Emit message if R.h included, can turn to warning

ChangeLog

@@ -1,3 +1,14 @@
+2025-11-24  Dirk Eddelbuettel  <edd@debian.org>
+
+	* inst/include/Rcpp/r/check_r_headers.h: Add RCPP_NO_R_HEADERS_CHECK
+	override to skip new check
+
+2025-11-23  Dirk Eddelbuettel  <edd@debian.org>
+
+	* inst/include/Rcpp/r/check_r_headers.h: New header to check if R.h
+	or related R headers were installed first
+	* inst/include/RcppCommon.h: Call new header as first thing
+
 2025-11-04  Dirk Eddelbuettel  <edd@debian.org>
 
 	* .github/workflows/macos.yaml (jobs): Roll macos-13 to macos-14

inst/include/Rcpp/r/check_r_headers.h

@@ -0,0 +1,44 @@
+// check_r_headers.h: Rcpp R/C++ interface class library -- R header check
+//
+// Copyright (C) 2025 - current Dirk Eddelbuettel
+//
+// This file is part of Rcpp.
+//
+// Rcpp is free software: you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 2 of the License, or
+// (at your option) any later version.
+//
+// Rcpp is distributed in the hope that it will be useful, but
+// WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Rcpp.  If not, see <http://www.gnu.org/licenses/>.
+
+#ifndef RCPP__CHECK__R__HEADERS__H
+#define RCPP__CHECK__R__HEADERS__H
+
+// Allow an escape hatch
+#if !defined(RCPP_NO_R_HEADERS_CHECK)
+
+  #if defined(R_R_H) && defined(USING_R)
+    #pragma message "R.h has been included before any Rcpp headers. This can lead to hard-to-debug errors, and is not necessary. See https://github.com/RcppCore/Rcpp/issues/1410"
+  #endif
+
+  #if defined(RINTERFACE_H_)
+    #pragma message "Rinterface.h has been included before any Rcpp headers. This can lead to hard-to-debug errors, and is not necessary. See https://github.com/RcppCore/Rcpp/issues/1410"
+  #endif
+
+  #if defined(R_INTERNALS_H_)
+    #pragma message "Rinternals.h has been included before any Rcpp headers. This can lead to hard-to-debug errors, and is not necessary. See https://github.com/RcppCore/Rcpp/issues/1410"
+  #endif
+
+  #if defined(R_DEFINES_H_)
+    #pragma message "Rdefines.h has been included before any Rcpp headers. This can lead to hard-to-debug errors, and is not necessary. See https://github.com/RcppCore/Rcpp/issues/1410"
+  #endif
+
+#endif  // escape hatch '!defined(RCPP_NO_R_HEADERS_CHECK)'
+
+#endif  // header guard

inst/include/RcppCommon.h

@@ -4,7 +4,7 @@
 //
 // Copyright (C) 2008 - 2009  Dirk Eddelbuettel
 // Copyright (C) 2009 - 2020  Dirk Eddelbuettel and Romain Francois
-// Copyright (C) 2021         Dirk Eddelbuettel, Romain Francois and Iñaki Ucar
+// Copyright (C) 2021 - 2025  Dirk Eddelbuettel, Romain Francois and Iñaki Ucar
 //
 // This file is part of Rcpp.
 //
@@ -27,6 +27,7 @@
 // #define RCPP_DEBUG_LEVEL 1
 // #define RCPP_DEBUG_MODULE_LEVEL 1
 
+#include <Rcpp/r/check_r_headers.h>
 #include <Rcpp/r/headers.h>
 #include <Rcpp/r/compat.h>