nmake: support long comments for custom commands

Fixes: #27980

Source/cmLocalUnixMakefileGenerator3.cxx

@@ -1235,6 +1235,48 @@ void cmLocalUnixMakefileGenerator3::AppendDirectoryCleanCommand(
   }
 }
 
+std::string cmLocalUnixMakefileGenerator3::TrimLongCommand(
+  std::string cmd, std::string& line) const
+{
+  cm::static_string_view longCommandFinalizer = "..."_s;
+
+  size_t commandLineLimit = cmSystemTools::CalculateCommandLineLengthLimit();
+  if (commandLineLimit == 0) {
+    // No limit, just return the command as is.
+    return cmStrCat(std::move(cmd), this->EscapeForShell(line));
+  }
+
+  size_t usedCommandLineLength = cmd.size() + longCommandFinalizer.size();
+
+  if (usedCommandLineLength >= commandLineLimit) {
+    cmSystemTools::Error(cmStrCat(
+      "CMake command line length limit exceeded for command: ", cmd, line));
+    return cmd;
+  }
+
+  size_t remainingSpace = commandLineLimit - usedCommandLineLength;
+  if (remainingSpace < line.size()) {
+    line.erase(remainingSpace);
+    line.append(longCommandFinalizer.data(), longCommandFinalizer.size());
+  }
+
+  // re-escape the line: we cannot just trim the line, because it may contain
+  // characters that cannot be removed without their pairs:
+  // * quotes in the end of the line
+  // * backslash-escaped characters in the middle of the line
+  std::string escapedLine = this->EscapeForShell(line);
+  size_t overflowSize = escapedLine.size() > remainingSpace
+    ? escapedLine.size() - remainingSpace
+    : 0;
+  if (overflowSize > 0) {
+    line.erase(remainingSpace - overflowSize);
+    line.append(longCommandFinalizer.data(), longCommandFinalizer.size());
+    escapedLine = this->EscapeForShell(line);
+  }
+
+  return cmStrCat(std::move(cmd), std::move(escapedLine));
+}
+
 void cmLocalUnixMakefileGenerator3::AppendEcho(
   std::vector<std::string>& commands, std::string const& text, EchoColor color,
   EchoProgress const* progress)
@@ -1288,7 +1330,7 @@ void cmLocalUnixMakefileGenerator3::AppendEcho(
                              progress->Dir, cmOutputConverter::SHELL),
                            " --progress-num=", progress->Arg, ' ');
           }
-          cmd += this->EscapeForShell(line);
+          cmd = this->TrimLongCommand(std::move(cmd), line);
         }
         commands.emplace_back(std::move(cmd));
       }

Source/cmLocalUnixMakefileGenerator3.h

@@ -250,6 +250,8 @@ protected:
                         cmDepends::DependencyMap& validDeps);
   void CheckMultipleOutputs(bool verbose);
 
+  std::string TrimLongCommand(std::string cmd, std::string& line) const;
+
 private:
   std::string MaybeConvertWatcomShellCommand(std::string const& cmd);
 

Tests/RunCMake/add_custom_command/LongCommentAutoGenerated.cmake

@@ -0,0 +1,8 @@
+file(GLOB_RECURSE FILES RELATIVE "${CMAKE_ROOT}" "${CMAKE_ROOT}/Modules/*")
+
+add_custom_command(
+    OUTPUT ${FILES}
+    COMMAND "${CMAKE_COMMAND}" -E copy_directory "${CMAKE_ROOT}/Modules" "${CMAKE_CURRENT_BINARY_DIR}/Modules"
+)
+
+add_custom_target(LongCommentAutoGenerated ALL DEPENDS ${FILES})

Tests/RunCMake/add_custom_command/LongCommentUserProvided.cmake

@@ -0,0 +1,12 @@
+set(manualComment "Message X times:")
+foreach(i RANGE 1 10850)
+    set(manualComment "${manualComment} hello")
+endforeach()
+
+add_custom_command(
+    OUTPUT out.txt
+    COMMAND "${CMAKE_COMMAND}" -E touch out.txt
+    COMMENT "${manualComment}"
+)
+
+add_custom_target(LongCommentUserProvided ALL DEPENDS out.txt)

Tests/RunCMake/add_custom_command/RunCMakeTest.cmake

@@ -24,6 +24,15 @@ run_cmake(LiteralQuotes)
 if(NOT RunCMake_GENERATOR STREQUAL "Xcode")
   run_configure_and_build(LotsOutputs)
 endif()
+# FIXME(#28019): LongCommentAutoGenerated test fails with Xcode generator on macOS 10.15 only.
+# FIXME(#28013): LongCommentAutoGenerated test fails with Borland Makefiles generator.
+if(NOT RunCMake_GENERATOR STREQUAL "Borland Makefiles" AND NOT RunCMake_GENERATOR STREQUAL "Xcode")
+  run_configure_and_build(LongCommentAutoGenerated)
+endif()
+# FIXME(#28013): LongCommentUserProvided test fails with Borland Makefiles generator.
+if(NOT RunCMake_GENERATOR STREQUAL "Borland Makefiles")
+  run_configure_and_build(LongCommentUserProvided)
+endif()
 run_cmake(NoArguments)
 run_cmake(NoOutputOrTarget)
 run_cmake(OutputAndTarget)