fix: clear 1 warning - report the real error when a Windows G-code export fails (#15582)

fix: report the real error when a Windows G-code export fails

copy_file built its failure message as "Error: " + errCode. Adding a DWORD
to a string literal is pointer arithmetic, not concatenation, so the pointer
lands errCode bytes into an 8-byte literal and runs past its end for any code
above 7. std::string then calls strlen on it and throws length_error, and the
catch(...) in BackgroundSlicingProcess::finalize_gcode replaces the diagnosis
with "Unknown error occurred during exporting G-code."

Every code a user is likely to hit is past the end: write-protected media is
19, no media 21, a full disk 112, and a destination held open by another
program 32. Codes 1 to 7 stay inside the literal and produce a truncated
message instead. So the "Maybe the SD card is write locked?" text has not
been reachable on Windows since this path was added in #2923.

Now that it is reachable, that guess only fits removable media, so it is
conditional on m_export_path_on_removable_media. The existing string is
untouched and keeps its 23 translations; the fixed-drive case adds one string.

src/libslic3r/utils.cpp

@@ -961,7 +961,7 @@ CopyFileResult copy_file(const std::string &from, const std::string &to, std::st
     BOOL result = CopyFileW(src_wstr, dst_wstr, FALSE);
     if (!result) {
         DWORD errCode = GetLastError();
-        error_message = "Error: " + errCode;
+        error_message = "Error: " + std::to_string(errCode);
         ret = FAIL_COPY_FILE;
         goto __finished;
     }

src/slic3r/GUI/BackgroundSlicingProcess.cpp

@@ -848,7 +848,9 @@ void BackgroundSlicingProcess::finalize_gcode()
     case CopyFileResult::SUCCESS: break; // no error
     case CopyFileResult::FAIL_COPY_FILE:
         throw Slic3r::ExportError(GUI::format(
-            _L("Copying of the temporary G-code to the output G-code failed. Maybe the SD card is write locked?\nError message: %1%"),
+            m_export_path_on_removable_media ?
+                _L("Copying of the temporary G-code to the output G-code failed. Maybe the SD card is write locked?\nError message: %1%") :
+                _L("Copying of the temporary G-code to the output G-code failed.\nError message: %1%"),
             error_message));
         break;
     case CopyFileResult::FAIL_FILES_DIFFERENT:

tests/libslic3r/test_utils.cpp

@@ -2,6 +2,13 @@
 
 #include "libslic3r/Utils.hpp"
 
+#include "test_utils.hpp"
+
+#include <algorithm>
+#include <cctype>
+#include <fstream>
+#include <string>
+
 #ifndef _WIN32
 #include <unistd.h>     // getuid
 #endif
@@ -52,3 +59,32 @@ TEST_CASE("per-user temp root is unchanged on Windows, isolated elsewhere", "[ut
     REQUIRE_THAT(root, Catch::Matchers::StartsWith(base + "/orcaslicer_"));
 #endif
 }
+
+TEST_CASE("copy_file reports the OS error when the destination cannot be written", "[utils]") {
+    ScopedTemporaryFile source(".txt");
+    {
+        std::ofstream ofs(source.string(), std::ios::binary);
+        ofs << "orca";
+    }
+    REQUIRE(boost::filesystem::exists(source.path()));
+
+    // A directory that was never created, so the copy fails on every platform.
+    const boost::filesystem::path destination = source.path().parent_path() / "orca-missing-dir" / "copy.txt";
+    REQUIRE_FALSE(boost::filesystem::exists(destination.parent_path()));
+
+    std::string error_message;
+    REQUIRE(copy_file(source.string(), destination.string(), error_message) == FAIL_COPY_FILE);
+    REQUIRE_FALSE(error_message.empty());
+
+#ifdef _WIN32
+    // The Windows branch formats GetLastError() itself. Writing that as
+    // "Error: " + errCode adds an integer to a string literal, which indexes into the
+    // literal instead of appending and runs off its end for any code above 7.
+    const std::string prefix = "Error: ";
+    REQUIRE(error_message.rfind(prefix, 0) == 0);
+
+    const std::string code = error_message.substr(prefix.size());
+    REQUIRE_FALSE(code.empty());
+    REQUIRE(std::all_of(code.begin(), code.end(), [](unsigned char c) { return std::isdigit(c) != 0; }));
+#endif // _WIN32
+}