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.

3 files changed, 114 insertions, 1 deletions

diff --git a/compiler/rustc_mir_transform/src/check_null.rs b/compiler/rustc_mir_transform/src/check_null.rs
new file mode 100644
index 00000000000..0b6c0ceaac1
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/check_null.rs
@@ -0,0 +1,110 @@
+use rustc_index::IndexVec;
+use rustc_middle::mir::interpret::Scalar;
+use rustc_middle::mir::*;
+use rustc_middle::ty::{Ty, TyCtxt};
+use rustc_session::Session;
+
+use crate::check_pointers::{BorrowCheckMode, PointerCheck, check_pointers};
+
+pub(super) struct CheckNull;
+
+impl<'tcx> crate::MirPass<'tcx> for CheckNull {
+    fn is_enabled(&self, sess: &Session) -> bool {
+        sess.ub_checks()
+    }
+
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        check_pointers(tcx, body, &[], insert_null_check, BorrowCheckMode::IncludeBorrows);
+    }
+
+    fn is_required(&self) -> bool {
+        true
+    }
+}
+
+fn insert_null_check<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    pointer: Place<'tcx>,
+    pointee_ty: Ty<'tcx>,
+    local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
+    stmts: &mut Vec<Statement<'tcx>>,
+    source_info: SourceInfo,
+) -> PointerCheck<'tcx> {
+    // Cast the pointer to a *const ().
+    let const_raw_ptr = Ty::new_imm_ptr(tcx, tcx.types.unit);
+    let rvalue = Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(pointer), const_raw_ptr);
+    let thin_ptr = local_decls.push(LocalDecl::with_source_info(const_raw_ptr, source_info)).into();
+    stmts
+        .push(Statement { source_info, kind: StatementKind::Assign(Box::new((thin_ptr, rvalue))) });
+
+    // Transmute the pointer to a usize (equivalent to `ptr.addr()`).
+    let rvalue = Rvalue::Cast(CastKind::Transmute, Operand::Copy(thin_ptr), tcx.types.usize);
+    let addr = local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into();
+    stmts.push(Statement { source_info, kind: StatementKind::Assign(Box::new((addr, rvalue))) });
+
+    // Get size of the pointee (zero-sized reads and writes are allowed).
+    let rvalue = Rvalue::NullaryOp(NullOp::SizeOf, pointee_ty);
+    let sizeof_pointee =
+        local_decls.push(LocalDecl::with_source_info(tcx.types.usize, source_info)).into();
+    stmts.push(Statement {
+        source_info,
+        kind: StatementKind::Assign(Box::new((sizeof_pointee, rvalue))),
+    });
+
+    // Check that the pointee is not a ZST.
+    let zero = Operand::Constant(Box::new(ConstOperand {
+        span: source_info.span,
+        user_ty: None,
+        const_: Const::Val(ConstValue::Scalar(Scalar::from_target_usize(0, &tcx)), tcx.types.usize),
+    }));
+    let is_pointee_no_zst =
+        local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
+    stmts.push(Statement {
+        source_info,
+        kind: StatementKind::Assign(Box::new((
+            is_pointee_no_zst,
+            Rvalue::BinaryOp(BinOp::Ne, Box::new((Operand::Copy(sizeof_pointee), zero.clone()))),
+        ))),
+    });
+
+    // Check whether the pointer is null.
+    let is_null = local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
+    stmts.push(Statement {
+        source_info,
+        kind: StatementKind::Assign(Box::new((
+            is_null,
+            Rvalue::BinaryOp(BinOp::Eq, Box::new((Operand::Copy(addr), zero))),
+        ))),
+    });
+
+    // We want to throw an exception if the pointer is null and doesn't point to a ZST.
+    let should_throw_exception =
+        local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
+    stmts.push(Statement {
+        source_info,
+        kind: StatementKind::Assign(Box::new((
+            should_throw_exception,
+            Rvalue::BinaryOp(
+                BinOp::BitAnd,
+                Box::new((Operand::Copy(is_null), Operand::Copy(is_pointee_no_zst))),
+            ),
+        ))),
+    });
+
+    // The final condition whether this pointer usage is ok or not.
+    let is_ok = local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
+    stmts.push(Statement {
+        source_info,
+        kind: StatementKind::Assign(Box::new((
+            is_ok,
+            Rvalue::UnaryOp(UnOp::Not, Operand::Copy(should_throw_exception)),
+        ))),
+    });
+
+    // Emit a PointerCheck that asserts on the condition and otherwise triggers
+    // a AssertKind::NullPointerDereference.
+    PointerCheck {
+        cond: Operand::Copy(is_ok),
+        assert_kind: Box::new(AssertKind::NullPointerDereference),
+    }
+}
diff --git a/compiler/rustc_mir_transform/src/check_pointers.rs b/compiler/rustc_mir_transform/src/check_pointers.rs
index 95d3ce05000..72460542f87 100644
--- a/compiler/rustc_mir_transform/src/check_pointers.rs
+++ b/compiler/rustc_mir_transform/src/check_pointers.rs
@@ -17,6 +17,7 @@ pub(crate) struct PointerCheck<'tcx> {
 /// [NonMutatingUseContext::SharedBorrow].
 #[derive(Copy, Clone)]
 pub(crate) enum BorrowCheckMode {
+    IncludeBorrows,
     ExcludeBorrows,
 }
 
@@ -168,7 +169,7 @@ impl<'a, 'tcx> PointerFinder<'a, 'tcx> {
             ) => true,
             PlaceContext::MutatingUse(MutatingUseContext::Borrow)
             | PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow) => {
-                !matches!(self.borrow_check_mode, BorrowCheckMode::ExcludeBorrows)
+                matches!(self.borrow_check_mode, BorrowCheckMode::IncludeBorrows)
             }
             _ => false,
         }
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index 0df9ce0c3d3..c6a9e3ee0e6 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/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.