Rollup merge of #3540 - matthiaskrgr:rustup, r=flip1995

rustup rust-lang/rust#56092

fix ui test cast_alignment failure by adding #![feature(rustc_private)]

Author: flip1995

tests/ui/cast_alignment.rs

@@ -9,8 +9,8 @@
 
 //! Test casts for alignment issues
 
-#![feature(libc)]
 
+#![feature(rustc_private)]
 extern crate libc;
 
 #[warn(clippy::cast_ptr_alignment)]