Insert null checks for pointer dereferences when debug assertions are enabled

Similar to how the alignment is already checked, this adds a check
for null pointer dereferences in debug mode. It is implemented similarly
to the alignment check as a MirPass.

This is related to a 2025H1 project goal for better UB checks in debug
mode: rust-lang/rust-project-goals#177.

Author: 1c3t3a

src/driver.rs

@@ -166,6 +166,8 @@ impl rustc_driver::Callbacks for ClippyCallbacks {
         // MIR passes can be enabled / disabled separately, we should figure out, what passes to
         // use for Clippy.
         config.opts.unstable_opts.mir_opt_level = Some(0);
+        config.opts.unstable_opts.mir_enable_passes =
+            vec![("CheckNull".to_owned(), false), ("CheckAlignment".to_owned(), false)];
 
         // Disable flattening and inlining of format_args!(), so the HIR matches with the AST.
         config.opts.unstable_opts.flatten_format_args = false;