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/lib.rs

@@ -168,7 +168,7 @@ pub const MIRI_DEFAULT_ARGS: &[&str] = &[
     "-Zmir-emit-retag",
     "-Zmir-keep-place-mention",
     "-Zmir-opt-level=0",
-    "-Zmir-enable-passes=-CheckAlignment",
+    "-Zmir-enable-passes=-CheckAlignment,-CheckNull",
     // Deduplicating diagnostics means we miss events when tracking what happens during an
     // execution. Let's not do that.
     "-Zdeduplicate-diagnostics=no",