Suggest using ptr::null_mut when user supplied ptr::null to a function expecting ptr::null_mut

compiler/rustc_hir_typeck/messages.ftl

@@ -87,6 +87,8 @@ hir_typeck_suggest_boxing_note = for more on the distinction between the stack a
 
 hir_typeck_suggest_boxing_when_appropriate = store this in the heap by calling `Box::new`
 
+hir_typeck_suggest_ptr_null_mut = consider using `core::ptr::null_mut` instead
+
 hir_typeck_union_pat_dotdot = `..` cannot be used in union patterns
 
 hir_typeck_union_pat_multiple_fields = union patterns should have exactly one field

compiler/rustc_hir_typeck/src/errors.rs

@@ -298,6 +298,17 @@ pub enum SuggestBoxing {
     },
 }
 
+#[derive(Subdiagnostic)]
+#[suggestion(
+    hir_typeck_suggest_ptr_null_mut,
+    applicability = "maybe-incorrect",
+    code = "core::ptr::null_mut()"
+)]
+pub struct SuggestPtrNullMut {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(hir_typeck_no_associated_item, code = "E0599")]
 pub struct NoAssociatedItem {

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1,4 +1,5 @@
 use crate::coercion::CoerceMany;
+use crate::errors::SuggestPtrNullMut;
 use crate::fn_ctxt::arg_matrix::{ArgMatrix, Compatibility, Error, ExpectedIdx, ProvidedIdx};
 use crate::gather_locals::Declaration;
 use crate::method::MethodCallee;
@@ -814,6 +815,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 );
             }
 
+            self.suggest_ptr_null_mut(
+                expected_ty,
+                provided_ty,
+                provided_args[*provided_idx],
+                &mut err,
+            );
+
             // Call out where the function is defined
             self.label_fn_like(
                 &mut err,
@@ -1271,6 +1279,28 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         err.emit();
     }
 
+    fn suggest_ptr_null_mut(
+        &self,
+        expected_ty: Ty<'tcx>,
+        provided_ty: Ty<'tcx>,
+        arg: &hir::Expr<'tcx>,
+        err: &mut rustc_errors::DiagnosticBuilder<'tcx, ErrorGuaranteed>,
+    ) {
+        if let ty::RawPtr(ty::TypeAndMut { mutbl: hir::Mutability::Mut, .. }) = expected_ty.kind()
+            && let ty::RawPtr(ty::TypeAndMut { mutbl: hir::Mutability::Not, .. }) = provided_ty.kind()
+            && let hir::ExprKind::Call(callee, _) = arg.kind
+            && let hir::ExprKind::Path(hir::QPath::Resolved(_, path)) = callee.kind
+            && let Res::Def(_, def_id) = path.res
+            && self.tcx.get_diagnostic_item(sym::ptr_null) == Some(def_id)
+        {
+            // The user provided `ptr::null()`, but the function expects
+            // `ptr::null_mut()`.
+            err.subdiagnostic(SuggestPtrNullMut {
+                span: arg.span
+            });
+        }
+    }
+
     // AST fragment checking
     pub(in super::super) fn check_lit(
         &self,