[Build System] Replace IMPLICIT_DEPENDS with DEPFILE_Fixes #21203

[DhanashreePetare]

This Pull request:

Replaces the broken IMPLICIT_DEPENDS with modern DEPFILE support in ROOT_GENERATE_DICTIONARY macro to enable cross-generator compatibility and proper dependency tracking.

Changes or fixes:

Modified cmake/modules/RootMacros.cmake:

• Removed IMPLICIT_DEPENDS usage (only works with Unix Makefiles, broken for cross-directory deps)
• Added DEPFILE parameter pointing to ${CMAKE_CURRENT_BINARY_DIR}/${dictionary}.d
• Passes -MF ${depfile_path} flag to rootcling

Modified core/dictgen/src/rootcling_impl.cxx:

• Added -MF command-line option to rootcling
• Implemented dependency file generation using Clang's FileManager API
• Writes Makefile-format dependency files listing all included headers
• Proper error handling for file operations

Implementation flow:

1. CMake creates depfile path: build/dir/G__Dict.d
2. CMake invokes: rootcling ... -MF build/dir/G__Dict.d
3. rootcling parses headers using Clang and tracks all included files
4. rootcling writes: G__Dict.cxx: header1.h header2.h ...
5. CMake reads the depfile for dependency tracking
6. Incremental builds only rebuild when dependencies actually change

Benefits:

• ✅ Works with Ninja generator (faster builds)
• ✅ Works with Visual Studio generator
• ✅ Works with Unix Makefiles
• ✅ Proper cross-directory dependency tracking
• ✅ Follows CMake best practices (DEPFILE is recommended over IMPLICIT_DEPENDS)
• ✅ Backward compatible (no interface changes to ROOT_GENERATE_DICTIONARY)

References:

• CMake Discourse recommendation
• CMake Issue #27521

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #21203

[pcanal]

Consider also removing the creation of the _implicitdeps list.

[github-actions]

Test Results

    22 files      22 suites   3d 6h 12m 37s ⏱️
 3 798 tests  3 749 ✅   0 💤 49 ❌
76 400 runs  76 041 ✅ 304 💤 55 ❌

For more details on these failures, see this check.

Results for commit 7da1877.

[guitargeek]

That is pretty cool! By better tracking of the dictionary dependencies, we'll be able to also avoid some unnecessary rebuilds of the dictionaries later, so this is an important change. @DhanashreePetare, can you please look at the CI failures and fix the implementation also for Windows and Ninja? Thanks!

[pcanal]

I wonder if the file ending in .d is leading to it being used 'also' in different context.

[pcanal]

Were you able to run the test locally successfully?