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

Diff for: compiler/rustc_codegen_cranelift/src/base.rs

@@ -417,6 +417,16 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
                             Some(source_info.span),
                         );
                     }
+                    AssertKind::NullPointerDereference => {
+                        let location = fx.get_caller_location(source_info).load_scalar(fx);
+
+                        codegen_panic_inner(
+                            fx,
+                            rustc_hir::LangItem::PanicNullPointerDereference,
+                            &[location],
+                            Some(source_info.span),
+                        )
+                    }
                     _ => {
                         let location = fx.get_caller_location(source_info).load_scalar(fx);
 

Diff for: 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()`, and
+                // `#[track_caller]` adds an implicit third argument.
+                (LangItem::PanicNullPointerDereference, vec![location])
+            }
             _ => {
                 // It's `pub fn panic_...()` and `#[track_caller]` adds an implicit argument.
                 (msg.panic_function(), vec![location])

Diff for: compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -509,6 +509,7 @@ impl<'tcx> interpret::Machine<'tcx> for CompileTimeMachine<'tcx> {
                     found: eval_to_int(found)?,
                 }
             }
+            NullPointerDereference => NullPointerDereference,
         };
         Err(ConstEvalErrKind::AssertFailure(err)).into()
     }

Diff for: compiler/rustc_hir/src/lang_items.rs

@@ -317,6 +317,7 @@ language_item_table! {
     PanicAsyncFnResumedPanic, sym::panic_const_async_fn_resumed_panic, panic_const_async_fn_resumed_panic, Target::Fn, GenericRequirement::None;
     PanicAsyncGenFnResumedPanic, sym::panic_const_async_gen_fn_resumed_panic, panic_const_async_gen_fn_resumed_panic, Target::Fn, GenericRequirement::None;
     PanicGenFnNonePanic, sym::panic_const_gen_fn_none_panic, panic_const_gen_fn_none_panic, Target::Fn, GenericRequirement::None;
+    PanicNullPointerDereference, sym::panic_null_pointer_dereference, panic_null_pointer_dereference, Target::Fn, GenericRequirement::None;
     /// libstd panic entry point. Necessary for const eval to be able to catch it
     BeginPanic,              sym::begin_panic,         begin_panic_fn,             Target::Fn,             GenericRequirement::None;
 

Diff for: compiler/rustc_middle/messages.ftl

@@ -17,6 +17,9 @@ middle_assert_gen_resume_after_panic = `gen` fn or block cannot be further itera
 middle_assert_misaligned_ptr_deref =
     misaligned pointer dereference: address must be a multiple of {$required} but is {$found}
 
+middle_assert_null_ptr_deref =
+    null pointer dereference occurred
+
 middle_assert_op_overflow =
     attempt to compute `{$left} {$op} {$right}`, which would overflow
 

Diff for: compiler/rustc_middle/src/mir/syntax.rs

@@ -1012,6 +1012,7 @@ pub enum AssertKind<O> {
     ResumedAfterReturn(CoroutineKind),
     ResumedAfterPanic(CoroutineKind),
     MisalignedPointerDereference { required: O, found: O },
+    NullPointerDereference,
 }
 
 #[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]

Diff for: compiler/rustc_middle/src/mir/terminator.rs

@@ -195,6 +195,7 @@ impl<O> AssertKind<O> {
             ResumedAfterPanic(CoroutineKind::Desugared(CoroutineDesugaring::Gen, _)) => {
                 LangItem::PanicGenFnNonePanic
             }
+            NullPointerDereference => LangItem::PanicNullPointerDereference,
 
             BoundsCheck { .. } | MisalignedPointerDereference { .. } => {
                 bug!("Unexpected AssertKind")
@@ -260,6 +261,7 @@ impl<O> AssertKind<O> {
                     "\"misaligned pointer dereference: address must be a multiple of {{}} but is {{}}\", {required:?}, {found:?}"
                 )
             }
+            NullPointerDereference => write!(f, "\"null pointer dereference occured\""),
             ResumedAfterReturn(CoroutineKind::Coroutine(_)) => {
                 write!(f, "\"coroutine resumed after completion\"")
             }
@@ -330,7 +332,7 @@ impl<O> AssertKind<O> {
             ResumedAfterPanic(CoroutineKind::Coroutine(_)) => {
                 middle_assert_coroutine_resume_after_panic
             }
-
+            NullPointerDereference => middle_assert_null_ptr_deref,
             MisalignedPointerDereference { .. } => middle_assert_misaligned_ptr_deref,
         }
     }
@@ -363,7 +365,7 @@ impl<O> AssertKind<O> {
                 add!("left", format!("{left:#?}"));
                 add!("right", format!("{right:#?}"));
             }
-            ResumedAfterReturn(_) | ResumedAfterPanic(_) => {}
+            ResumedAfterReturn(_) | ResumedAfterPanic(_) | NullPointerDereference => {}
             MisalignedPointerDereference { required, found } => {
                 add!("required", format!("{required:#?}"));
                 add!("found", format!("{found:#?}"));

Diff for: compiler/rustc_middle/src/mir/visit.rs

@@ -636,7 +636,7 @@ macro_rules! make_mir_visitor {
                     OverflowNeg(op) | DivisionByZero(op) | RemainderByZero(op) => {
                         self.visit_operand(op, location);
                     }
-                    ResumedAfterReturn(_) | ResumedAfterPanic(_) => {
+                    ResumedAfterReturn(_) | ResumedAfterPanic(_) | NullPointerDereference => {
                         // Nothing to visit
                     }
                     MisalignedPointerDereference { required, found } => {

Diff for: compiler/rustc_mir_transform/src/check_null.rs

@@ -0,0 +1,73 @@
+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::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);
+    }
+}
+
+fn insert_null_check<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    local_decls: &mut IndexVec<Local, LocalDecl<'tcx>>,
+    block_data: &mut BasicBlockData<'tcx>,
+    pointer: Place<'tcx>,
+    _: Ty<'tcx>,
+    source_info: SourceInfo,
+    new_block: BasicBlock,
+) {
+    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();
+    block_data
+        .statements
+        .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();
+    block_data
+        .statements
+        .push(Statement { source_info, kind: StatementKind::Assign(Box::new((addr, rvalue))) });
+
+    // Check if the pointer is null.
+    let is_ok = local_decls.push(LocalDecl::with_source_info(tcx.types.bool, source_info)).into();
+    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),
+    }));
+    block_data.statements.push(Statement {
+        source_info,
+        kind: StatementKind::Assign(Box::new((
+            is_ok,
+            Rvalue::BinaryOp(BinOp::Ne, Box::new((Operand::Copy(addr), zero))),
+        ))),
+    });
+
+    // Set this block's terminator to our assert, continuing to new_block if we pass
+    block_data.terminator = Some(Terminator {
+        source_info,
+        kind: TerminatorKind::Assert {
+            cond: Operand::Copy(is_ok),
+            expected: true,
+            target: new_block,
+            msg: Box::new(AssertKind::NullPointerDereference),
+            // This calls panic_misaligned_pointer_dereference, which is #[rustc_nounwind].
+            // We never want to insert an unwind into unsafe code, because unwinding could
+            // make a failing UB check turn into much worse UB when we start unwinding.
+            unwind: UnwindAction::Unreachable,
+        },
+    });
+}

Diff for: compiler/rustc_mir_transform/src/lib.rs

@@ -117,6 +117,7 @@ declare_passes! {
     mod add_subtyping_projections : Subtyper;
     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
@@ -656,6 +657,7 @@ 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.

Diff for: compiler/rustc_monomorphize/src/collector.rs

@@ -814,6 +814,9 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
                 mir::AssertKind::MisalignedPointerDereference { .. } => {
                     push_mono_lang_item(self, LangItem::PanicMisalignedPointerDereference);
                 }
+                mir::AssertKind::NullPointerDereference => {
+                    push_mono_lang_item(self, LangItem::PanicNullPointerDereference);
+                }
                 _ => {
                     push_mono_lang_item(self, msg.panic_function());
                 }

Diff for: compiler/rustc_smir/src/rustc_smir/convert/mir.rs

@@ -484,6 +484,9 @@ impl<'tcx> Stable<'tcx> for mir::AssertMessage<'tcx> {
                     found: found.stable(tables),
                 }
             }
+            AssertKind::NullPointerDereference => {
+                stable_mir::mir::AssertMessage::NullPointerDereference
+            }
         }
     }
 }

Diff for: compiler/rustc_span/src/symbol.rs

@@ -1456,6 +1456,7 @@ symbols! {
         panic_location,
         panic_misaligned_pointer_dereference,
         panic_nounwind,
+        panic_null_pointer_dereference,
         panic_runtime,
         panic_str_2015,
         panic_unwind,

Diff for: compiler/stable_mir/src/mir/body.rs

@@ -251,6 +251,7 @@ pub enum AssertMessage {
     ResumedAfterReturn(CoroutineKind),
     ResumedAfterPanic(CoroutineKind),
     MisalignedPointerDereference { required: Operand, found: Operand },
+    NullPointerDereference,
 }
 
 impl AssertMessage {
@@ -306,6 +307,7 @@ impl AssertMessage {
             AssertMessage::MisalignedPointerDereference { .. } => {
                 Ok("misaligned pointer dereference")
             }
+            AssertMessage::NullPointerDereference => Ok("null pointer dereference occured"),
         }
     }
 }

Diff for: compiler/stable_mir/src/mir/pretty.rs

@@ -296,6 +296,9 @@ fn pretty_assert_message<W: Write>(writer: &mut W, msg: &AssertMessage) -> io::R
                 "\"misaligned pointer dereference: address must be a multiple of {{}} but is {{}}\",{pretty_required}, {pretty_found}"
             )
         }
+        AssertMessage::NullPointerDereference => {
+            write!(writer, "\"null pointer dereference occured.\"")
+        }
         AssertMessage::ResumedAfterReturn(_) | AssertMessage::ResumedAfterPanic(_) => {
             write!(writer, "{}", msg.description().unwrap())
         }

Diff for: compiler/stable_mir/src/mir/visit.rs

@@ -426,7 +426,10 @@ pub trait MirVisitor {
             | AssertMessage::RemainderByZero(op) => {
                 self.visit_operand(op, location);
             }
-            AssertMessage::ResumedAfterReturn(_) | AssertMessage::ResumedAfterPanic(_) => { //nothing to visit
+            AssertMessage::ResumedAfterReturn(_)
+            | AssertMessage::ResumedAfterPanic(_)
+            | AssertMessage::NullPointerDereference => {
+                //nothing to visit
             }
             AssertMessage::MisalignedPointerDereference { required, found } => {
                 self.visit_operand(required, location);

Diff for: library/core/src/panicking.rs

@@ -291,6 +291,22 @@ fn panic_misaligned_pointer_dereference(required: usize, found: usize) -> ! {
     )
 }
 
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold, optimize(size))]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
+#[track_caller]
+#[cfg_attr(not(bootstrap), lang = "panic_null_pointer_dereference")] // needed by codegen for panic on null pointer deref
+#[rustc_nounwind] // `CheckNull` MIR pass requires this function to never unwind
+fn panic_null_pointer_dereference() -> ! {
+    if cfg!(feature = "panic_immediate_abort") {
+        super::intrinsics::abort()
+    }
+
+    panic_nounwind_fmt(
+        format_args!("null pointer dereference occured"),
+        /* force_no_backtrace */ false,
+    )
+}
+
 /// Panics because we cannot unwind out of a function.
 ///
 /// This is a separate function to avoid the codesize impact of each crate containing the string to

Diff for: src/tools/rust-analyzer/crates/hir-def/src/lang_item.rs

@@ -410,6 +410,7 @@ language_item_table! {
     PanicLocation,           sym::panic_location,      panic_location,             Target::Struct,         GenericRequirement::None;
     PanicImpl,               sym::panic_impl,          panic_impl,                 Target::Fn,             GenericRequirement::None;
     PanicCannotUnwind,       sym::panic_cannot_unwind, panic_cannot_unwind,        Target::Fn,             GenericRequirement::Exact(0);
+    PanicNullPointerDereference, sym::panic_null_pointer_dereference, panic_null_pointer_dereference, Target::Fn, GenericRequirement::None;
     /// libstd panic entry point. Necessary for const eval to be able to catch it
     BeginPanic,              sym::begin_panic,         begin_panic_fn,             Target::Fn,             GenericRequirement::None;
 

Diff for: tests/ui/mir/null/addrof_null.rs

@@ -0,0 +1,13 @@
+//@ run-pass
+//@ compile-flags: -C debug-assertions
+
+struct Field {
+    a: u32,
+}
+
+fn main() {
+    unsafe {
+        let ptr: *const Field = std::ptr::null();
+        let _ptr = core::ptr::addr_of!((*ptr).a);
+    }
+}

Diff for: tests/ui/mir/null/null_lhs.rs

@@ -0,0 +1,10 @@
+//@ run-fail
+//@ compile-flags: -C debug-assertions
+//@ error-pattern: null pointer dereference occured
+
+fn main() {
+    let ptr: *mut u32 = std::ptr::null_mut();
+    unsafe {
+        *(ptr) = 42;
+    }
+}

Diff for: tests/ui/mir/null/null_rhs.rs

@@ -0,0 +1,10 @@
+//@ run-fail
+//@ compile-flags: -C debug-assertions
+//@ error-pattern: null pointer dereference occured
+
+fn main() {
+    let ptr: *mut u32 = std::ptr::null_mut();
+    unsafe {
+        let _v = *ptr;
+    }
+}

Diff for: tests/ui/mir/null/place_without_read.rs

@@ -0,0 +1,9 @@
+//@ run-pass
+//@ compile-flags: -C debug-assertions
+
+fn main() {
+    let ptr: *const u16 = std::ptr::null();
+    unsafe {
+        let _ = *ptr;
+    }
+}

Diff for: tests/ui/mir/null/two_pointers.rs

@@ -0,0 +1,12 @@
+//@ run-fail
+//@ compile-flags: -C debug-assertions
+//@ error-pattern: null pointer dereference occured
+
+fn main() {
+    let ptr = std::ptr::null();
+    let mut dest = 0u32;
+    let dest_ptr = &mut dest as *mut u32;
+    unsafe {
+        *dest_ptr = *(ptr);
+    }
+}

Diff for: tests/ui/mir/null/use_null.rs

@@ -0,0 +1,8 @@
+//@ run-fail
+//@ compile-flags: -C debug-assertions
+//@ error-pattern: null pointer dereference occured
+
+fn main() {
+    let ptr: *const u32 = std::ptr::null();
+    let _ptr = unsafe { *ptr };
+}