Suggest null ptr if 0 is given as a raw ptr arg

compiler/rustc_hir_typeck/src/demand.rs

@@ -60,6 +60,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             || self.suggest_clone_for_ref(err, expr, expr_ty, expected)
             || self.suggest_into(err, expr, expr_ty, expected)
             || self.suggest_floating_point_literal(err, expr, expected)
+            || self.suggest_null_ptr_for_literal_zero_given_to_ptr_arg(err, expr, expected)
             || self.note_result_coercion(err, expr, expected, expr_ty);
         if !suggested {
             self.point_at_expr_source_of_inferred_type(err, expr, expr_ty, expected, expr.span);

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -13,6 +13,7 @@ use rustc_hir::{
 use rustc_hir_analysis::astconv::AstConv;
 use rustc_infer::traits::{self, StatementAsExpression};
 use rustc_middle::lint::in_external_macro;
+use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::{
     self, suggest_constraining_type_params, Binder, DefIdTree, IsSuggestable, ToPredicate, Ty,
     TypeVisitable,
@@ -1244,6 +1245,49 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
+    /// Suggest providing `std::ptr::null()` or `std::ptr::null_mut()` if they
+    /// pass in a literal 0 to an raw pointer.
+    #[instrument(skip(self, err))]
+    pub(crate) fn suggest_null_ptr_for_literal_zero_given_to_ptr_arg(
+        &self,
+        err: &mut Diagnostic,
+        expr: &hir::Expr<'_>,
+        expected_ty: Ty<'tcx>,
+    ) -> bool {
+        // Expected type needs to be a raw pointer.
+        let ty::RawPtr(ty::TypeAndMut { mutbl, .. }) = expected_ty.kind() else {
+            return false;
+        };
+
+        // Provided expression needs to be a literal `0`.
+        let ExprKind::Lit(Spanned {
+            node: rustc_ast::LitKind::Int(0, _),
+            span,
+        }) = expr.kind else {
+            return false;
+        };
+
+        // We need to find a null pointer symbol to suggest
+        let null_sym = match mutbl {
+            hir::Mutability::Not => sym::ptr_null,
+            hir::Mutability::Mut => sym::ptr_null_mut,
+        };
+        let Some(null_did) = self.tcx.get_diagnostic_item(null_sym) else {
+            return false;
+        };
+        let null_path_str = with_no_trimmed_paths!(self.tcx.def_path_str(null_did));
+
+        // We have satisfied all requirements to provide a suggestion. Emit it.
+        err.span_suggestion(
+            span,
+            format!("if you meant to create a null pointer, use `{null_path_str}()`"),
+            null_path_str + "()",
+            Applicability::MachineApplicable,
+        );
+
+        true
+    }
+
     pub(crate) fn suggest_associated_const(
         &self,
         err: &mut Diagnostic,

tests/ui/suggest-null-ptr.fixed

@@ -0,0 +1,31 @@
+// run-rustfix
+
+// Suggest providing a std::ptr::null{,_mut}() to a function that takes in a raw
+// pointer if a literal 0 was provided by the user.
+
+extern "C" {
+    fn foo(ptr: *const u8);
+
+    fn foo_mut(ptr: *mut u8);
+
+    fn usize(ptr: *const usize);
+
+    fn usize_mut(ptr: *mut usize);
+}
+
+fn main() {
+    unsafe {
+        foo(std::ptr::null());
+        //~^ mismatched types [E0308]
+        //~| if you meant to create a null pointer, use `std::ptr::null()`
+        foo_mut(std::ptr::null_mut());
+        //~^ mismatched types [E0308]
+        //~| if you meant to create a null pointer, use `std::ptr::null_mut()`
+        usize(std::ptr::null());
+        //~^ mismatched types [E0308]
+        //~| if you meant to create a null pointer, use `std::ptr::null()`
+        usize_mut(std::ptr::null_mut());
+        //~^ mismatched types [E0308]
+        //~| if you meant to create a null pointer, use `std::ptr::null_mut()`
+    }
+}

tests/ui/suggest-null-ptr.rs

@@ -0,0 +1,31 @@
+// run-rustfix
+
+// Suggest providing a std::ptr::null{,_mut}() to a function that takes in a raw
+// pointer if a literal 0 was provided by the user.
+
+extern "C" {
+    fn foo(ptr: *const u8);
+
+    fn foo_mut(ptr: *mut u8);
+
+    fn usize(ptr: *const usize);
+
+    fn usize_mut(ptr: *mut usize);
+}
+
+fn main() {
+    unsafe {
+        foo(0);
+        //~^ mismatched types [E0308]
+        //~| if you meant to create a null pointer, use `std::ptr::null()`
+        foo_mut(0);
+        //~^ mismatched types [E0308]
+        //~| if you meant to create a null pointer, use `std::ptr::null_mut()`
+        usize(0);
+        //~^ mismatched types [E0308]
+        //~| if you meant to create a null pointer, use `std::ptr::null()`
+        usize_mut(0);
+        //~^ mismatched types [E0308]
+        //~| if you meant to create a null pointer, use `std::ptr::null_mut()`
+    }
+}

tests/ui/suggest-null-ptr.stderr

@@ -0,0 +1,83 @@
+error[E0308]: mismatched types
+  --> $DIR/suggest-null-ptr.rs:18:13
+   |
+LL |         foo(0);
+   |         --- ^ expected `*const u8`, found `usize`
+   |         |
+   |         arguments to this function are incorrect
+   |
+   = note: expected raw pointer `*const u8`
+                     found type `usize`
+note: function defined here
+  --> $DIR/suggest-null-ptr.rs:7:8
+   |
+LL |     fn foo(ptr: *const u8);
+   |        ^^^
+help: if you meant to create a null pointer, use `std::ptr::null()`
+   |
+LL |         foo(std::ptr::null());
+   |             ~~~~~~~~~~~~~~~~
+
+error[E0308]: mismatched types
+  --> $DIR/suggest-null-ptr.rs:21:17
+   |
+LL |         foo_mut(0);
+   |         ------- ^ expected `*mut u8`, found `usize`
+   |         |
+   |         arguments to this function are incorrect
+   |
+   = note: expected raw pointer `*mut u8`
+                     found type `usize`
+note: function defined here
+  --> $DIR/suggest-null-ptr.rs:9:8
+   |
+LL |     fn foo_mut(ptr: *mut u8);
+   |        ^^^^^^^
+help: if you meant to create a null pointer, use `std::ptr::null_mut()`
+   |
+LL |         foo_mut(std::ptr::null_mut());
+   |                 ~~~~~~~~~~~~~~~~~~~~
+
+error[E0308]: mismatched types
+  --> $DIR/suggest-null-ptr.rs:24:15
+   |
+LL |         usize(0);
+   |         ----- ^ expected `*const usize`, found `usize`
+   |         |
+   |         arguments to this function are incorrect
+   |
+   = note: expected raw pointer `*const usize`
+                     found type `usize`
+note: function defined here
+  --> $DIR/suggest-null-ptr.rs:11:8
+   |
+LL |     fn usize(ptr: *const usize);
+   |        ^^^^^
+help: if you meant to create a null pointer, use `std::ptr::null()`
+   |
+LL |         usize(std::ptr::null());
+   |               ~~~~~~~~~~~~~~~~
+
+error[E0308]: mismatched types
+  --> $DIR/suggest-null-ptr.rs:27:19
+   |
+LL |         usize_mut(0);
+   |         --------- ^ expected `*mut usize`, found `usize`
+   |         |
+   |         arguments to this function are incorrect
+   |
+   = note: expected raw pointer `*mut usize`
+                     found type `usize`
+note: function defined here
+  --> $DIR/suggest-null-ptr.rs:13:8
+   |
+LL |     fn usize_mut(ptr: *mut usize);
+   |        ^^^^^^^^^
+help: if you meant to create a null pointer, use `std::ptr::null_mut()`
+   |
+LL |         usize_mut(std::ptr::null_mut());
+   |                   ~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0308`.