Auto merge of #134424 - 1c3t3a:null-checks, r=saethlin

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 inserts checks in the same places as the `CheckAlignment` pass and additionally
also inserts checks for `Borrows`, so code like
```rust
let ptr: *const u32 = std::ptr::null();
let val: &u32 = unsafe { &*ptr };
```
will have a check inserted on dereference. This is done because null references
are UB. The alignment check doesn't cover these places, because in `&(*ptr).field`,
the exact requirement is that the final reference must be aligned. This is something to
consider further enhancements of the alignment check.

For now this is implemented as a separate `MirPass`, to make it easy to disable
this check if necessary.

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.

r? `@saethlin`

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -713,6 +713,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 // and `#[track_caller]` adds an implicit third argument.
                 (LangItem::PanicMisalignedPointerDereference, vec![required, found, location])
             }
+            AssertKind::NullPointerDereference => {
+                // It's `fn panic_null_pointer_dereference()`,
+                // `#[track_caller]` adds an implicit argument.
+                (LangItem::PanicNullPointerDereference, vec![location])
+            }
             _ => {
                 // It's `pub fn panic_...()` and `#[track_caller]` adds an implicit argument.
                 (msg.panic_function(), vec![location])