add more tests for the offset_of macro

Implements what I [suggested in the tracking issue](rust-lang/rust#106655 (comment)), plus some further improvements:

* ensuring that offset_of!(Self, ...) works iff inside an impl block
* ensuring that the output type is usize and doesn't coerce. this can be changed in the future, but if it is done, it should be a conscious decision
* improving the privacy checking test
* ensuring that generics don't let you escape the unsized check

r? `````@WaffleLapkin`````

Author: Dylan-DPC

core/tests/mem.rs

@@ -386,6 +386,21 @@ fn offset_of() {
     // Layout of tuples is unstable
     assert!(offset_of!((u8, u16), 0) <= size_of::<(u8, u16)>() - 1);
     assert!(offset_of!((u8, u16), 1) <= size_of::<(u8, u16)>() - 2);
+
+    #[repr(C)]
+    struct Generic<T> {
+        x: u8,
+        y: u32,
+        z: T
+    }
+
+    // Ensure that this type of generics works
+    fn offs_of_z<T>() -> usize {
+        offset_of!(Generic<T>, z)
+    }
+
+    assert_eq!(offset_of!(Generic<u8>, z), 8);
+    assert_eq!(offs_of_z::<u8>(), 8);
 }
 
 #[test]