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: https://github.com/rust-lang/rust-project-goals/pull/177.

compiler/rustc_mir_transform/src/lib.rs

@@ -119,6 +119,7 @@ declare_passes! {
     mod check_call_recursion : CheckCallRecursion, CheckDropRecursion;
     mod check_alignment : CheckAlignment;
     mod check_const_item_mutation : CheckConstItemMutation;
+    mod check_null : CheckNull;
     mod check_packed_ref : CheckPackedRef;
     mod check_undefined_transmutes : CheckUndefinedTransmutes;
     // This pass is public to allow external drivers to perform MIR cleanup
@@ -643,6 +644,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         &[
             // Add some UB checks before any UB gets optimized away.
             &check_alignment::CheckAlignment,
+            &check_null::CheckNull,
             // Before inlining: trim down MIR with passes to reduce inlining work.
 
             // Has to be done before inlining, otherwise actual call will be almost always inlined.