Don't lint on derefs of null pointers to ZSTs

Author: saethlin

compiler/rustc_lint/src/builtin.rs

@@ -2726,6 +2726,16 @@ impl<'tcx> LateLintPass<'tcx> for DerefNullPtr {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &hir::Expr<'_>) {
         /// test if expression is a null ptr
         fn is_null_ptr(cx: &LateContext<'_>, expr: &hir::Expr<'_>) -> bool {
+            let pointer_ty = cx.typeck_results().expr_ty(expr);
+            let ty::RawPtr(pointee, _) = pointer_ty.kind() else {
+                return false;
+            };
+            if let Ok(layout) = cx.tcx.layout_of(cx.typing_env().as_query_input(*pointee)) {
+                if layout.layout.size() == rustc_abi::Size::ZERO {
+                    return false;
+                }
+            }
+
             match &expr.kind {
                 hir::ExprKind::Cast(expr, ty) => {
                     if let hir::TyKind::Ptr(_) = ty.kind {

tests/ui/lint/lint-deref-nullptr.rs

@@ -1,13 +1,14 @@
 // test the deref_nullptr lint
 
-#![deny(deref_nullptr)]
-
 use std::ptr;
 
 struct Struct {
     field: u8,
 }
 
+#[derive(Clone, Copy)]
+struct Zst;
+
 fn f() {
     unsafe {
         let a = 1;
@@ -32,6 +33,11 @@ fn f() {
         // ^^ OKAY
         let offset = ptr::addr_of!((*ptr::null::<Struct>()).field);
         //~^ ERROR dereferencing a null pointer
+
+        // Make sure the lint permits derefs of null pointers to ZSTs
+        let ok: Zst = *ptr::null();
+        let ok: Zst = *ptr::null_mut();
+        let ok: Zst = *(0 as *const Zst);
     }
 }
 

tests/ui/lint/lint-deref-nullptr.stderr

@@ -1,53 +1,49 @@
 error: dereferencing a null pointer
-  --> $DIR/lint-deref-nullptr.rs:15:18
+  --> $DIR/lint-deref-nullptr.rs:16:18
    |
 LL |         let ub = *(0 as *const i32);
    |                  ^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
    |
-note: the lint level is defined here
-  --> $DIR/lint-deref-nullptr.rs:3:9
-   |
-LL | #![deny(deref_nullptr)]
-   |         ^^^^^^^^^^^^^
+   = note: `#[deny(deref_nullptr)]` on by default
 
 error: dereferencing a null pointer
-  --> $DIR/lint-deref-nullptr.rs:17:18
+  --> $DIR/lint-deref-nullptr.rs:18:18
    |
 LL |         let ub = *ptr::null::<i32>();
    |                  ^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
 
 error: dereferencing a null pointer
-  --> $DIR/lint-deref-nullptr.rs:19:18
+  --> $DIR/lint-deref-nullptr.rs:20:18
    |
 LL |         let ub = *ptr::null_mut::<i32>();
    |                  ^^^^^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
 
 error: dereferencing a null pointer
-  --> $DIR/lint-deref-nullptr.rs:21:18
+  --> $DIR/lint-deref-nullptr.rs:22:18
    |
 LL |         let ub = *(ptr::null::<i16>() as *const i32);
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
 
 error: dereferencing a null pointer
-  --> $DIR/lint-deref-nullptr.rs:23:18
+  --> $DIR/lint-deref-nullptr.rs:24:18
    |
 LL |         let ub = *(ptr::null::<i16>() as *mut i32 as *mut usize as *const u8);
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
 
 error: dereferencing a null pointer
-  --> $DIR/lint-deref-nullptr.rs:25:19
+  --> $DIR/lint-deref-nullptr.rs:26:19
    |
 LL |         let ub = &*ptr::null::<i32>();
    |                   ^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
 
 error: dereferencing a null pointer
-  --> $DIR/lint-deref-nullptr.rs:27:19
+  --> $DIR/lint-deref-nullptr.rs:28:19
    |
 LL |         let ub = &*ptr::null_mut::<i32>();
    |                   ^^^^^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed
 
 error: dereferencing a null pointer
-  --> $DIR/lint-deref-nullptr.rs:33:36
+  --> $DIR/lint-deref-nullptr.rs:34:36
    |
 LL |         let offset = ptr::addr_of!((*ptr::null::<Struct>()).field);
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^^ this code causes undefined behavior when executed